Transterrestrial Musings

Comments

A first stage with a scramjet may be designed such to eliminate large launch pads. This would lower launch costs.

Posted by Leland at June 16, 2007 08:44 AM

You don't need a scramjet to air launch.

Posted by Rand Simberg at June 16, 2007 08:48 AM

It's not the raw material cost of oxygen that makes it desirable to leave behind- it's the weight. Every pound of inexpensive LOX is another pound of multi-million-dollar satellite that you can't lift.

Scramjets are also somewhat throttlable, and I think their nozzles can be made more easilly efficient. Those bell-shaped rocket nozzles are 'stuck,' efficient only at one altitude. Along with the weight savings of a scramjet, I believe it is easier to incorporate tunable nozzles (cooler exhaust gasses, maybe?).

Lots of reasons to like scramjets. Now, if we can only get them to work...

Posted by rc at June 16, 2007 09:07 AM

Rand:

You wrote: Proponents claim that by allowing airbreathing up to high Mach numbers, there is no need to take along as much oxygen for the rocket engines, because they can gather it for "free." This argument assumes that space transportation is expensive because propellants are, but those aren't the cost driver. If they were, space would already be affordable, because liquid oxygen is actually about as cheap as milk. Propellant costs are such a tiny fraction of launch costs that they're down in the noise. If we ever get to the point where they become a real issue (as they are for airlines), we'll have solved the problem.

The real issue isn't cost as in price, but cost as in weight. If you have to carry LOX into space, that is weight that cannot be devoted to other tasks (payload or other equipment). If the scramjet can do a substantial portion of the lifting, that means that you could theoretically devote more to payload or equipment to get into space.

Posted by lawhawk at June 16, 2007 09:09 AM

Every pound of inexpensive LOX is another pound of multi-million-dollar satellite that you can't lift.

No, it's not. This kind of simplistic (and incorrect) analysis is the reason that scram jets have such appeal. But anyone who has actually attempted to design one always finds out that rockets turn out to be much more effective.

Posted by Rand Simberg at June 16, 2007 09:10 AM

If the scramjet can do a substantial portion of the lifting, that means that you could theoretically devote more to payload or equipment to get into space.

I'm sorry, but that is true in neither theory, or practice. Any weight savings resulting from getting oxygen from the atmosphere is more than eaten up by the heavier engines, the heavy wings needed to operate in the atmosphere, and the additional velocity required to compensate for all the drag. No matter how appealing the concept might be, it alway falls apart upon actual analysis of an actual vehicle design. There was a good reason that NASP never got built, and it wasn't just because we didn't have the scram jet technology.

Posted by Rand Simberg at June 16, 2007 09:14 AM

Dead spot on, Rand:

Solving the real problem--getting enough market and enough activity to get operational economies of scale, the lack of which is the real cause of the high cost of space access.

Not only is the fuel inexpensive, but rockets engines would also be inexpensive (relatively speaking) if produced in bulk.

I recall reading an assertion that an RL-10 is at about the same level of manufacturing sophistication as the gas turbine engines used in helos and M-1 Abrams tanks. But RL-10s are hand built with prices measured in the millions of dollars per unit while excellent quality gas turbine engines routinely sell for about $100,000.

Of course, more than ten thousand gas turbine engines are produced each year while RL-10 production can be counted using one's fingers.

Moving on, the real expense and risk is systems integration, as Rand correctly notes in his SpaceX post.

The answer to that, it seems to me, is to build a robust launch system with ample margins and then use it over and over to eliminate the bugs. And therefore, high launch rates are the solution yet high launch rates will require more missions than the taxpayers can ever afford to buy.

So we come full circle.

Low launch costs will require commercial demand for space access.

Posted by Bill White at June 16, 2007 09:16 AM

Let me nitpick myself;

And therefore, high launch rates are the solution yet high launch rates will require more missions than the taxpayers can ever afford to buy.

High launch rates will require more missions than the taxpayers are likely to buy. Lower costs enough and it would be affordable, but there just aren't enough worthwhile non-commercial missions to fly.

Posted by Bill White at June 16, 2007 09:19 AM

...and the additional velocity required to compensate for all the drag...

I think Mary Shafer (and Ken Iliff) would promote this to the top of the list of show-stoppers. Air-breather enthusiasts are so thrilled by the "free lunch" of oxidizer that they simply refuse to look at the fundamental curves of drag with increasing speed. For nearly all flight profiles, the extra thrust needed eats up all your mass savings -- and more -- unless you go so high that you'd need an intake the size of Kansas.

IOW, TANSTAAFL.

Posted by Monte Davis at June 16, 2007 09:51 AM

Therefore, convincing rich people to subsidize high flight rates by buying tourist flights puts the burden of developing cheap spaceflight (a socially useful public good) on the rich instead of the general taxpayer population. Progressives ought to like it.

Of course, people would be having fun. Now, there's the problem!

Posted by Jim Bennett at June 16, 2007 10:44 AM

Humor someone who's only science class in college was Introduction to Oceanography: A 747 piggybacks a scram-jet to 45,000 feet. The scram-jet takes the payload to the edge of the atmosphere at mach 10 and flings it into space -- how is that not more efficient than spending a million dollars a pound sending LOX from the launchpad?

Posted by wayne s at June 16, 2007 11:07 AM

I'm glad someone pointed out some actual engineering reasons as disadvantages for airbreathing SSTOs rather than Rand's wacky "LOX costs" notion.

Edison thought commercial airtravel wouldn't happen until aircraft could be made VTOL. He had a point, but it didn't work out that way and we're stuck with a clunky alternative. Similarly, rockets may be more efficient, but in the long term I don't think passengers will enjoy sitting on top of a bomb that depends on it's engines working for them to survive take off and landings. No doubt we'll get past blowing up over one percent of all our orbital rocket launches someday, but if someone can get an airbreather to orbit, that's the company I'll be flying to space on.

