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.
Postscript: Simberg has a series of reports on the recent Space Access Conference. On one of the panels, Clark Lindsey talks about history:
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.
23 comments
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10 April 2011 at 11:10 pm
montejo
What’s the logic in man-rating Falcon Heavy? Couldn’t SpaceX just man-rate Falcon 9 and use that to ferry astronauts to orbit? Man-rating Falcon Heavy will make it less cost effective in getting mass to orbit–mass like fuel, supplies, water, modules, and infrastructure. The crew is only a small fraction of the mass needed to conduct manned space exploration, so any savings for the rest of the mass will make a huge difference.
10 April 2011 at 11:20 pm
Ric Locke
As I understand it, Falcon-9 can lift SpaceX’s capsule but not Orion, especially if you add the mass of an escape system (please correct me if I’m wrong). Man-rating Heavy would open up more opportunity. Man-rating -9 would mean opportunity in space tourism and possibly other non-Government markets, but -9 is more expensive per pound, which cuts into the market big time.
12 April 2011 at 3:26 pm
Robert Conley
From the press conference the Falcon Heavy) was built to all published NASA human spaceflight. Apparently rockets are built with a 25% margin into the structure and the Falcon Heavy will be 40% which apparently one of the key factors to man-rating a rocket.
13 April 2011 at 10:58 pm
mikelorrey
FH can put a Dragon capsule on a lunar flyby trajectory. They might even upgrade the trunk of the Dragon capsule to have a Kestrel engine and some fuel tanks to give it more dV to do a lunar orbit insertion. Current F9 can only put the basic Dragon and its payload/passengers in low earth orbit.
11 April 2011 at 4:53 am
Murgatroyd
Wikipedia gives the Isp of LOX and RP-1 kerosene as 358.
And as for man-rating Falcon Heavy: Elon Musk has talked about wanting to retire on Mars. I think he’s serious. He’ll be 40 this June. Imagine what he could do in the next thirty years …
11 April 2011 at 5:12 am
Murgatroyd
I’ve read claims of two other delightful advantages of fuel transfer, both a bit specuulative:
* The Falcon boosters were intended to be recoverable and reusable, which would really reduce costs. So far SpaceX hasn’t succeeded in recovering them, though. But the two stages of Flacon Heavy won’t go as high or as fast as the center stage or a single Falcon 9 booster, so it may be practical to recover and refurbish them even if the center stage is a throw-away item.
* Suppose you don’t put a second stage on the beast. Then the center stage may be able to reach orbit (perhaps if it has additional fuel tanks replacing the payload). Then you’ll have a refuellable vehicle, complete with fuel transfer plumbing, in orbit. If there’s a fuel depot up there too, where could that vehicle go?
I like Elon Musk.
11 April 2011 at 8:33 am
Ric Locke
I like Musk, too (obviously).
The first thing you mention is at least a notional advantage, although I reckon SpaceX are busy finding ways to locate and recover rockets. The problem up to now is that they tend to sink.
As for the second — in the near term, I cannot see the use for a vehicle with a million-plus pounds of thrust and an Isp of 350 in near Earth orbit. Almost anything imaginable could be done better by something more efficient, simply by boosting longer. As the fuel tank component of the depot itself it’s got real possibilities, in which case the engines would be along for the ride. Perhaps the gas-station attendants could dismount them in their spare time, if the EVA capabilities are good enough. An engine with 100K-lb plus thrust that’s already in orbit ought to be worth something to somebody. Back of the envelope says it’s the basis for a Lunar lander with over 30 tonnes gross mass… or they could bring ’em home two at a time on Dragons deadheading home from ISS resupply, for refurb & re-use.
13 April 2011 at 9:05 am
Jason Bontrager
If they’re not useful as spacecraft after achieving orbit, they’re still *mass*. Mass that can be used as a spaceship hull or a space-station hull, or even reaction mass for a mass-driver. The whole *purpose* or a rocket is to put mass in orbit, why shouldn’t the rocket itself be considered part of that payload?
13 April 2011 at 11:01 pm
mikelorrey
FH can put some serious equipment in orbit. It can loft some SERIOUS nuclear engines to build a truly interplanetary spaceship. It can put up mining equipment needed to exploit resources on the moon and Mars. And it can do those things at a cost per pound that was previously impossible.
11 April 2011 at 11:34 am
Murgatroyd
Almost anything imaginable could be done better by something more efficient ….
That’s the argument for hydrogen as fuel, too.
Refueling the Falcon core and using it has the advantage of being doable relatively soon without the additional time and expense of developing and testing “something more efficient.” It may not be the best way, but it might be the fastest and cheapest way.
What could you do with it? You could transfer fuel to it in LEO, burn a bit of kerosene and LOX, and use it as a fuel depot in lunar orbit or EML1. You could burn more of that kerosene and LOX and put a fuel depot into orbit around Mars. You could launch an Aldrin Cycler or a Mars expedition.
12 April 2011 at 2:47 pm
SpaceX Thoughts - Transterrestrial Musings
[…] …from Ric Locke. […]
12 April 2011 at 4:32 pm
Author
Couple of nitpicks;
1. Isp of Shuttle’s SRB is 269 at sea level, which is more than 250.
2. R-7 family flew more than 1700 times, which is far from 2700.
And honestly F9H is rather unnecessary until there’s a flight rate. Elon is doing it for 2 reasons: to compete for NRO contracts, and because he’s fixated on Mars. There are no payloads for it and won’t be for a long time, and when there are payloads, they won’t be going up 10 times a year. It’s a stunt.
