Orion: Have You Called Jenny Yet?

My suspicions have been confirmed, sort of, by a post over at Transterrestrial Musings. The new Orion spacecraft is having some [ahem] girth issues. Now, to a degree, this is a chicken-and-egg problem: is Orion too damn heavy, or is the Ares I booster too puny? Most experienced spacecraft engineers will tell you it's the former. There are a number of good reasons for that.

As I said earlier, I was rather less than astonished to see that the estimated mass of the Orion spacecraft grow during the various iterations of the design process. There hasn't been a flying vehicle yet from the Wright Flyer on up that hasn't experienced weight creep. It just happens. You think you've hit all the angles, and toted up everything you're going to put on the vehicle. But every time you turn the crank to re-compute the gross weight, more mass appears almost as if by magic. Because there's always something you didn't consider. There's always a bracket that doesn't quite fit, and needs re-design. Always, re-design means that the part in question gets heavier.

No, that's not what's got people talking. Some weight creep is expected. What's causing the uproar is the amount of weight creep. And more to the point, the amount of weight creep relative to where we are in the design process.

What I find somewhat unnerving is the number of changes being made, as suggested by this document. They're fiddling with a whole bunch of things that really ought to have been nailed down tight already. I worry rather less about the specifics of the trade-offs than I do about the fact that they're being made NOW, not two years ago. This ain't good. But on the other hand, it's been thirty years since we last designed a manned spacecraft. To an extent, we're having to re-learn the discipline. The guys what did it last time are retired, dead, or both.

Doing it for real isn't the same as doing it on paper.

That said, we're just going to have to face facts, and accept that the schedule is going to slip to the right as they iron out these problems. They're going to have to force Orion onto a strict diet, and find a way to shed some unsightly pounds. And as I said earlier, there are plenty of good reasons they're going that route, rather than either beefing up the Ares I booster or upgrading to a more powerful model.

Every extra pound at the top of the stack has many, many unwanted effects. You have to burn that much more fuel to get it there. The extra fuel translates into bigger fuel tanks, more structure, more weight, and so on. It's easy to get into such a vicious spiral that you no longer have enough power to leave the ground at all, much less reach orbit. Plus, every pound you shave off of Orion is an extra pound of payload that can go to the Moon lander. More equipment, more sample return capacity, more flexibility on that end of the mission. These savings can also have unexpected benefits. The Apollo spacecraft had similar weight problems, and a design decision was made to shift some of the mass budget over to the lunar lander. The extra fuel and consumables ended up saving the lives of the Apollo 13 crew, since the upgraded LEM now had the capability to function as a lifeboat.

So, bearing all that in mind, my mood is slightly annoyed, but not truly alarmed. These are some pretty sharp folks. They might have to go through a few more iterations to get it just right, but they'll get the job done.