Posted by K at June 16, 2007 11:15 AM

The short answer is that it doesn't cost a "million dollars a pound to send LOX from the launchpad."

Not to mention that you can't "fling something into space" from Mach 10 at the "edge of the atmosphere." You need a rocket to get into space. Your proposal is not only not "efficient," it doesn't work at all.

Look, if you don't understand the science (and the engineering), then you're going to have to take our word for it. No matter how appealing it sounds on the surface, air breathers don't work to get to orbit, at least not as well, or cheaply, as rockets. As I said, this is a distraction from the real (and difficult) problems of markets, and investment, and insurance, and scale. I guess, though, that it's more fun and easier to imagine that all we need is the right propulsion system.

Posted by Rand Simberg at June 16, 2007 11:16 AM

Just to flog my favorite mare, Tom Ligon did a presentation at ISDC on IEC fusion-powered spacecraft using polyhedral dynamic magnetic well (Polywell) fusion technology. The numbers on how it works for space travel are just staggering. A permanent moonbase for $12B, Mars for $15B.

If (and a big if) Polywell fusion works the way Bussard's theory predicts, and can be engineered for space travel, we could start colonizing the solar system in our lifetimes.

http://isdc2.xisp.net/~kmiller/isdc_archive/fileDownload.php/?link=fileSelect&file_id=422

Posted by TallDave at June 16, 2007 11:19 AM

No doubt we'll get past blowing up over one percent of all our orbital rocket launches someday, but if someone can get an airbreather to orbit, that's the company I'll be flying to space on.

Then you're going to wait an awful long time (perhaps forever) to get to space, and many thousands of others will be there before you.

Posted by Rand Simberg at June 16, 2007 11:20 AM

There is a much more fundamental problem with scramjets. They need to start engine operation at many times the speed of sound. No conventional airplane (such as a 747) can provide that launch. An article that gives more technical details is here: http://www.newscientisttech.com/article/dn12075-scramjet-hits-mach-10-over-australia-.html and includes a picture of the rocket which provided the needed boost.

Thus the "scramjet" part of the trip is in the form: rocket, scramjet, rocket. Further, the design of a rocket includes a carefully-shaped nose cone that minimizes drag. A scramjet nacelle practically maximizes drag, since it is designed as an air scoop.

The key quote in the article says that the "ultimate goal of the tests is to design an engine that produces more thrust than drag." If you said that was the purpose of a rocket design, you would be laughed out of the room.

Posted by docduke at June 16, 2007 11:41 AM

"Look, if you don't understand the science (and the engineering), then you're going to have to take our word for it."

Yeah, that's exactly the argument that Al Gore uses to explain why we should all move to the city and ride the bus. "Take my word for it."

What kind of engineer are you if you can't explain what you're working on? If you can't explain it to someone else, then you don't fully understand it yourself.

The guy didn't say "I'm stupid." He said "I haven't spent seven years of college and a twenty-year career living with this stuff."

Posted by DensityDuck at June 16, 2007 12:37 PM

Yeah, that's exactly the argument that Al Gore uses to explain why we should all move to the city and ride the bus. "Take my word for it."

Well, then again, I actually am an aerospace engineer who's worked on a lot of launch vehicle designs, and Al Gore is...well...the guy who flunked out of Divinity School.

The guy didn't say "I'm stupid."

I didn't say he was stupid, either. But he shouldn't try to design launch systems when by his own admission (and as demonstrated by his question) he knows nothing about them.

It's not an issue of being unable to explain what I'm working on (not that I'm currently working on it). It's an issue of not having time to write a long treatise and tutorial on launch system parametrics just to convince people who want to continue to believe in a technological tooth fairy.

Posted by Rand Simberg at June 16, 2007 12:42 PM

Similarly, rockets may be more efficient, but in the long term I don't think passengers will enjoy sitting on top of a bomb that depends on it's engines working for them to survive take off and landings.

Do you think airplanes are bombs, too?

Or don't you realize they depend on their engines working?

Posted by Edward Wright at June 16, 2007 12:50 PM

Those bell-shaped rocket nozzles are 'stuck,' efficient only at one altitude.

Adjustable bell nozzles have been around for decades.

Lots of reasons to like scramjets.

Yes, but very few of them are valid.

Posted by Edward Wright at June 16, 2007 12:52 PM

Building a launch vehicle designed to get to near-orbital velocities while using atmospheric oxygen is about as easy--and maybe as sensible--as a supersonic submarine.

Posted by tbrosz at June 16, 2007 01:21 PM

The bottom line on airbreathing propulsion is that it NEEDS to interact with the air in order to work (N.B. the clue is in the name), whereas rockets see air as just an obstruction!

As a result, ALL airbreathers are designed to work at a specific speed - their Design Point - and an awful lot of "bells and whistles" (i.e. mass) must be added to allow them to accelerate up to that design speed (e.g. variable intakes, variable compressor inlet guide vanes and variable exhaust nozzles). This is why airbreathing engines have a thrust-to-weight ratio of 10:1 or less compared to the 100:1 or more for rockets... so guess which type launcher designers prefer to carry into orbit if they have to.

The basic consequence of all this physics and engineering is that airbreathers are best suited for cruising and rockets are best suited for accelerating. So, if you want to go from 0 to 17500 mph in around 500 seconds, guess which form of propulsion is best!

Clearly, there's more to it than this and other considerations like staging, the need for wider launch windows or landing go-around may mean that some degree of airbreathing could be an advantage, though this is most likely to be sub-sonic.

Yes, if someone gave you (i.e. you didn't have to pay for it's development) a vehicle able to carry your "rocket" to Mach 10 - and could separate you safely at that altitude (>30Km) - then maybe you'd want to use it as a first stage. However, as no such vehicle is likely to exist for quite some time, this is not something real launch vehicle designers spend too much time dreaming about.

Dave
P.S. The HOTOL/Skylon concepts DO NOT use scramjets and so fall outside this debate.

