Elon Musk is a really bright guy. He agrees with me.
No, I don’t think Musk or anybody at SpaceX has ever heard of me or knows (or cares) who I am or what my opinions are; still less do I pretend to have the chops to design a rocket. Still, I do try to keep up from a (hopefully intelligent) layman’s point of view, and I like to think that if I’d followed up on my teen-age dreams of becoming a rocket scientist, I would have avoided Apollo-thinking and come up with something like what SpaceX produces. Their current and proposed output conforms so closely to my current set of prejudices and opinions that it’s like one of us was reading the other’s mind.
Right at the top: they’re burning kerosene (and liquid oxygen, of course), not hydrogen. The single most useful metric for a rocket, overall, is specific impulse, or Isp, which is a measure of how efficiently the rocket uses its fuel. Isp depends on exhaust velocity, and exhaust velocity depends on how “hot” the fuel burns and how heavy the combustion products are (lighter is better). NASA is fixated on using hydrogen for fuel, because hydrogen-oxygen is the hottest fire with the lightest combustion product possible with the chemical elements that exist. The Space Shuttle, burning hydrogen and oxygen in a very high-tech engine, achieves an Isp of 450 or so, somewhere close to the limit for chemical fuels. I don’t have a good number for a kerosene-oxygen rocket, but I’d guess it has to be below 400 (but not by much). NASA wants the most for the least, so for them, hydrogen is the way to go.
It comes at an awful cost. Hydrogen is the lightest element, and is dreadfully difficult to get cold enough to be a liquid. Even as a liquid it isn’t very dense, so the fuel tank has to be big. The molecule of hydrogen is so tiny that the concept of “tank” isn’t really applicable — the stuff will leak (actually seep) through solid metal, and on the way combines with the metal to form hydrides, which are weak and brittle; this is one of the factors controlling the life of a nuclear reactor, which produces hydrogen as part of its functioning. Producing and storing liquid hydrogen is expensive, complicated, and hazardous to all concerned. Then there’s the matter of fuel pumps.
Oh, yes, rockets have fuel pumps. A portion of the fuel/oxidizer mixture is diverted to run a motor, usually a turbine, and that motor drives the pumps. So with liquid hydrogen, on one side we have the hottest fire possible — and a few inches away, we’re pumping huge quantities of a substance so cold most materials shatter like icicles when it touches them! Now you know one of the reasons the Space Shuttle Main Engine is so darned expensive.
With kerosene many of those problems go away. No doubt a rocket needs something more pure than what you’d get by whistling up a Diesel-fuel tanker or the Jet-A truck from the airport, but the stuff is very similar and can be handled by the same sort of machinery. Just being able to store the fuel in an ordinary tank and pump it with gear you can order from a catalog translates into big savings in per-launch costs. The rocket will be heavier, because kerosene is heavier than hydrogen for the same amount of energy, but it will actually be smaller, because the kerosene needs a smaller tank.
Second, they aren’t using solid-propellant boosters. Solid propellants are excellent for rockets that have to be stored for later use, and be dependable when uncrated — air-to-air missiles, rocket-propelled grenades, and ICBMs that have to wait in their silos (or aboard a submarine) for a long time and go off on command. If you want something storable and dependable, a solid is the way to go — but the main advantages of solids are things that don’t make any difference to a satellite-launching rocket. For that you need something light and powerful. In a liquid-fuel rocket, only the engine has to take the awful pressure and heat. In a solid, the whole thing is the “engine”, and has to be tough enough to stand the gaff. That translates into something heavy, and the Isp of a solid doesn’t go much above 250, if it gets there. Solids are referred to as “strap-ons”, and if that makes you think of heavy breathing and substitutes for the Real Thing, you’re pretty much on target.
