Sometimes the lads at NASA are slow learners. Back in 1989, George Koopman of AMROC offered to replace the dangerous Solid Rocket Boosters (SRB’s) of the Space Shuttle with a safe, throttleable hybrid version. NASA wasn’t interested and not long after George’s tragic death in a car accident, AMROC folded.
But never fear! Fourteen years later, NASA has discovered hybrids! Better late than never I suppose.
Why, you may ask, am I making such a big deal about a hybrid replacement for the SRB’s? They fixed all the problems on the Shuttle after the Challenger didn’t they?
No. They did not. Not because they didn’t want to, but because there is one problem inherent in the STS design which can’t be fixed without a big change: SRB’s cannot be shut down. Once those candles light, there is no survivable abort until they have burned out and SRBSEP has occured. (That’s “Solid Rocket Booster Seperation” in laymanese). You can’t do an early SEP either. I’ll try to explain why.
The current SRB’s are basically very large skyrockets. So large they have to be built in segments (with O-ring sealing gaskets in between the bolted together sections) because quality control on pouring the fuel/oxidizer mixture inside would be a nightmare on something that big. The stuff must be perfectly regular inside and have no voids (bubbles). There is a shaped void down the centerline which must be of the right shape. The SRB’s are ignited from the top and since the mixture contains both the fuel and the oxidizer, once they start burning, there is no stopping them until the gunk is all gone.
There is a way to stop the thrust however; there are explosive charges that blow the endcaps at an appropriate time so that dropped SRB’s don’t go flying off on their last legs somewhere they shouldn’t; opening the tube can also act as a brake. The recovery chutes are up there as well.
Now for the scenarios. Let’s assume some pending Cat1 (category 1, failure leading to loss of vehicle, loss of payload, loss of crew) shows up on the consoles of the commander or ground control.
- Before launch. The crew open the hatch, run out along the gantry, jump into escape baskets that whisk them to ground level and an armoured vehicle some distance from the pad.
- At launch. The shuttle is too low to reach a landing attitude, let alone an ET (External Tank) and SRB SEP, and get on a glide path to the runway. Either the whole stack collapses back onto the pad in a big fireball or it blows at low altitude or falls into the sea and then blows. Survivors? Are you joking?
- During ascent. The SRB’s are at thrust and cannot be shut down. If you do a premature SEP, the odds are rather high the shuttle will be ripped apart by aerodynamic stresses far above design limits. It wasn’t built to ride the wake of two hot SRB’s passing beneath its’ wings. If they blow the endcaps to “put out the fire”, the stress of the sudden deceleration rips the ET apart and we get a Challenger type cloud in the sky with bits falling out of it. No survivors.
- After SRB burnout. They can throttle back the SSME’s (Space Shuttle Main Engines), do an ET SEP, drop out from underneath (they fly upside down to orbit), do a half roll and proceed with an RTLS abort. (Return To Launch Site).
- Too far for RTLS, too low for Spain… If they can’t make the runway and have to ditch in the Atlantic, they get into parachutes and extend a long pole out the hatch which guides the jumpers away from the wings. The commander has to keep the shuttle flying level and make sure everyone gets out. He also may not have time to escape himself, but that’s Pilot’s Burden and acceptable to most who understand the responsibility of being a Pilot.
- Just a close call. If an SSME or two is shut down, they may still be able to Abort To Orbit (ATO) or Abort Once Around (AOA), depending on what low orbit they can reach. This is much preferable because it gives valuable time to work through the situation. There has been one ATO so far. The engine shut down was due to a faulty sensor and most of the mission was completed in the lower, less than optimal orbit.
You are wondering, “Why can’t they eject or bail out?” Well… at least two ideas were examined years ago. They could put in ejection seats for the flight deck. But what do you do about the lower deck? On the other side of the pressure vessel from them is the ET; nowhere to eject to. So only the flight deck crew could eject. The idea was dropped. No pilot was going to want to be in a position of abandoning the people they’d trained with for years. They simply wouldn’t eject.