Posted by Dave Salt at June 16, 2007 02:01 PM

Perhaps used, but a new jet engine of the same thrust as an RL-10 will probably cost over a million dollars. However, its operational life will be thousands of hours instead of hundreds of seconds. From what I've read, even the small turbofans being used on the new VLJs cost more than $100K. Perhaps their price will come down as production increases. Once can hope.

The article trots out the same canard about a mythical long range hypersonic airliner "capable of flying from London to Sydney in 2 hours". Sorry, but that isn't going to happen. A coworker of mine was an SR-71 RSO for 5 years. He said that they had to let the plane cool before landing (often shooting touch-n-goes) or it would've been too hot to exit. That plane flew at about Mach 3.15 and parts of it got over 1000 degrees Fahrenheit. It was also mostly fuel tank and had to in-flight refuel about every 3,000 miles. A hypersonic airliner would get much hotter. Even if you could carry enough fuel for such a long range flight, you'd probably end up cooking your passengers (if you had room for them).

Posted by Larry J at June 16, 2007 02:19 PM

Forgot to add:

About the only way a scramjet would be useful for space launch might be to have a lightly fueled jet aircraft take off horizontally carrying a rocket (X-15/SS1 on steroids). Meet a tanker or two to pick up the rest of your fuel, then use jet engines to accelerate to perhaps Mach 3 or so. If you could light off scramjets that slow and use them to accelerate to Mach 7 (give or take), then launch the rocket to take the payload the rest of the way to orbit.

The question is whether the cost of developing and building the scramjets to accelerate to Mach 7 (and developing a vehicle capable of carrying a large payload that fast) would gain you enough to offset staying in a very high temperature region longer than you would if you simply launched a somewhat larger rocket at Mach 3.

Posted by Larry J at June 16, 2007 02:24 PM

Rand: "It's an issue of not having time to write a long treatise and tutorial on launch system parametrics just to convince people who want to continue to believe in a technological tooth fairy."

So you're saying you're just lazy? :P

tbrosz: "Building a launch vehicle designed to get to near-orbital velocities while using atmospheric oxygen is about as easy--and maybe as sensible--as a supersonic submarine."

Now _that_ is an idea whose time has come! And sound goes so much faster in water... could you imagine how fast a supersonic sub would be? Seattle to Sydney in a coupla hours! I see no problem with developing this technology as soon as possible.

Of course, this is about the time that Rand's going to chime in and try to rain on my parade. Likely, he's just jealous that he didn't think of that it first...

(subtext: so I guess scramjet to orbit isn't such a hot idea after all...)

Posted by rc at June 16, 2007 02:53 PM

So you're saying you're just lazy?

No, I'm saying what I said. That I don't have time, or rather, I have better uses for my time.

Posted by Rand Simberg at June 16, 2007 02:57 PM

Rand, I know what you said. That was just my (apparently unsuccesful) attept at humor. I believe you now, when you say that there are very significant, likely insurmountable obstacles to scramjet to orbit vehicles.

Posted by rc at June 16, 2007 03:17 PM

I assumed that it was, based on the smiley, but it was sufficiently unsuccessful to me that I thought I'd clarify for others. ;-)

Posted by Rand Simberg at June 16, 2007 03:25 PM

Of course, people would be having fun. Now, there's the problem!

The other problem is that they'd be allowed to spend their own money in unapproved ways, not in proper comportment with social justice.

Posted by Rand Simberg at June 16, 2007 03:29 PM

"I didn't say he was stupid, either. But he shouldn't try to design launch systems when by his own admission (and as demonstrated by his question) he knows nothing about them."

I wasn't trying to horn in on your turf, I was trying to provide some insight as to how the technically unsophisticated would react to your original post. I'm confident if you popped off on estates and trust administration I could eat your lunch, too. Nothing is harder for a member of a guild than to imagine how his efforts are perceived by the less enlightened...but I did appreciate you taking the time to reply.

Posted by wayne s at June 16, 2007 03:36 PM

Bravo. Finally, someone talking sense about the difficulties posed by trying to turn a launch vehicle into an airplane.

Fuel mass is dirt cheap. Development cost ain't. If you have a vehicle that pushes the boundaries of technology to save you a few tons of liquified air, while turning your vehicle into an over-complicated nightmare of ductwork operating in hideously harsh aerodynamic regimes, then the one thing you aren't saving is cash.

Posted by Aaron at June 16, 2007 04:13 PM

You know, if you had a vehicle that was somewhat like the shuttle (ducks behind bulletproof barricade), but had a better sacrificial heat shield on a pod that brought the engines back, and simplified engines (by which I mean the turbomachinery) that sacrifiece Isp for reduced part count, and an expendable phenolic chamber/nozzle, you just might be able to swing the reusable launch vehicle thing while avoiding all the mistakes of the shuttle.

If you have to reuse, bring back the only things worth reusing - the turbomachinery, and perhaps some sort of seperate pod for the crew, and throw out the drop tanks, heat shield, and engine nozzle.

Posted by Aaron at June 16, 2007 04:18 PM

;-)

So, next question. And I hope you haven't already blogged on this. Aerospike or linear aerospike: viable technology or fanboy wet dream?

Posted by rc at June 16, 2007 05:25 PM

Aerospike or linear aerospike: viable technology or fanboy wet dream?

Sorry, but I've gotten tangled in enough theological discussions for one day...

Posted by Rand Simberg at June 16, 2007 06:17 PM

Jim Bennett writes:

I agree with Jim 100% (here at least) notwithstanding my less enlightened liberal bretheren.