The SRBs on the Shuttle are a political concession, not a technical advantage. There have been several proposals to replace them with liquid-fuel boosters, either kerosene-oxygen (for simplicity) or hydrogen-oxygen (for commonality with the rest of the fueling), and for the same weight and size the result would have been up to a 40% improvement in booster performance; in one report I saw, the Executive Summary concluded with a laconic statement to the effect that there were no technical barriers to the replacement. It was not to be, and solids were also an integral part of the (now-failed) Ares program, and not the least problematic part. This is one of the reasons SpaceX has a real advantage. If the U.S. Government builds a rocket, it will have solid-propellant boosters; Musk doesn’t have to answer to Sen. Hatch, so he and his people are free to use what makes sense technically.
Third, they use multiple small engines instead of one (or a few) big ones. Falcon-9 has nine engines, thus the name; Falcon-Heavy starts out by being three Falcon-9s strapped together, and might well be called “Falcon-27”. There’s no doubt that multiple engines means greater cost — but it also means that they’ll make enough engines to run something like a production line, and since each engine is relatively small the parts can be handled by normal production machinery, or even people, and that translates into “economies of scale”. The Shuttle only has three engines, and there have only ever been five of them; counting test articles and the like, there have still never been more than about twenty space shuttle engines, and they were extremely high tech, so they had to be hand-made one at a time. Falcon-9 has already flown twice, so SpaceX has already built (and flown) more rocket engines than the entire Shuttle program.
That’s huge in terms of reliability, in two ways. First, the way you get good at doing something is to do it over and over — practice — and building rocket engines is no different. They can control the quality at every step, and become thoroughly familiar with all the processes necessary to get the product out the door. That often falls down in the case of consumer products, because the metric there is how fast you can push them out and sell them. In a rocket-engine factory, with fewer time and money constraints, they can apply the full force of modern manufacturing and quality-control techniques, and let practice make perfect. As noted, eighteen SpaceX engines have actually flown, and none of them failed. To be fair, I don’t know that any Space Shuttle engines have failed, either — but SpaceX manufactured those engines; NASA built theirs one at a time, with an army of craftsmen and inspectors SpaceX didn’t need, and didn’t pay for.
The second advantage of multiple engines is redundancy. There is no bean-counter in the whole wide world who can’t prove to you, with reams of statistics and innumerable PowerPoint slides, that One Big [whatever] is more cost-effective than many little ones. They’re right, too — right up to the point where the One Big [whatever] hiccups, whereupon we learn once more why putting all the eggs in one basket is a sub-optimal procedure.
Shuttle has three engines, plus the two SRBs which don’t quite add up to a fourth. If one engine fails, there’s no choice but to abort — it can’t go on if it loses a quarter of its total thrust. Falcon has nine engines. If one craps out, that’s only an 11% loss in thrust — and the rest still have the same amount of fuel available. If that happens after it’s well on its way, it might not even mean loss of the mission. The same amount of fuel means the same amount of “change of motion”, and a little less thrust just means the rocket has to burn longer to achieve the same final speed. There’s even the possibility that the engineers have sandbagged a bit, that the engines can throttle up a little above their designed power level, and that might mean nobody but the engineers controlling it would ever notice the burned-out pod. Falcon-H will have twenty-seven engines. If one croaks, that’s less than 4% loss of thrust, and the mission goes on as scheduled with hardly a hiccup.
This factors into the debate about safety, euphemized as “man-rating”. SpaceX has already launched an unmanned capsule, similar to the Mercury/Apollo ones, and recovered it successfully on the first try. This isn’t good enough; if people are to be stuck atop a rocket to go into space, their capsule must have an emergency system to yank them away in case things go to worms. This sounds quite plausible, and the arguers in favor can pull on heartstrings with ease — but Falcon’s redundancy means things are much less likely to go wrong than they are with big-engine designs running on hydrogen; the only case in which an escape system might be useful is an explosion on the pad, and that’s enormously less likely with kerosene fuel. So why the insistence? Well, an escape system is an ideal application for solid propellant rocket motors. Sen. Hatch still has his fingers on the purse-latch, even if his grip isn’t as tight as it was.