Another idea was a B-70 or B-1 like crew compartment seperation. Blow the entire pressure vessel off and put a big frigging parachute or parasail on it. This was dropped as being not very feasible. Removing payload capacity from an already overpriced and unecomical vehicle was a non-starter. Payload mass is a terrible thing to waste.
Now we get to the point. Why are hybrid’s so great? And what the hell is a hybrid anyway?
A hybrid is superficially like a solid rocket. It has fuel coating the inside of the tube in a way that looks just like the solid. The difference is this coating is only fuel, not a fuel/oxidizer combination. The oxidizer is usually LOX (Liquid OXygen) that is fed from the top in gaseous form. Things burn in pure Oxygen like you would not believe. AMROC’s hybrids used butyl rubber for a fuel. That’s basically a truck tire. In pure oxygen the stuff burns very cleanly. No black smelly smoke, just water and CO2. The nice part is you now have control. Slow the LOX flow and the thrust goes down; increase the LOX flow and the thrust goes up; cut it off and the fire goes out.
This is all assuming, of course, you have heaters on your valves. If you don’t they’ll freeze in position and nothing will happen. Don’t laugh: it happened to AMROC on the pad at Vandenberg AFB. The valve stuck partially open during the start up and would neither go to the full open or the full closed position. Their rocket sat there pouring black smoke out the bottom. When enough fuel burned to upset the balance – there was a payload on top – the thing fell over ala Firesign Theatre (“She’s no fun, she fell right over”). Afterward the AMROC guys walked up to the used and now empty rocket laying on the ground and retrieved the payload.
The fuel was so safe AMROC were legally allowed to drive a fully fueled rocket through a city centre at rush hour. It was no more explosive than the tires of the trucks beside it. Technicians could smoke cigarettes while working on it on the pad any time until the LOX was pumped in during the countdown. Their cigarettes were more dangerous to them than the rocket.
The only thing better for a manned rocket is an all liquid booster. The problem is the cost to develop a complex reuseable engine whose plumbing gets dunked in salt water after every use. I don’t know about you, but the idea scares the hell out of me.
I must admit I like the image the new NASA hybrid gives me though. We really will be lighting candles. The fuel, you see, is paraffin.
Note: I have vastly oversimplified and glossed over detail. One could write books on this topic. I haven’t the time to write enough to really cover it accurately; most of you neither would care nor have the time to read it; and those of you who know what I’ve left out or simplified don’t need to read it anyway.
I agree! NASA is a slow learner because it is a large publicly-funded bureacracy, subject to massive political interference. It constitutes the single biggest obstacle to the exploration and commercial exploitation of space (the NHS plays a similar role wrt health care in the UK). The shuttle, as Freeman Dyson and many others have argued, is a turkey. It achieves only low-earth orbit, it costs too much to launch, and costs too much and takes months to maintain/repair in between launches. The space station is a giant and colossally expensive ‘tin can in space’, which benefits pretty much nobody. It certainly doesn’t seem to have helped science at all, or the ball bearing industry (as promised at one point). As a means of employing former soviet rocket scientists, it might have had a small initial benefit (but nothing that a few US visas wouldn’t have sorted). To begin the space race in earnest we need to develop some more biotech and come up with a ground-based alternative to rocket fuel (e.g. laser propulsion or em cannons) The ‘Smaller, faster, cheaper’ idea was probably the best recent thinking to permeate NASA’s high command. Unfortunately, a few failed missions later, and the approach was squashed. I suspect those missions cost less than the rivets on the space station. ‘Smaller faster cheaper’ is of course is the default approach of start-up companies. Everytime they lost a robotic probe, NASA should have rejoiced. Hurray! We’ve found a new way NOT to design a probe. Instead, well, bad PR is bad for Congressmen……… EOR 🙂
Note that in the Challenger explosion the crew compartment and the crew survived the explosion just fine–they were killed when they hit the water. If there had been parachutes on the compartment they would have survived.
“Note that in the Challenger explosion the crew compartment and the crew survived the explosion just fine–they were killed when they hit the water.”
Lnkplz
http://www.faqs.org/faqs/space/controversy/
Scroll down a bit (or use search) for “HOW THE CHALLENGER ASTRONAUTS DIED.”