And remember, Church is for sinners not saints and therefore leftie space advocates need to be encouraged. ;-)

Posted by Bill White at June 16, 2007 07:21 PM

This thread has gotten me a little depressed. I accept the arguments about scramjets not being physically useful for getting to orbit, and from what I read it isn't feasible to have suborbital aircraft that are economically viable. Does this mean that, for the foreseeable future we are stuck with either orbital rockets or subsonic passenger/freight aircraft? I'm getting ready for my first trip to Japan, and I'm really not relishing the idea of being stuck in a plane for nearly 14 hours. Do the laws of physics and economics really mean we will never have a viable, economical alternative less than orbital but more than subsonic? I guess I'm young enough to still have had some hope, but old enough to live with the loss of yet another possibility.

Posted by Captain Nerd at June 16, 2007 08:37 PM

for the foreseeable future we are stuck with either orbital rockets or subsonic passenger/freight aircraft?

That's my view sadly, and as I spend several hundred hours a year in planes over the Atlantic and Pacific, it bugs the hell out of me that my one flight in Concorde might be about the only time in my life I fly to somewhere faster than about 1000kph.

Posted by Dave at June 16, 2007 09:10 PM

It is the propellant combination that is as cheap as milk, at least for kerosene and liquid oxygen. Liquid oxygen is about four cents per pound, maybe less in large quantity. The propellant cost per pound to orbit for the Zenit booster is about twelve dollars.

Posted by Lee Valentine at June 16, 2007 10:27 PM

Rand;

I am unfortunatly unable to find it on-line again, but I have an paper I printed out a year or so ago en-titled:
"External Burning as a Means of Extended Global Reach" (I'm recalling but it's 'early' and I could be mixing titles.

In the report a hypothetical first stage of a two stage system is described. It uses turbojets, ramjets, and finally rockets to reach speeds for launching the actual orbital vehicle portion of the spaceplane.

It doesn't actually use 'scramjets' as a listed propulsion system, however...

By using 'skip-glide' techniquie and by burning part of its hydrogen fuel externally during the 'skip' portion of the flight it manages to build up to about 85% of orbital speed and with a high hypersonic L/D can fly around the world and back to it's launch site in as little as two hours.

Just thought I'd mention it.

Randy

Posted by Randy Campbell at June 17, 2007 03:35 AM

aerospikes ? as its very likely that first orbital RLVs are going to be TSTO designs, with large hydrocarbon first stage to get above the atmosphere and high-ISP vacuum optimized second stage to get to orbital velocity, the advantages of aerospike in this configuration would be pretty much minimal.

however, when Armadillo, Masten and so on start flying their VTVL vehicles into vacuum, im expecting somebody to fly a lit low-power aerospike as a payload ( like the one Garvey Areospace and California students flew a few years ago ) to test its actual performance in actual flight to space.

Posted by kert at June 17, 2007 03:58 AM

Do the laws of physics and economics really mean we will never have a viable, economical alternative less than orbital but more than subsonic?

Never is a long time. Eventually some combination of increasing wealth and technology will do it -- more quickly, as Rand has noted elsewhere, if the sonic boom can be reduced enough to make overland flight acceptable.

Given the proven markets and much larger budgets of military/civil aviation, it seems to me the sequence of viable hypersonic aircraft -> flyback hyperstage is much more likely than the reverse.
NASA backed off from the latter for a fully reusable STS in 1970-71 not (as wistful folklore has it) because it couldn't be done on the available budget, but because it couldn't be done at any price on anything like their timeline. I think that's still true today, although I'll cheerfully eat my words if someone wants to invite me to a Blackstar flyby.

Posted by Monte Davis at June 17, 2007 06:19 AM

**aerospikes ? as its very likely that first orbital RLVs are going to be TSTO designs, with large hydrocarbon first stage to get above the atmosphere**

Best re-think the use of hydrocarbon fuels as they produce CO2, which with high launch rates (and thus >publicity) will bring down the global warming police....justified or not.

Posted by Terry in Michigan at June 17, 2007 10:12 AM

I guess I am jumping into this thread kind of late.

As to airbreathing orbital launch, there are two historical precedents. One is the 1960's Aerospaceplane, which only got as far as a paper study, and the other is the National Aerospace Plane (NASP) of the Reagan 1980's, which I guess guess was also only a paper airplane but spent a lot more money. Both concepts were to use 1) airbreathing, 2) onboard LH2 fuel, 3) HTHL as a means to single-stage-to-orbit.

The Aerospaceplane was supposed to use LACE - liquid-air cycle engine. The idea was to take off with only LH2 and empty O2 tanks and to sacrifice some of the LH2 to liquify air and separate out more of the O2 component to fill up those O2 tanks in flight, and I read that somewhere in Culver City they had a successful ground test of get this liquifier to work without completely icing up.

LACE is what the gummit wanted; all of the contractors wanted to take the scramjet approach. The problem with LACE is that to get to orbit you have to go so far, freakin' fast that for whatever you gain by collecting O2 from the air, you lose by the air scoop that has to accelerate air molecules to the speed of the vehicle. Many other comments are about drag, but it is more than just some parasitic drag that can be reduced by streamlining -- the faster you go, the more inherent penalty you get scooping your O2 out of the air because that scoop has to accelerate all that air up to vehicle speed.

Think of the conventional ramjet. You scoop in intake air and then you have a diffuser with multiple sets of shock waves so slow the air down and raise its pressure. You squirt in fuel into the compressed air and expand it out a nozzle for a jet exhaust, forming a thermodynamic cycle. The faster you go, the more compression you get in a ram jet, and the more efficient the thermodynamic cycle -- up to a point. A point is reached at some Mach number that the compression is so high that you reach the dissociation temperature of your fuel -- the combustion inlet is so hot that your fuel won't combine with the O2 to raise the temperature any further to add energy to the system.

That Mach number is reached well below orbital velocity. Think of it; the reason rockets are so hard and why you need multiple stages or ridiculously high mass fraction is that orbital speed is some multiple of H2-O2 combustion exhaust velocity, and that exhaust velocity roughly corresponds to the Mach number where the compression temperature means the ram jet stops working.

