Michael Belfiore updates his previous post, to indicate that Rocketplane (as I was quite confident was the case) has in fact been in discussions with the FAA. But he persists in his misguided (in my opinion) fear of liquid propulsion:

I say a good healthy dose of skepticism never hurt anyone about to climb into a commercial spaceship fueled with explosive liquids.

While not denying that skepticism is always appropriate to some degree, he still seems to think that hybrids cannot explode. That would come as a shock to many (including me) who watched an Amroc 250,000-lb-thrust motor launch itself down the mountain up at the rocket lab in the early 90s, as a chunk of rubber got caught in the throat, blocking the flow and causing the internal pressure to build up to the point that it blew the bolts on the aft bulkhead, with spectacular results. Hell, even steam boilers can explode (this killed many people in the early days of river transportation).

It's true that a hybrid can't achieve total combustion in the same way that mixing liquids can, but it's a big mistake to think of them as intrinsically "safe" (a term that is always relative, and never absolute). I would personally feel just as comfortable on a vehicle powered by one of (for example) XCOR's rocket engines as by any hybrid, because I'd be confident that they would build adequate margins and safeguards into it to make it as safe as reasonably possible.

Well, if he's got a healthy dose of skepticism on climbing into a commercial spaceship (read suborbital hopper, for now ;) fueled by explosive liquids how does he feel about climbing into ...

Airplains! No, fueled by explosive liquids.

Trains! No, fueled by explosive liquids.

Cars! No, fueled by explosive liquids.

Horses! No, they expel explosive gases.

Which is were I think these issues are going.

Myth was perpetuated by Virgin Galactic. As if we needed LOX to blow something up. My parents bought a '72 Ford Pinto. I think the argument that lighting off on the pad is more dangerous to the extent it applies at all is limited to VTOL. It applies very little to self-ferrying ships like Rocketplane. XCOR is interesting to mention since Dan's igniter may be as safety improving as Burt's carefree reentry. (Full disclosure, I am working with one and investing in the other and competing with the third.)

Do hybrids also go catastrophic if they have cracks in the solid propellant?

I know that with solid propellants they are usually X-rayed to make sure that there are no cracks in the propellant. But do hybrids share the same concern?

No, unless oxidizer could pool there for some reason. Solids go boom because they are surface burners - any surface, exposed to oxidizer or not. A hybrid would burn faster if there was more surface area exposed to oxidizer, but that would be pretty hard to arrange.

Even a pure-solid fuel rocket could explode, couldn't it? At least a vapor-expansion explosion, if not a fuel based explosion.

It is just that anything like that would be a _major_ design flaw on a solid rocket.

Rocket engines, by their very nature are both energy and power intensive. Whether solid, liquid, or hybrid propellants, sometimes they go 'bang'. Whether that 'bang' is an 'explosion' or not is an argument I refuse to recognize. There are several examples of hybrids going 'bang' or having a case burnthrough bad enough to destroy the vehicle surrounding it, including two of the X-Prize contenders.

Turbine and compressor wheels on commercial jet airliners are in this same category, and large commercial airliners have engine containment structure to mitigate this problem. It is XCOR's policy to have blast containment around its rocket engines for the same reasons. We have had no explosions after 2389 rocket engine runs totalling 17,422 seconds of runtime on 5 different engine designs. We are confident in our engine design, but still plan to always have blast containment on all vehicles.

I agree with Michael that healthy skepticism is good; but that skepticism must include all aspects of vehicle design and operation, not just propellant choice.

Dan DeLong
Chief Engineer
XCOR Aerospace

I totally agree with that (especially the part about containment failure being totally equivalent to an explosion), and would add that the underlying technology does not matter - only the application of it. The are many examples of safe liquid propulsion devices, there are many examples of safe solid propulsion devices, and if they were common I'm sure there would be examples of safe hybrids.

Safe just means you've worked on safety, and you've flown a lot. (How on earth are helicopters safe enough to use as abulances? Lose engine on takeoff, everybody dies? Why not?)