Wow. Thanks. Didn’t know that.
I’m afraid I’ll pass. I already know in absolutely excruciating detail. Yes, I know that too. I pretty much expended myself on this subject at the time. I was on the Evening news in Pittsburgh the day after, using a model I built for Space Day exhibits of Pittsburgh L5 to show the sequence of events.
I went to school with Judy btw. One year ahead of me in undergrad EE at CMU. Same advisor at different times. When I ran the ISDC in Pittsburgh in 1987, we had the Civil Air Patrol fly in the rabbi from her home town for a Sunday dawn ceremony in Point Park with a colour guard.
Yes, I know exactly how they died and when.
January 27th and 28th are days on which I always stop for a few moments in remembrance.
Shame if you don’t know why two days…
Another problem with the “ejection seat” idea is that you can’t use one at speeds like Mach 6 (and the shuttle goes a lot faster than that). The impact of the air on the person as he emerges from the ship would be more than enough to kill him. He’d actually have to be enclosed in some sealed vessel, and that’s not practical.
In case #3, Dale, would “blowing the endcaps” actually cause a reverse-thrust situation to create the “sudden deceleration” that you mention? If the SRBs simply extinguish and, of course, the liquid fuel engines also shut down, the most deceleration you would get is 1g from the pull of gravity.
There are two factors. One is the jerk, da/dt; but I suspect the more important is the drag introduced by the now open ended
SRB’s in a supersonic flow. That is going to be a real slam in the face.
There are other ways to stop the thrust than blowing the ends off. You can just blow vent holes in the case in such a way that the hot gases don’t impinge on the shuttle body. This technique is used quite often to thrust-terminate ballistic missiles that use solid fuel.
I know this doesn’t address the how-to-save-the-astronauts question, but I think others have done that better than I could.
Lots of people have worked on the problem. There are solutions but they are too costly. The vehicle just can’t be economicaly made much safer.
Thrust termination of a free flying BM is a whole different kettle of fish. You have to blow holes in all directions such that the resulting side thrusts are totally balanced. Other wise you rip the structures apart. They weren’t, and can’t be built to take loads far above the expected levels. Aerospace strength margins are very low.
Blow holes on the outside of the SRB’s and the two SRB’s will ram the connecting rods through the ET structure on the other side, at which point the ET will structurally fail and you have a Challenger event as the upper LOX tanks breaks through the lower LH tank. The turbulence of the burnoff did in the Challenger (not blown up, ripped apart by structural failure due to aerodynamic loads not designed for), and it will do in your idea seconds after the termination holes are blown.
Much, much better to go to something you can smoothly throttle down to idle over a period of tens of seconds.
What’s the state of the art in SSTO designs? That implies restartable, throtlable engines.
Restartable engines aren’t the problem here. We’ve got loads of restartable liquid fueled engines. But designing a liquid fuel engine for a rebuildable booster is extremely difficult and probably not worth the money. You have two options: ocean revovery or flyback.
Building an add on flyback to the existing shuttle stack in place of the SRB’s strikes me as a project that would cost more than what a private company could build a lunar outpost for.
Ocean recovery is equally dicey. You’ve got all the stresses of flight, plus the stresses of landing and floating in salt water and being towed back to KSC. Solvable, but… I can think of lots better ways to spend the money.
Government SSTO’s programs (other than DCX which was so successful they didn’t go to Phase II) have all been lemons. NASP never got off the drawing board; the X33 was so big a technology jump it was doomed at birth.
Also, I’d much rather the government just keep doing moderate things to the Shuttle. I hope we force them to live with it until it’s so old they have to buy a replacement – perhaps a used passenger rocket from USSpace…
We really don’t want NASA competing with all the brilliant projects that are cropping up all over. People are out there in the deserts bending metal and putting rockets under arses guys. That’s where the real thing will come from.
Faster, better, cheaper was a fraud — like so much in Goldin’s NASA.
A few good things came out of it — adoption of a few smaller missions, for example.
But space station? Please.
Not to mention abuse of staff who wouldn’t go along with the lies.