The scramjet idea, and this goes back to Aerospaceplane and was the basis of NASP, is that you don't attempt to slow the incoming air all of the way and compress it all the way to the stagnation point. In some proposals, you don't even have what looks like an inlet and a jet engine. You dip this fin or strake into the hypersonic airstream to only slow it down a little bit and raise its pressure, you squirt in fuel, and the expanding burning stream downstream of this strake is supposed to give thrust somehow. You are essentially trying to feed a flame fire in a hypersonic airstream -- that is why you need H2 because it is one of the few fuels to burn without the fire getting blown out.

In principle, if all of the pieces came together -- scramjet design, thermal design, LH2 tankage - you could build an SSTO on scramjet power, and that was the idea behind NASP before someone pulled to plug on it. In theory, if you had composite materials and a high enough mass fraction, you could dispense with the difficulty of developing scramjet, especially since there are so many known-unknowns and unknown-unknowns of whether the scramjet could give enough thrust over a wide enough speed range and whether you could survive the thermal environment; if you had enough mass fraction, you could just build an LH2-LO2 rocket, and that was the X-33 Venture Star approach. But after some difficulties with the composite LH2 tanks, the plug was pulled on that too.

In my opinion, if you were able to throw enough money at the problem, you could probably get either a scramjet SSTO or an LH2-LO2 rocket SSTO to work. Current thinking is that the LH2-LO2 SSTO is the way to go because composite materials could give you the mass fraction and the rest of the design is pretty much a known quantity. The scramjet approach could work too, but the thermal environment for airbreathing is so much harder than thrusting out of the atmosphere and accelerating in space with a rocket, and scramjets are bleeding-edge aerodynamics as well. The rocket SSTO is purely a structures problem and a safer bet.

The problem with either approach is that it would take an unknown X-billion dollars to pull if off, and the gummit doesn't have the will to do this and LockMart and others don't have an economic model to pay down the development cost with enough LEO space missions.

Another problem with SSTO is that it is single-stage to low-Earth orbit, and all of the interesting destinations (apart from ISS) are farther out -- GEO, lunar orbit, lunar surface, Mars, asteroid belt, and so on. If you had SSTO, you would have to do some kind of on-orbit propellant transfer to reach the farther destinations. I would have NASA use the ISS to do engineering development of on-orbit propellant transfer, both storable and cryo, and perhaps create some space infrastructure that some private concern would develop SSTO or other means to supply.

Posted by Paul Milenkovic at June 17, 2007 02:33 PM

Current thinking is that the LH2-LO2 SSTO is the way to go because composite materials could give you the mass fraction

Whose current thinking? Max Hunter made a convincing case that the mass fraction is easier to achieve with dense propellants rather than hydrogen.

Interesting to whom?

I don't understand where all this denigration of LEO (always by people who've never been there) comes from.

Polls show that millions of Americans would love the chance to visit LEO. Maybe it's so popular that no one's interested in it any more? :-)

Posted by Edward Wright at June 17, 2007 08:25 PM

The scramjet cycle is important when related to RBCC as in "thermally integrated rocket ejector ram scramjet rocket" combined with LACE, ACES, or deeply cooled air collection. If you only focus on the scramjet alone then one missing the whole point and advantage of airbreathers. It must all function as one unit the vehicle becomes the propulsion unit, the heat is utilized to breakdown the hydrogen fuel "thermally coupled" the compression is utilized to create lift etc.... One utilizes the rocket, and airbreathers cycles at different points in the launch process to maximize efficiencies of each.

Posted by Doug Gard at June 18, 2007 10:09 AM

> Humor someone who's only science class in college was Introduction to Oceanography: A 747 piggybacks a scram-jet to
> 45,000 feet. The scram-jet takes the payload to the edge of the atmosphere at mach 10 and flings it into space -- how is
>that not more efficient than spending a million dollars a pound sending LOX from the launchpad?
> Posted by wayne s at June 16, 2007 11:07 AM

An excellent and basic question everyone seemed to busy (or snoty) to answer, likely because they didn’t realize the answer invalidates most of this discussion.

A air breather Launcher (like one using a scramjet or something up to some fraction of orbital speed) doesn’t need to carry the heavy mass of oxygen to feed its rocket engine, but the air breathing engines are much heaver then rocket engines, plowing through the air at Mach 10 – 24 (orbital velocity is about Mach 24 in the air) generates a huge amount of drag and heat on the hull a rocket can just avoid, and the craft gets a bit more complicated. So you just trade big LOx tanks and weight in LOx for weight of other things the airbreather needs.

However the above has no real impact on, or has any relationship to, costs. Bottom line is the $ cost to orbit has virtually nothing to do with fuel/LOx mass fractions to cargo weight, because the fuel to get a pound of cargo to orbit only costs ($) a couple times (at worst) the cost of the Fuel a 747 would take hauling it from US to Australia. The cost to orbit has to do with things like repair costs, how many flights the fleet of craft are likely to do. You divide the heavy upfrount costs to develop the craft, the overhead costs to keep a fleet of them going, purchase cost of the fleet, by the numbers of flights. Then add the servicing costs to repair the craft and prep it for another flight. That’s how you get the cost per flight.

One plus airbreathers have is they are likely to crash less (current craft crash 1 time out of 50 flights) and potentially be more serviceable and usable since they can fly around. That’s not a universal for all designs, but its far more critical then the mass ratios – which are utterly irrelevant to the costs (..spending a million dollars a pound sending LOX from the launchpad?_).

Oh, and actually the cost of LOx from the Launchpads a lot less then that, and at $0.05-$0.1 a pound in bulk, LOx isn’t a price factor.

Sorry you get dised by the locals with attitude, especially the site host.

Kelly Starks

Posted by Kelly Starks at June 18, 2007 11:09 AM

Beware: scramjet marketing hype. :-)

Claims that scramjets "are likely to crash less" and "potentially more serviceable" are not supported by fact or sound engineering analysis.

