There's a fairly in depth look at scramjets in the latest edition of The Industrial Physicist. It's got enough meat on it to be worth reading, though I have a visceral dislike of scramjets. It's nothing to do with the technical merits - it's just that they are yet another technology that's constantly being held out as the technical breakthrough needed to bring down launch costs. Someday I suspect scramjet powered vehicles (at least missiles) will be practical. In the meantime they are a kind of interesting technology that's worth understanding just for curiousity's sake.

Incidentally, I'm not singling out scramjets here - space elevators also trigger my "here we go again" reaction. Ditto electromagnetic accelerators.

Like Fusion, it's always within 10 years for the past 30 years...

Is there anything which isn't "here we go again" inducing? It's one of the reasons sci.space.policy gets annoying at times; the number of people who suggest $flavour_of_the_month to solve any technical issue, no matter how fundamentally unrelated.

(VASMIR. Beanstalks. CEV. Suborbital planes, generally. Falcon V. Tethers of every shape & size. ... I wonder if you could draw a graph, and predict when, say, laser-launch will next cycle back up into the slot?)

I'd add Lunar He3 mining to the list. The Space Elevator gets press because it is another "neat" idea. Right now it makes about as much sense as building a bridge between South America and Antarctica: We don't have the technology and there isn't the traffic to support it.

It does annoy me that tethers are usually ignored, though - short (as in miles) tethers have potential near term uses, and can develop into something far more useful.

Well, I watch too much Gundam, so Jupiter He3 mining is in line with mine thinking :)

Well, scramjets for transit to orbit are generally a losing proposition but for rapid point-to-point travel on the Earth, I expect the trade studies would come out differently.

When it comes to space elevators, if that's helping drive forward the CNT technology, great -- they may produce actual elevators, but at least they'll produce the tech for more mundane tethers.

(BTW, not that it's a generality, but all too often those that have the "here we go again" reaction are folks that *think* they know all kinds of stuff about a particular technology, but they really know very little other than 20+ year old 'conventional wisdom'.)

- Eric.

I don't see scramjets launching people into orbit. I mean, ramjets are their nearest cousin, and they don't see much use; and scramjets are orders of magnitude less easy to design, build and operate.

Space elevators are looking more promising; but I can probably give you 6 reasons why that may never work either. And it only takes one.

But if we can get our collective asses together, we can actually grow the launch market enough that more exotic launch technologies might actually see the light of day.

Personally, I'd love to see someone build a Skylon; that's pretty close to the state of the art; it might actually work in our lifetime.

"Well, scramjets for transit to orbit are generally a losing proposition but for rapid point-to-point travel on the Earth, I expect the trade studies would come out differently."

There are many problems with operational use of hypersonic vehicles for commercial purposes. As one person involved with NASP once said to me "What good is it flying to Tokyo in two hours when it takes two hours for the plane to cool down before you can exit?" Of course, four hours to Tokyo is still a deal, but there are other concerns. It wasn't clear that the aircraft could be scaled up to carry enough people to make sense.

"When it comes to space elevators, if that's helping drive forward the CNT technology, great -- they may produce actual elevators, but at least they'll produce the tech for more mundane tethers."

Even if a space elevator doesn't result, the technologies that would have contributed to it have other applications. Better suspension bridges, more earthquake-proof buildings, perhaps even a laser-based missile defense shield, tougher fabrics - all would result from the pursuit of the goal of a space elevator.

Eric: In my experience it's not that I know about them, but that I can see very easily that the person waving them knows even less about them than I do...

(You could analogise it to politics. Every now and again you hear someone say "We should fix healthcare. XYZ could fix healthcare for us", because XYZ gave the last interview about healthcare they read, so...)

Andrew Gray wrote:
>I wonder if you could draw a graph, and predict
>when, say, laser-launch will next cycle back up
>into the slot?

It's late... Dr. Kare released the "Phase 1: Final Report" on Laser Launch last month.
(NASA Institute for Advanced Concepts link to paper:
http://www.niac.usra.edu/studies/study_template.jsp?id=897&cp_num=02-02&phase=I&last=Kare&first=Jordin&middle=T.&title=Modular+Laser+Launch+Architecture%3A+Analysis+and+Beam+Module+Design&topic=Advanced+Propulsion&sub_topic=null&organization=Kare+Technical+Consulting&active=false&begin_date=Oct+1,+2003&end_date=Mar+31,+2004

Actually in late March/early May.)

I heard all the updates in late May at a presentation he gave at a local convention. The paper is a nice read though. Points away from needing a massive, (and massivly expensive) Free-Electron laser, and towards more 'off-the-shelf' equipment.

Not much payload 'per-launch' (120kg IIRCC) but it could be done.

The question, of course, remains "Will it?"

Randy

My view on scramjets is that they may or may not make nice missiles; but if Concorde flying across the Atlantic only just makes New York due to the fuel fraction; exactly what makes anyone think a scramjet can reach Tokyo from America at 3x the drag per mile?

The answer should be: nothing

I doubt the space elevator concept is a big motivator for most of the folks working on carbon nanotube fiber/cable technology - there are so many existing markets for superstrength fiber.

And if you are interested in space elevators, please research "rotovators" and other advanced tether concepts that can do most of what an elevator can and have many other applications for orbital and planetary transfer.

Look at Tethers Unlimited for a start - from relatively nearterm tether applications to advanced ones.

There's much, MUCH more to tethers than space elevators.

Andrew Case
" space elevators also trigger my "here we go again" reaction"

Maybe - but I see the technology catching up and making an SE possible. Earlier in the 'here we go again' cycle wasn't it more like "we have no idea how to make the stuff to produce the ribbon".

What's frustrating is that the dozen odd things that _could_ stop an SE aren't technical but political.

Not really.

- finance (as in as or more expensive than rockets!)
- power beaming (expected efficiency is about 0.5% currently- you only need a few dollars of electricity- but multiply by 1/0.5% = 200 and how much does it come to?)
- energy density of taut fiber (I believe it is comparable to an explosive- it snaps- it blows up?)
- micrometeorites
- space junk
- van allen belts more or less preclude human usage

And then you get onto the politics.

Ian,

Your objections have answers - but I'm not sure Rand's comment section on an unrelated topic is the best place for an extended discussion. Liftport has opened it's forum software if you'd (or anyone else) would like to drop by - http://www.liftport.com/forum/

I'll briefly note that

One of your objections (finance) isn't technical; but it is a booger. More expensive than rockets however? More expensive that the model rocket motors in my garage, sure. Dr. Edwards estimated that a SE can be launched for 6-10 billion. Not chicken feed, but not as expensive as (say) Shuttle.

I've not heard of the CNT material exploding under stress - that is, it's not been raised as a problem that I'm aware of.

No one wants to put human beings aboard a climber, yet. People are fussy about things like breathable air, radiation, temperature and so on. Plus they kvetch if you don't give them a window seat. Who needs that? We're looking at cargo only usage.

- Scramjets: Fine for in-atmosphere travel, lousy
for space flight. Requires Unobtanium Mk I.
- Space elevator: Good idea IMHO.
Requires Unobtanium Mk X.
- VASIMR, laser/microwave propulsion: Very nice.
If you have a big enough wall socket close to
the propulsion unit and electricity is $1/MW.

My favourites:
- Orion like drive: Neat. The only catch is
radioactive fallout and the Greens.
I suppose it could always launch from some
deserted place.
- Matter/Antimatter propulsion: Requires dirt
cheap electricity, some minor Unobtanium.
Generates gamma rays (I can hear the Greens
screaming with delight now).