Hybrids are definitely safer, if not perfectly safe. I worked at AMROC during the launch attempt of SET-1, our single-stage suborbital vehicle. We all put one of our business cards in the nose cone, hoping to recover them afterwards as souvenirs.

SET-1 failed because the LOX valve iced up (all our testing was in the dry air of Edwards AFB -- the launch attempt was at Vandenberg, near the ocean). It never developed enough thrust to liftoff and just fell over and burned.

But I got my business card back. If it had been all solid or all liquid that wouldn't have been possible.

And I would like to add one thing that reading part of Dan's comment brought to my mind to go with cars and fueled by explosive liquids.

Cars! No, there is an explosive device in the center of just about every steering wheel and just above most glove compartments (and are being instailed by the heads of the backseat passangers now). They just have a hightech pillow case to catch the expanding gasses from the explosion*.

They're called airbags.

*I will point out that at the speed you need to get them to inflate in order to do any good you pass 'fast gas generater' (read solid rocket engine) and move into the realm of 'explosive'.

"...I agree with Michael that healthy skepticism is good; but that skepticism must include all aspects of vehicle design and operation, not just propellant choice.

Dan DeLong
Chief Engineer
XCOR Aerospace"

Given identical oxidizers with both a liquid motor and a hybrid motor, anybody who’s thinking about it would prefer to be standing next to a rubber based fuel over a kerosene or alcohol fueled motor any day! There IS clearly a difference between a case over pressurization failure or insulation burn-thru potential of a hybrid verse the catastrophic fireball detonation potential of a liquid motor. It’s my understanding that the DoD gave the AMROC hybrid rocket a TNT equivalent rating of ZERO!

I think it is safe to say that ANY motor malfunction on a flight vehicle is a life threatening event for the crew and as Apollo 13 proved, even certified pressure vessels like tanks and rocket motors have that potential for catastrophic failure.

As SS1 has demonstrated, Hybrids can be developed faster and with more thrust (15K) with a higher degree confidence of success than liquids as the recent Starchaser motor (5K) firing pretty clearly demonstrated. That doesn’t mean that with the proper design and more importantly *testing*, a liquid can’t be made to be safe and reliable for a commercial spaceship! Liquids certainly should have some turn around time advantages over hybrids. Until we see a comparable liquid as far as Total Impulse built and flown by one of these other Alt. Space companies in question, the issue really seems to have answered itself. I’m not sure that 2000ish liquid teacart firings are even relevant as evidence of liquids safety as compared to the tens of thousands of hobby hybrid HPR flights!

I would admit that certain commercial hybrid proponents are over playing the “Safety Card” as a marketing tool. But at least it has a thread of truth and certainly no worse than the other outrageous claims I read on many of the other Alt. Space teams web pages and press releases everyday! I would agree that the statement “Hybrids Are Safe” is too absolute! With everything else being equal, perhaps “SAFER” or “SAFEIST” would be the more accurate term. A funny thing about myths, they tend of have an observed ring of truth.

In the end if I were spending the $200K for a joyride to the edge of space, I would ONLY be looking at the track record of actual flights not the propulsion system!.....Advantage Rutan & VG, although I really like Rocketplane’s concept and wish them both success!

K2
eAc

Quote from David Summer: "How on earth are helicopters safe enough to use as abulances? Lose engine on takeoff, everybody dies? Why not?"

Well helicopters are pretty safe even in the event of a engine failure. Without engine power a helicopter turns into an Autogyro. The pilot will put the rotor pitch flat out with no lift and keep forward airspeed up. As a gyrocopter the pilot will still have plenty of control over the aircraft to pick a safe landing area. As the ground approaches the pilot should have several seconds of energy stored up to make a safe and gentle landing.

The real thing to be fearful of in a helicopter is not an engine failure, its a tail rotor failure. Loss of the tail rotor almost instantly puts the copter into a deadly spin. Hence the reason those naughty Somalians were trained by Al Qaeda to shoot at the tail rotors of helicopters with their air bursting RPG rounds.