Conventional airbreathers (turbojets and turbofans) do not achieve reliability and serviceability from some magic substance in the air they breathe. They achieve it through adequate margins and mature technology with lot of operating experience.

Rockets are less mature than turbojets, but are starting to catch up. Scramjets, however, are on the bleeding edge. Scramjet flight experience is almost nonexistant, compared to rocket engines. The few flight experiments to date have mostly been expendable. The one time NASA tried flying a scramjet mockup on the X-15, the shock interaction damaged the X-15 so badly that it was grounded permanently. No hypersonic airbreather has ever been serviced and reflown, so there is no data to substantiate Kelly's marketing hype. The margins for orbital scramjet vehicles are not just small, they are negative, and achieving positive margins requires breakthroughs in materials science (unobtainium).

Scramjet salesmen say they can make vehicles more reliable and more serviceable by replacing a relatively mature technology (rockets) with a new technology that's much less mature but more complex, has smaller margins, and requires expensive, rare, difficult-to-produce materials.

Has that trick ever worked?

Posted by Edward Wright at June 18, 2007 06:25 PM

unobtainium . . . not necessary with a regenerative thermally cooled vehicle (hydrogen fuel is circulated around the vehicle skin to control the heat). The concept was tested in (mock-up produced) in the sixties/seventies timeframe utilizing existing materials that were available at that time. In essence the entire vehicle and propulsion unit becomes an energy transfer medium.

Posted by Doug gard at June 18, 2007 08:43 PM

The concept was tested in (mock-up produced) in the sixties/seventies timeframe utilizing existing materials that were available at that time.

The result of the test was that NASA and DoD spent a classified number of billions but never managed to produce a working vehicle.

Posted by Edward Wright at June 18, 2007 11:02 PM

ScramJet" should not be refereed to as a stand-alone propulsion unit (that is ancient thinking) but rather as a propulsion cycle as in RBCC. The airbreathing/LACE cycle is used in the M3 (starts with RAM cycle switches to SCRAM/LACE cycle) to M12 'mid velocity" range. The lower accell and final orbital insertion cycles are rocket powered. Most of the negatives I see mentioned here are based on 50's era scramjet thinking, design and application. RBCC is an all –in-one propulsion unit with few moving parts (simple lightweight mechanical design) materials are cooled by pumping hydrogen through them. The entire vehicle is an all-in-one wingless lifting body/propulsion/intake/exhaust/energy exchange medium design, no external podded engines etc.... LACE reduces the oxidizer volume (smaller lighter vehicle design) as mentioned it has nothing to do with oxidizer cost but everything to do with reduction of vehicle size(oxidizer tank)and weight.

Posted by Doug Gard at June 19, 2007 04:40 AM

“The result of the test was that NASA and DoD spent a classified number of billions but never managed to produce a working vehicle”.

Yes NASA and DoD spent billions on many programs over the years and never produced a working vehicle . . . . most were not based on airbreathing technology (X-33 for example) that’s nothing unusual for either of those agencies!

Hypersonic research in labs, universities, and aerospace companies was well funded in the sixties and early seventies. They produced several workable designs and many of the subsystems, including regenerative cooling (similar to that designed for the X-20) were tested and proven. For the most part those vehicle designs were never built due to managerial, bureaucracy and McNamara type philosophy than actual failure of the airbreathing technology.

Posted by Doug Gard at June 19, 2007 05:17 AM

> Claims that scramjets "are likely to crash less" and "potentially more
> serviceable" are not supported by fact or sound engineering analysis.
>
> Conventional airbreathers (turbojets and turbofans) do not achieve
> reliability and serviceability from some magic substance in the air
> they breathe. They achieve it through adequate margins and mature
> technology with lot of operating experience. ==
>Posted by Edward Wright at June 18, 2007 06:25 PM

My point was that scramjet craft generally have wings so engine outs don’t involve free falling on your tail – hence you are less likely to crash. Also they tend to be less bulky and srtucturaly stronger (less tanks, more wings) which aids in reliability and serviceability of the craft.

As to the reliability (or perhaps safty would be a better word) of the scramjets themselves. Jets of any type have lower internal pressures then rockets – so they are less likely to blow up, and the pumps and such are lower pressure ones more standards then the high pressure turbopumps in rockets. Since we have a lot more experience with low pressure pumps, and they are under less stress, they are a lot easier to make reliable.

I don’t by the way think that highly of scramjets. Last I heard the conclusions from NASA were that the cooling systems needed to sustain a craft flying at more then mach 6 get heavy and outweigh the advantage of airbreathing. Bellow Mach 6 more traditional Ram-jets work better.
However I think a LV that could take off and land normally, fly normally through airspace until reaching high altitude for acceleration to Mach 6 and then boosting out, would have significant operational and marketability advantages – and need a lot less infrastructure. Not sure a scram-jet would help that much (I think the PDEs hold a lot more promise) but at least were experimenting with SOME new engines!!

Posted by Kelly Starks at June 19, 2007 08:38 AM

Yes NASA and DoD spent billions on many programs over the years and never produced a working vehicle . . . . most were not based on airbreathing technology (X-33 for example)

No, I'm referring to the National AeroSpace Plane (NASP). Hence the reference to the classified budget.

Hypersonic research in labs, universities, and aerospace companies was well funded in the sixties and early seventies. They produced several workable designs

They produced designs that promoters claimed were workable. When they actually got the money to build one, they didn't get very far. I've talked to quite a few people who worked on NASP. Some are still hyping airbreathers but most recognize it was a dead end.

and many of the subsystems, including regenerative cooling (similar to that designed for the X-20) were tested and proven.

I didn't recall X-20 used regenerative cooling, but even if it did, you're making a mistake. The TPS requirements for DynaSoar are much less severe than those of a hypersonic airbreather, which experiences greater thermal challenges during ascent than it does on reentry.

Posted by Edward Wright at June 19, 2007 12:11 PM

My point was that scramjet craft generally have wings so engine outs don’t involve free falling on your tail – hence you are less likely to crash.