And just to air-check this reasoning: ‘way back in the Fifties, while idling his time away in one of Stalin’s less-uncomfortable labor camps, Sergei P. Korolev did the preliminary designs for what later became the Vostok/Molnya/Soyuz series of rockets. They all have multiple motors instead of one big one; they all run on kerosene and liquid oxygen; they don’t use SRBs. Taken all together, there have been something like two thousand seven hundred flights of those rockets, with so few failures that the Russians have separate memorials for each such, and if you buy a ride to space from the Russians, either for tourism or to go man the Space Station, that’s what you’ll ride. According to the European Space Agency, the Soyuz launch vehicle is the most frequently used and most reliable launch vehicle in the world. Folks, that’s what you call a “track record”. It makes sense to build on and improve what’s been demonstrated to work, no?
And finally, Falcon-H will have a feature that delights me because I didn’t think of it: cross-feeding. When they put three Falcon-9s together to make a Falcon-H, they’ll add pumps that transfer fuel and oxygen from the outer two to the middle one. That makes the outer ones partially into fuel tanks, but with engines of their own. When taking off from the ground, a rocket needs lots of thrust to lift the weight, and in the dense lower atmosphere it needs thrust to overcome air drag. Once it’s well away from the ground and passes out of the thickest parts of the atmosphere, the need for high levels of thrust goes away; at that point it becomes a matter of fuel remaining and how much mass it has to push, and if it needs to go faster it simply burns longer because it isn’t lifting weight off the ground or shoving air aside. So when the boosters have burned their fuel and passed some along to the center rocket, they fall away (less mass to push, less drag in the little bit of air remaining at that altitude) and the core section goes merrily on, with plenty of fuel left to satisfy the Need for Speed by burning as long as it needs to. How much fuel gets transferred to the middle “stage” will depend on the mission, where it’s going and how much weight it’s lifting, but the ability to make the tradeoff will itself make the rocket more flexible and useful.
And that’s the only real innovation. Falcon-9 has been demonstrated; it works. There is no reason to believe three of them hooked together won’t work, and the rest of it is tried-and-true technology (much of which was invented by NASA and its contractors, which is how it’s supposed to go). Booster separation goes all the way back to WWII “drop tanks”; there are always details, but the basic idea is familiar, even old-fashioned. Falcon-9 has a second stage, a smaller rocket that goes into action when the big one has finished its work; that, too, is well understood, invented by Konstantin Tsiolkovski and developed to a fine art by Korolev, von Braun, and a host of engineers since space programs started up. If cross-feeding works it’s a significant innovation, but for me (and, I’ll bet, for a lot of engineers) it’s a facepalm thing — “Now why the [expletive] didn’t I think of that?” This is not to say the engineers at SpaceX didn’t work both hard and smart — they did and have, but it’s a matter of getting their version right (which is hard enough), not striking off into the Wild Blue Yonder.
Now all SpaceX needs is customers. NASA needs to service the Space Station, and right now the Russians are the only game on the planet; if Musk can get them to buy some Falcon-9 flights to deliver consumables he’ll have money to go on, in addition to what he can get from private customers (TV satellites, etc.), the Air Force, and the spooks at NSA’s National Reconnaissance Office. If the escape system can be built and tested, allowing Falcon-H to lift people, SpaceX can compete with the Russians for crew delivery and space tourism, too. Just don’t be too terribly surprised if Falcon-H sports a pair of solid boosters (in addition to everything else) some time in the future. It needs them like a competition motorcycle needs training wheels, but the Government is still the customer with the most money — and if it must have something on it that’s made in Promontory, Utah, I’m sure Musk and the other clever folk at SpaceX will be able to find a way.
By Apollo 17, things had gotten frozen in space, including attitudes (space is expensive and always will be, and only government can do it)…. Institutions were frozen as well — had gotten locked into Big Project mode by Apollo.
I think I’m entitled to preen a little. Modestly and decorously, of course.