Just 2 comments:

If we are going to compare hobby activity safety rates, I think the winner has to be solids. I'm not sure anyone has been hurt by an Estes rocket in years, with thousands of flights! I'm just saying that the amount of damage done to a person standing next to an exploding rocket of type A, B, C is irrelevant - because the rocket needs to not explode. You can take any rocket type to that point - heck we have premixed, explosive rocket engines that can be sent by mail, for goodness sake!

As for the helicopter comment, that is exactly my point - the reason an engine failure doesn't kill you is that someone designed around it. The rotor is not firmly attached to the engine, it has a slip mount so that if power is removed it keeps spinning. If problems with the tail rotor were similarly bad, someone would have engineered a similar auto throttle disconnect. (Not saying that isn't a problem, just that it is not seen as an unacceptable safety issue)

K2,
The 250,000 lb Amroc hybrid failure was a memorable experience by all who witnessed it.
(I did not witness this, this comment is 2nd hand)

I can imagine no failure of a Liquid Motor that
could be as explosive as the worrysome failure of a hybrid.

The scary hybrid failure mode is as follows...

The solid fuel fails structurally and a Large piece clogs the Rocket throat. You now have far more energy in a contained space than in any liquid failure that I can imagine. Gun powder is not a good exposive unless you contain it in some kind of pressure vessel. when it builds up enough pressure to rupture the vessel and the rapid
expansion of the rupturing vessel creates the explosive shock. Gun powder lit on the ground just looks like a energetic sparkler.

A liquid tank or pump failure can create an uncontained fireball, and a hard start can blow a liquid chamber, but the amount of energy contained in a blown chamber is a small fraction of the energy availible in a plugged hybrid.

For a liquid the fireball is not contained in any rupturing pressure vessel.

For the Nitrous oxide Rubber hybrid like SS1 there is anouther failure... if your hybrid plugs or otherwise generates enough pressure to push hot gas back into the nitrous tank the nitrous
will thermally decompose and again you have an explosion in a contained pressure vessel.

I personally think that the liquid failure modes
are more survivable, given an escape system, than solid or hybrid failures. One can design to stand a brief high heat flash, and the Liquid won''t make as severe shock front.

Paul

> I’m not sure that 2000ish liquid teacart firings are even relevant
> as evidence of liquids safety as compared to the tens of thousands of
> hobby hybrid HPR flights!

The fallacy of that comparison is obvious. High-power rocketry rules require spectators to stand way back to ensure safety. That doesn't prove anything about the safety of high-power rocket motors. You can test the most dangerous devices in perfect safety, if you keep people far enough away.

We blew two 4000 lb thrust engines off the Space America vehicle in 1998 because of the jelling of Kerosene that made it get to the engines just slightly after the oxygen did. It made a very cool pressure wave across the desert, started a fire on the vehicle that burned most of the wiring, and then it just sat there venting the LOX upward away from the fire. The kerosene, in steel tanks, just sat there.

This was a pressure fed engine with obviously oversized tanks to contain the 300 + psi pressures.

It seemed to us that the pressure fed system had a lot of safety factors built into it (including the burn wire that safed the electrical system). Any payload that was on the system would have been perfectly safe.

Now there are probably modes that still would have blown up this vehicle but to say that hybrids are inherently safe versus liquid systems absolutely, is not really a valid statement.

There are many other failures other than explosion that would leave payload and or customers just as dead with either technology.

Dennis

Tractor trailers loaded with tons of LOX and tens of gallons of kerosene continually crisscross the nation's highways and pull into hospital parking lots. They are safe because the LOX and kerosene are in separate tanks a few feet apart.In that way they are no different from LOX/kerosene rockets.

Aside from hard starts, which should can and should be engineered out,it seems that the danger is solely in tank rupture and propellant mixing.For that reason, a vertical takeoff rocket can explode if it loses thrust and falls back to the ground.

Similarly, a horizontal takeoff rocket must crash to explode. With provision for LOX dump even that crash danger is mitigated.

Maybe there is a way to prevent hybrids' exploding from a plugged nozzle but I'm not sure how it'd be done.When I ride a rocket, you can be sure it'll be LOX/ liquid hydrocarbon.

Lee Valentine

And it'll be sooner than you think.