Your reasoning has multiple malfunctions here.

First, rockets can have wings.

Second, not all scramjet concepts have wings.

Third, wings are not magic. Most aircraft have a critical period after takeoff when losing the engine(s) will probably cause a loss of aircraft. The wings on an orbital scramjet would need to be small for aerodynamic and TPS reasons. It would not be a good glider.

Also they tend to be less bulky and srtucturaly stronger (less tanks, more wings) which aids in reliability and serviceability of the craft.

No, you need much larger tanks because all of your propellant is low-density hydrogen. The mass of a tank varies with the volume of the contents, not the weight of the contents.

I don't know why you think scramjets will have "more wings." I presume you mean "larger wings" (unless you're suggesting a hypersonic biplane :-) but due to the more severe TPS problems and lower weight margins you can't afford larger wings.

As to the reliability (or perhaps safty would be a better word) of the scramjets themselves. Jets of any type have lower internal pressures then rockets – so they are less likely to blow up,

You should read a bit about the history of jet engines. When turbojets were first developed, people said the same thing about them that you do about rockets. The pressures were too high, they would always blow up... not to mention the short lifetime compared to piston engines. It was doubtful that they'd ever be useful in passenger aircraft.

Yet, engine manufacturers developed turbojets that didn't blow up, just as companies are now building rocket engines that don't blow up.

Of course, this is a red herring because any scramjet you build is going to require rockets for orbital insertion (scramjets don't work in a vacuum) and possibly takeoff (scramjets don't work at low speeds). So, if you think the only rocket engines are crappy low-quality engines that blow up, your scramjet is going to blow up along with them.

and the pumps and such are lower pressure ones more standards then the high pressure turbopumps in rockets. Since we have a lot more experience with low pressure pumps, and they are under less stress, they are a lot easier to make reliable.

Pump pressure is a manageable problem as long as you stay away from staged combustion. The RL-10, for example, operates at a pressure of only about 30 atmospheres. If that still scares you, there are options like piston pumps that do away with turbines entirely.

However I think a LV that could take off and land normally, fly normally through airspace until reaching high altitude for acceleration to Mach 6 and then boosting out, would have significant operational and marketability advantages – and need a lot less infrastructure.

That does not require hypersonic airbreathers. SpaceShip One did all of that (except for reaching Mach 6).

Not sure a scram-jet would help that much (I think the PDEs hold a lot more promise) but at least were experimenting with SOME new engines!!

I would prefer experimenting with something useful.

Posted by Edward Wright at June 19, 2007 01:03 PM

Looks like I stand alone here in defense of and support of future SERJ and RBCC propulsion incorporating SCRAM cycle airbreather applications. Perhaps the technology has languished for so long in the repressive black world (generating Aurora hype) that the mainstream has dismissed it. I have my sources, my data, my contacts . . . plus recent AFRL and DARPA activity. Based on that I believe those Aussies out there in the god-for-shaken outback are pursuing something big . . . something transformational. They are on the cusp of the next propulsion revolution.

Posted by Doug Gard at June 19, 2007 08:40 PM

>> Yes NASA and DoD spent billions on many programs over the years
>> and never produced a working vehicle . . . . most were not based
>> on airbreathing technology (X-33 for example)
> No, I'm referring to the National AeroSpace Plane (NASP).
> Hence the reference to the classified budget.
> Posted by Edward Wright at June 19, 2007 12:11 PM

NASP wasn’t classified? It was a joint DOD NASA program that was defiantly not handeled in a classified manor.

>> My point was that scramjet craft generally have wings so engine outs
>> don’t involve free falling on your tail – hence you are less likely to crash.
>
> Your reasoning has multiple malfunctions here.
> First, rockets can have wings.
> Second, not all scramjet concepts have wings.
> Third, wings are not magic. Most aircraft have a critical period
> after takeoff when losing the engine(s) will probably cause a loss
> of aircraft. The wings on an orbital scramjet would need to be small
> for aerodynamic and TPS reasons. It would not be a good glider.
> Posted by Edward Wright at June 19, 2007 01:03 PM

I said generally. Generally rocker propelled LVs do not have wings, take off and ascend vertically (to get out of the atmosphere as quickly as possible), and have less graceful failure modes in a engine out situation.

How well a scramjet powered craft (or other hypersonic craft) would glide depends on the specific design. Some glide pretty well.

>> Also they tend to be less bulky and srtucturaly stronger (less tanks,
>> more wings) which aids in reliability and serviceability of the craft.
>
> No, you need much larger tanks because all of your propellant is low-density hydrogen. ==
> Posted by Edward Wright at June 19, 2007 01:03 PM
Only if you assume the scramjet is hydrogen powered and the rocket propelled craft is not. ScramJets and Rockets work with both hydrogen and hydrocarbons (the later preferable) but the rocket needs a lot of LOx, hence extra tank volume.

===
>> As to the reliability (or perhaps safty would be a better word) of
>> the scramjets themselves. Jets of any type have lower internal
>> pressures then rockets – so they are less likely to blow up,
>
> You should read a bit about the history of jet engines. When
> turbojets were first developed, people said the same thing about
> them that you do about rockets. The pressures were too high, they
> would always blow up... ==

Didn’t say always, said les likely. Given a scramjet – or any other jet that comes to mind – has low internal pressures compared to Rockets (in pumps and combustion chanbers), there less likely to have a explosion. You certainly could build a rocket as safe and reliable as a jet, though so far the higher pressures tend to be a problem for their systems compared to other systems like Jets of PDEs.

Course this doesn’t consider that scram jets are generally a combined cycle system with Ramjet if not ejector ramjet or Rocket modes built in – but that’s gets off on to many side threads.

