Sunday, September 16, 2007

Launch Abort!

Yesterday we examined some of the technical issues associated with the Emperor's launch vehicle. One gets the idea that this $6-9B investment may not be in the taxpayer's best interest after all. It's kind of like ordering lobster for dinner and ending up with lutefisk! In their attempt to eliminate complexity in the design (i.e., getting rid of the turbomachinery of a liquid first stage), the NASA "rocket scientists" at MSFC (more accurately, the NASA "vu-graph engineers"...no let's take that back, too...no one would ever accuse Steve Cook of being an engineer) have added complexity back into the design with band-aids to keep the wobbly spaghetti stick flying straight. That makes one piece of safety equipment, the launch abort system (LAS), even more critical in the Orion/ARES design.

The basic LAS design goes all the way back to the Mercury program. A small tower attached to the top of the crew capsule contains a rocket motor that can quickly pull the capsule away from its launch vehicle in the event of a catastrophic failure. The tower puts the safety system as far away as possible from any initiating event in the launch vehicle. The Russians have had some real world experience in that department. Soyuz T-10-1 and Soyuz 18a both employed a launch abort system similar to (based directly on Max Faget's design for which Max received an award acknowledging that fact from the Russians many years later) the American design. These early systems were small, simple, and got the job done.

Of course, on Orion/ARES, nothing is small or simple. We'll discuss the reasons for that later, but suffice to say, Orion is so heavy it requires a very large, very sophisticated LAS. The large system's early designs also suffered from a severe acoustics problem. Shocks coming off the tip of the tower and the escape rocket nozzles re-impact the capsule and give it an unacceptable "buzz." Protecting the capsule with a heavier blast protective cover only adds more weight to an already stocky system.

Large and sophisticated means heavy, so instead of solving the primary problem (Orion=heavy), NASA engineers once again started looking for point design band-aids. On the back of a napkin, Doc Horowitz (to be remembered as ARES' proud papa), once again illustrated his lack of engineering prowess by suggesting that a pusher system might serve better as an LAS for ORION.

Pushers suffer from multiple problems, some implementations are worse than others. First and most obvious, pushers are located behind the capsule and closer to the potentially offending launch vehicle failure. I don't know about you, but I'd rather have those couple extra milliseconds for the blast wave to find its way to the top of my safety system, than to give up that precious time and lose out with the system engulfed underneath me.

If its a liquid system, more precious time is taken spinning up the system to full thrust to get away. If its a solid propellant based system, its probable distributed around the circumference of the vehicle in separate motor pods. Now a complex ignition system must get all those rocket motors fired up at exactly the same time, and all of the hold downs released simultaneously, least the capsule hang up on one side and start to tip in unacceptable fashion.

By the way, if the pusher is to just carry away the capsule, then it must somehow be configured circumferentially around the capsule above the service module. That would seem to interfere with the launch aerodynamics of the system, and would suffer from severe heating and acoustics environments. So a more benign configuration would place the pusher under or inside the service module. Of course, this system must now carry not just the capsule, but also the service module off the top of the exploding rocket. More lift capability = more propellant = more weight.

If the whole thing is surrounded by a shroud, necessary to protect the capsule from the acoustics and heating brought on by ascent through the atmosphere, the shroud must at some point split in two, allowing the capsule to safely deploy its parachutes and land. Of course, the capsule must also now separate from the service module before safely returning to earth. Complexity just always to seem to creep back into NASA's simple solutions, doesn't it?

A freshman engineer could do some simple math in his head to see that such a pusher system won't be lighter than a tractor system. Consider that the capsule must be pulled away at a specific level of acceleration. That defines the thrust that must be delivered by the rocket motors. With all things being equal between a tractor system and a pusher you then have to add in the weight of the service module and shroud. Now you need an even bigger set of LAS rockets. Of course, those rockets have to push their own weight away when they are used, so make them even bigger to do that.

But it gets even worse. With the tractor system, after it would no longer be effective, you get rid of it. That extra mass doesn't have to get carried with you all the way to orbit, impacting overall payload performance. Not so with the pusher, as you have to carry that system all the way to orbit, maybe all the way to the moon. What engineer familiar with the rocket equation would even propose such an ill-formed solution? That's a penalty that the ORION can not afford to bear.

Pushers are losers.

And we didn't even mention that unlike Apollo's very simple passive system (light it off and it is able to bring the crew back safely without need for complex computers and flight sensors), Orion's system is loaded with avionics to "make it more versatile." In the old days, versatility meant "opportunity for failure."

Recall that this discussion resulted from reviewing a concept born on the back of an envelop. Your tax money is now being spent to assemble this deficient concept into a test article and keep folks employed at NASA's Langley Research Center The artist, Doc Horowitz has been leading the development of our next generation spacecraft and launch vehicles. What's wrong with the picture?

Maybe the Emperor should take his entire team to the department store for a fitting?

2 comments:

NASAwatcher said...

Very good blog. Another thing about the tractor system--when its use is over, it is ejected and the mass of the system is reduced. A pusher located between the CEV and the SM it is not ejected and its mass has to be carried to the moon (not good for mass and also safety--sure wouldn't like to sit on this system after its use is over); if it is located under the SM, then it has to be carried to orbit before it is ejected. The rocket equation shows that this is also not as effective as getting rid of a tractor before orbit is reached.

Jon Goff said...

Rocket Man,
I do have to (slightly) disagree. There are some situations where a "pusher" system can make sense. There is always the issue you brought up of the engine being closer to the explosion (if it was an explosion--there are abort situations not caused by an earth-shattering kaboom), I'll grant you that. However, by placing a pusher with the engine in the middle, it can actually greatly reduce the required worst-case thrust needed for safe escape. Basically, in a transonic abort, you end up getting a low pressure region between the capsule and the vehicle that try to force the two back together. With a centrally located pusher, that region between the two parts is actually at a higher pressure than ambient, which ends up reducing the force necessary by a large margin.

Also, there's nothing that says that a liquid-fueled engine has to have a turbopump. It could very well be pressure fed. And in some cases a pressure-fed liquid can have a faster "zero-to-hero" time than a solid.

In the case of Orion, you are probably right though. Pushers when you have a large and separate service module really don't make much sense. They make a lot more sense for smaller, more reusable capsule/stage designs. I'm just sayin' that you shouldn't throw the pusher "baby" out with the Shaft "bathwater".

~Jon