12 April 2011 at 6:27 pm
Mike Puckett
Your remarks about the Shuttle are not eintrely accurate. At certain points in it’s flight profile it can lose one and even two main engines and still make orbit. In fact, in 1985 a shuttle Spacelab mission STS-51F. did lose an engine. It was prematurely shut down due to a faulty temp sensor, and still made orbit and sucessfully completed it’s mission.
13 April 2011 at 4:10 pm
Don Rodrigo
A Falcon 9 upgraded with the Merlin 1-D of the Heavy could carry Orion, FWIW.
The orbital fuel depot/refueling concept would work fine with the Falcon Heavy upper (orbital) stage. Refueling the second stage in LEO would give the system as much payload cpacity to the Moon or escape velocity as a Saturn 5.
13 April 2011 at 6:07 pm
Ric Locke
How so?
The Orion Crew Module masses 8.5 tonnes; the fueled service module is 12 tonnes. I couldn’t find a mass for the abort module, which Wikipedia says has a thrust greater than that of the Atlas that launched John Glenn; it can’t be a lightweight. 20.5 tonnes is only a little less than twice the 23K pounds mass Falcon-9 can take to LEO — and if you think NASA will let them stack it without the abort module you live on a very different planet from mine.
C or D model Merlins makes only a tiny difference. Provided that it will lift and accelerate from a standing start on the surface, thrust is almost irrelevant. Yeah, it goes from 1.11 million pounds takeoff thrust to 1.26 million pounds, but the Ds don’t have significantly better Isp, so the total change of motion depends entirely on how much fuel it has — and the fuel load doesn’t change.
13 April 2011 at 11:03 pm
mikelorrey
Higher thrust means higher acceleration and lower gravity losses, also, the first stage tanks are being lengthened to carry more fuel, so there is more dV.
15 April 2011 at 7:59 pm
Nelson Bridwell
Rick:
So, using your logic, the ill-fated Soviet N-1, with 30 engines, was more reliable (4 failures, 0 successes) than the Saturn V, with 5 engines (0 failures, 13 successes)???
I suspect that some of what you assert is true. However, when it comes to probabilities, the devil is in the details. In particular, I would be concerned about the chance that failure in one engine could be totally catostrophic. In contrast, the Ares I design, with one solid engine in the main stage, came up with one of the better safety estimates.
It would be interesting to find out how real reliability estimates are calculated. (Anyone have a link to a relevant document/wiki???)
I could be wrong, but it almost sounds like you like Mr Musk enough that if he was, hypothetically, a lemming, you would gladly follow suit 😉
15 April 2011 at 10:45 pm
Ric Locke
As I said in the post, SpaceX’s work appears to follow most of my own prejudices and preconceptions, and it’s pretty well inevitable that I’d like it, now isn’t it?
It is perfectly possible to make a bad design starting with literally any set of concepts. As ESR says, “There is no God but Finagle, and Murphy is His prophet.” N-1 failed by fratricide in the engine clusters, because the Soviets didn’t have powerful enough engines and ran into the Law of Diminishing Returns trying to add more; they then overstressed them, resulting in rather more noise and smoke than intended. Saturn V was also a design with considerable redundancy, and as I understand it did lose engines without loss of the mission.
Engineering is a matter of tradeoffs, of accepting disadvantages here to gain more significant advantages there. As a matter of personal preference, I like trading off ultimate performance for reliability. Leaping off cliffs, with or without companions, is only “reliable” in the perverse sense of the concept.
21 April 2011 at 2:57 pm
Murgatroyd
In contrast, the Ares I design, with one solid engine in the main stage, came up with one of the better safety estimates.
In 1985 you could have made the same claim about the Shuttle.
(And unlike the Shuttle, the real Ares I has never flown.)
15 April 2011 at 8:32 pm
Nelson Bridwell
Click to access 20090007804_2009007046.pdf
Click to access 20100023397_2010025664.pdf
15 April 2011 at 8:33 pm
Nelson Bridwell
Some NASA reliability analysis reports:
Click to access 20100023397_2010025664.pdf
Click to access 20090007804_2009007046.pdf
21 April 2011 at 2:52 pm
Murgatroyd
More evidence that Elon Musk has set his sights on Mars:
http://www.theregister.co.uk/2011/04/21/musk_mars_dragon_claim/
23 April 2011 at 12:56 am
BravoRomeoDelta
A very thoughtful overview – as with many things the devil is in the details. I’d like to respond at more length later, but in the interim, a couple minor notes.
First, I agree that the ISP of solids is lousy, but they do have valid use in providing a lot of prompt, high-thrust propulsion. So yes, they do have a use beyond placating ATK (although this is an interesting bit in terms of who subsidizes what: http://bit.ly/efuT37 )
Secondly, Dragon will have a launch abort system. Much like their counterparts at Boeing’s team on the CST-100, they are pursuing a pusher system. In future, a pusher system does allow for the possibility of powered landing system, relegating parachutes to landing backup systems. This has the huge advantage of getting away from wet splashdown and all associated operational costs and maintenance burden of using space systems in a maritime role.
Past that, there are some serious questions about the broader business plan, but that’s a comment for another day. I really do hope that this all works, but there will need to be a few more rabbits in the hat for it to come together. I hope there are, and I’ve been surprised by the number of rabbits pulled forth. Nonetheless, from the cheap seats, the hat only looks so big and the number of rabbits therein can only be so large. The only important questions on the table are “How many big is big?” and “How large is large?”