===
and the pumps and such are lower pressure ones more standards then the high pressure
>> However I think a LV that could take off and land normally, fly
>> normally through airspace until reaching high altitude for acceleration
>> to Mach 6 and then boosting out, would have significant operational
>> and marketability advantages – and need a lot less infrastructure.
>
> That does not require hypersonic airbreathers. SpaceShip One did
> all of that (except for reaching Mach 6).
> Posted by Edward Wright at June 19, 2007 01:03 PM

SS1 was carried to high alt, didn’t accelerate to Mach 6, and did not from there boost out to orbit. And it could only do that much due to its airbreather mother ship.

>> Not sure a scram-jet would help that much (I think the PDEs hold a lot
>> more promise) but at least were experimenting with SOME new engines!!
> I would prefer experimenting with something useful.
> Posted by Edward Wright at June 19, 2007 01:03 PM

The PDEs (pulse detonation Engines) are generally considered the most usefull new engine concepts around, expected to replace turbines or Ram jets due to their dramatically reduced engine weight and complexity, ad increased

> Looks like I stand alone here in defense of and support of future SERJ
> and RBCC propulsion incorporating SCRAM cycle airbreather applications. ==
> Posted by Doug Gard at June 19, 2007 08:40 PM

It does have potential, and can dramatically improve the efficency of the rocket engine in atmospheric flight. Also integrateing in the rocket allows the ram.scramjet parst function at slow or zero speed. A significant advantage.

I think a 60%-70% ISP gain of a RBCC vrs a straight rocket was a expected average. Though I’m not sure of exactly where I read that?

Posted by Kelly Starks at June 20, 2007 08:14 AM

NASP wasn’t classified? It was a joint DOD NASA program that was defiantly not handeled in a classified manor.

The program wasn't classified but many aspects of it were. Including the total amount of money that was spent.

Generally rocker propelled LVs do not have wings,

There aren't enough examples of RLVs to draw valid generalizations. And ELVs are not really relevant.

have less graceful failure modes in a engine out situation.

Engine out? If you were at the X-Prize Cup in 2005, you would have seen a rocketplane repeatedly stop and restart its engine in flight.

No scramjets required.

> When turbojets were first developed, people said the same thing about them that you do about rockets. The pressures were too high, they would always blow up...

Didn’t say always, said les likely.

No, that is not what they said. Look it up. A lot of people very skeptical about jet engines. Their arguments were very similar to your arguments about rockets.

Given a scramjet – or any other jet that comes to mind – has low internal pressures compared to Rockets (in pumps and combustion chanbers), there less likely to have a explosion.

Pressure is not the problem. Margins are the problem. Even a low-pressure steam boiler can explode and kill you, if the safety margins are inadequate. If you think a scramjet with low pressures and low margins is going to be safer and more reliable than a rocket with high pressure and adequate margins, then you do not understand the problem.

SS1 was carried to high alt, didn’t accelerate to Mach 6, and did not from there boost out to orbit. And it could only do that much due to its airbreather mother ship.

Maybe that's why I wrote "except for reaching Mach 6"?

But I assume you know it's possible for a rocket to boost into orbit and weren't contesting that.

SpaceShip One did "take off and land normally, fly normally through airspace until reaching high altitude... have significant operational and marketability advantages – and need a lot less infrastructure."

No scramjets, PDE, or warp drive required.

Posted by Edward Wright at June 21, 2007 08:18 PM

>> -- have less graceful failure modes in a engine out situation.

> Engine out? If you were at the X-Prize Cup in 2005, you would have seen a
> rocketplane repeatedly stop and restart its engine in flight.

Yes its when you can’t restart it – and when you’re going to fall like a rock without it that you're worried about.

;)

Ofcourse rocket - if built to - can stop and restart.

====
>
> SpaceShip One did "take off and land normally, fly normally through airspace until
> reaching high altitude... have significant operational and marketability
> advantages – and need a lot less infrastructure."

> Posted by Edward Wright at June 21, 2007 08:18 PM

Yes and no.
SS1could not take off or assend to high alt., it was carried up there by White knight. After WK droped it at alt,SS1 boosted up to about Mach 3.5? and out of the atmosphere. Then it reentered like a glider. It could not fly normally through the airspace down, and I'm not sure it was above legal airspace when it boosted out (where its ability to play nice with trafic was limited). All this requires a lot of legal and procedural special handeling which hurt your operational flexibility and capacity, and limits the kind of markets it can serve. Though yes it doesn’t require the specialize launch center infrastructure of a traditional LV.

Not a big issue if you limit to its intended up and down within one restricted airspace “air box”, but that’s a MAJOR market limitation even for suborbital. For an orbital capable craft such restrictions lock it out of the likely VASTLY larger point to point transport markets, or operating as a orbital launcher, such a design limits where you can go and boost to orbit from.

A craft with airbreathers or higher ISP Rockets or air breathers isn't so awkward.

Posted by Kelly Starks at June 22, 2007 08:57 AM

>> -- have less graceful failure modes in a engine out situation.

> Engine out? If you were at the X-Prize Cup in 2005, you would have seen a
> rocketplane repeatedly stop and restart its engine in flight.

Yes its when you can’t restart it – and when you’re going to fall like a rock without it that you're worried about.

;)

Ofcourse rocket - if built to - can stop and restart.

> Posted by Edward Wright at June 21, 2007 08:18 PM

A craft with airbreathers or higher ISP pde Rockets or air breathers isn't so awkward.

Posted by Kelly Starks at June 22, 2007 08:57 AM

>> -- have less graceful failure modes in a engine out situation.

> Engine out? If you were at the X-Prize Cup in 2005, you would have seen a
> rocketplane repeatedly stop and restart its engine in flight.

Yes its when you can’t restart it – and when you’re going to fall like a rock without it that you're worried about.

;)

Ofcourse rocket - if built to - can stop and restart.

> Posted by Edward Wright at June 21, 2007 08:18 PM

A craft with airbreathers or higher ISP pde Rockets or air breathers isn't so awkward.

Posted by Kelly Starks at June 22, 2007 08:57 AM

Post a comment