Chair Force Engineer

Thursday, March 06, 2008

Was X-33 Really Such a Bad Idea?

I really don't even know what motivates me to post this, as X-33 is a long-dead program that hasn't held the industry on the edge of its seat in almost a decade. Yet the fact that the X-33 hardware is allegedly still in storage shows that, to some people, the concept still carries great hope.

Most observers of X-33 realize that the program was trying to accomplish too many test objectives, including:
--Metallic thermal protection systems
--Lifting Body aerodynamics and control
--Linear Aerospike engines for propulsion and steering
--Composite, multi-lobed cryogenic pressure vessels
--Rapid turnaround
--Autonomous runway landing
--On-board health management avionics

In hindsight, not all of the X-33 technologies turned out well. The aerospikes came in too heavy, and supposedly didn't meet their performance objectives during test firings. The composite hydrogen tanks proved difficult to construct without flaws, and failed their testing.

X-33's most glaring problem was the way it was grossly oversold. Lockheed Martin pitched the fantasy of VentureStar, twice as long as X-33 and over twice as fast--enough to achieve orbit without staging. Or so the fantasy went. Any aeronautical engineer could see through the ruse of the LockMart Public Relations machine and its blatant disregard for the laws of physics. But apparently it was enough to sway NASA into staking its hopes for replacing the shuttle on the mantle of X-33.

It's interesting to think about what would happen if X-33 was de-scoped early in the program. What if metallic propellant tanks were baselined from the beginning, rather than after the hydrogen tank failed? What if a conventional, off-the-shelf rocket engine were used? If these decisions had been made differently, it's possible that X-33 could have flown, and achieved a velocity sufficient to test the innovative Goodrich-developed heat shield (around Mach 12.) It's also possible that the repeated flights of X-33 would have taught us a lot about what is required to rapidly turn around and re-fly a spacecraft.

To be fair, there weren't a lot of choices for alternate engines on X-33. The Space Shuttle Main Engine was probably the best choice. Its vacuum performance was higher than that of the aerospike, but it's a finicky engine due to the complex staged-cycle combustion. Another option would have been the Ariane 5's Vulcain. Similar to the old J-2 in terms of thrust and specific impulse, it would be perfect for Ares I's upper stage (if only it was capable of air-start.) But the Vulcain wasn't designed for reusability, so it's uncertain if the engine would have been useful for the X-33.

So let's imagine for a minute that the X-33, after some moderate descoping, worked as advertised. It achieved a reasonable flight rate and performed successfully. What would be the next step towards replacing the shuttle and having a truly reusable launch vehicle? Forget about the VentureStar delusion and think about the possibility of strapping two or three X-33's together in a parallel configuration. Such a vehicle could probably make it to orbit with a usable payload. It probably wouldn't come close to being a shuttle replacement, but it would allow NASA (likely using the Air Force as a partner and end-user in development of this vehicle) to gain valuable experience with development and operations of a truly reusable spacecraft.

In a situation where two or three X-33's (or any bimese/trimese boosters) are flown in parallel, it is likely that the stack would lift off with all engines at max thrust, followed by throttle-down on the engines of the Stage 2 and Stage 3 boosters. A propellant cross-feed system might be employed, which would definitely add complexity to the system. When a booster would deplete its fuel, it could glide to a landing at a downrange airfield (depending on the chosen trajectory, burnout speed/altitude, and booster Lift/Drag ratio.) Because the X-33 was designed to be transported on a modified 747 (likely one of the two shuttle carriers,) recovering the boosters shouldn't be a problem.

But the "what if" questions about yesterday's choices fade away into the wispy clouds of memory. Instead, the Space Shuttle still flies; National Aerospace Plane, Shuttle II, DC-X, X-33, and Orbital Space Plane remain ideas whose promise was long ago discarded. And while the teams working on Ares and Orion might pride themselves on how far they've gotten, they need look no farther than the partially-assembled X-33 to realize how far along in the development cycle a program can go before it gets killed. The lesson is to determine what the requirements are, scope your program so it meets the thresholds with little risk, and never oversell what you're trying to do.