Consternation

Since the last scrub of Endeavor, NASA TV has been in something of a pickle. They were probably hoping for lots of groovy footage from the launch and in-orbit (you can say "on-orbit" if you like, but I won't join you) activities, but the LH2 leak and the subsequent beta-angle cutout till mid-July have seriously denuded the NASA TV schedule.

So they've been playing a sort of advertising video for the Constellation program in the dead time. Constellation, for those who don't know, is the overall name for the manned spacecraft (plural) that will replace the Shuttle when it is retired. Constellation consists of two launch vehicles, the Ares I and the Ares V, the Orion spacecraft (best thought of as an updated Apollo CSM) and the Altair lunar lander (best thought of as an updated LM).

There's a lot to like in all of this. I've always been fairly vocal in my belief that putting holes in your heat shield, such as landing gear doors or separate pieces for the wing leading edge, is too risky for a manned spacecraft, especially one intended to re-enter at hyperbolic speeds. So the Orion is a good move, I think. And I like the Ares V, because I think heavy lifters are always useful. You just never know when you might want to put 120 metric tons of stuff into low orbit in one mighty heave, and I enjoy the homage to the Saturn V implied by the name Ares V.

But the Ares I continues to bug me. It's bugged me since day one, but I sort of forgot about it until they started playing the Constellation pep rally video. The Ares I upper stage is fine - it's a liquid fueled J2X engine, derived from the J2 of S-IVB fame, and there's nothing wrong with that. But the first stage? Hrmm. I come from a generation of rank rocket amateurs that believes with near-religious faith the following commandment:

Thou Shalt Not Attempt To Man-Rate A Solid Fuel Booster.

Period. Full stop. Case closed.

Solid fuel rockets have lots of interesting and useful features. They're relatively inexpensive, especially compared to cyrogenic stages (those that use, say, liquid hydrogen and liquid oxygen). Though difficult to manufacture, they aren't as difficult to manufacture as liquid-fuel engines. Though they are not particularly efficient (meaning that they have a relatively low specific impulse) they can be made extremely large, and since thrust is a function of combustion chamber surface area, size means power, so they can be made extremely powerful indeed. They are also storable, which isn't all that useful in a space launcher, but often of vital import for a military rocket. And solid fuel rockets cannot by definition suffer from an annoying periodic variation in thrust commonly called "pogo" that often plagues liquid-fuel rockets. Problems with resonance are also nonexistent, and the stout steel casing makes for a nice strong fuselage.

But are they safe? Safe enough for launching unmanned things, certainly, where the worst consequence of a catastrophic launch failure is that you have to build a second payload and try again (and, if the failure happened at low altitude, you may also have to fix the parking lot and replace a bunch of burned-out cars). But are they safe enough for manned flights?

I don't think so. Solid-fuel rockets have several grave disadvantages when it comes to manned operations, upon which I dwell below:

1. In a solid fuel rocket, almost all failures are catastrophic ones. Most failures in liquid-fuel engines are also catastrophic, but there are cases of liquid-fuel engines being successfully shut down before exploding. This leads us to...

2. Solid-fuel rockets cannot be shut down. Once they are ignited, they burn until they're out of fuel. There is no way to shut the thing down and abort the launch. If there's a problem after ignition, well, that's just too bad. You can blow open ports in the chamber that vent the gas and prevent the booster from generating any thrust, but you cannot shut down the combustion process itself.

3. TVC (Thrust Vector Control) is much more difficult in solid-fuel rockets. A liquid-fuel rocket can be steered by gimbaling the engine, but a solid-fuel rocket has to use either a movable exhaust nozzle, or some variant of fluid-injection TVC. Fluid injection seems safer to me, but the Ares I is going to use a movable nozzle, which I think presents risks of the flexible gasket burning through, or the movable nozzle becoming jammed with aluminum from the exhaust plume. This has never happened to the Shuttle SRB, which forms the baseline for the Ares I first stage, but it has happened to other solid fuel rockets.

4. It strikes me that most solid-fuel rockets become erratic when they near burn-out. There's no way to completely control the pattern of combustion in the chamber, and the engine's performance varies as parts of the grain burn down to the steel liner ahead of other parts. All sorts of interesting combustion instabilities can occur, one of the most picturesque being "chuffing", often seen in things like the Honest John. Erratic thrust is bad. It makes navigation and guidance difficult, and it shakes the hell out of the spacecraft and its occupants. The only way to get around this, it seems to me, is to jettison the solid fuel booster long before it reaches the point of chuffing and other burnout hijinks, which means that you are having to lift a bunch of excess fuel that you can never use, and if enough of these inefficiencies build up, you start having to surrender capability.


In Summary

The Ares I first stage is based on the Shuttle's SRB, as previously mentioned. And the SRB was the direct cause of 50% of the Shuttle's fatal accidents. Let us not forget that it was a field joint burn-through that led to the loss of Challenger, while in no case has an SSME (Space Shuttle Main Engine) failure led to a loss of life. So in our new design we throw away the part of the propulsion system that has been proven to be efficient, reliable and safe, and we keep the part of the propulsion system that has already caused one fatal accident and to this day continues to suffer from O-ring blow-by in the field joints.

Does this sound like good engineering to you?

It sounds like "cost containment" to me...