SpaceX is a go for April 30th: 1st commercial launch to space station

[publiusr]

That crossfeeding stuff worries me about as much as Curiosity. Let's hope they both work fine.

[gturner]

I don't see any major issues that would arise with cross-feeding, and find it hard to believe everyone hasn't been doing it for the past 50 years.

I don't think Musk is going to lose much money pursuing at least first-stage re-usability unless he has a lot of crashes during grasshopper testing. I doubt the landing legs and minor enhancements add much to the cost of the stage, so when he loses one it's not much different than just expending what would normally be dumped in the ocean anyway. Upper stage re-usability will of course be trickier.

If he's successful with both first-stage re-usability and the F9 heavy, I think NASA should carefully consider the idea of mating 2 and 3 F9-Heavies together to give them a fully re-usable first stage that would outperform SLS, where six booster stages seperate and return, then three cores. It would be a shorter stack with less risk and very little added development costs, and the components could still be used individually, keeping their flight rate up by filling in with smaller launches.

[sojourner]

^That would be 81 engines for the first stage. At that point you've gone beyond the benefits of engine out right into the realm of unwieldy.

[YellowSubmarine]

Why is that?

[gturner]

Possibly, or perhaps not. There's a statistical hump you cross as the number of engines gets very large, though I'm not sure it offsets the costs of having the parts count so high.

Since the Merlins can fail without causing a daisy-chain failure (unlike the Soviet N-1), you can think of them as each providing a thrust increment to the overall total. Designs that use much less than the nine on the Falcon can't complete a mission with an early engine failure because each engine's contribution to the total is too great to compensate for. The Falcon is marginal in this regard, with an 11% drop in maximum thrust per engine failure.

As the number of engines gets very large, say a hundred, you can launch with single and double failures without much affect at all, as it would only be a 1 or 2% drop in maximum thrust, well within design margins that have to include engine-to-engine thrust variations. So it's like evaluating the risk that a bad sparkplug will prematurely terminate an airplane flight. Is it a 4-cylinder Cessna or a 112-cylinder B-50?

If you crunch through the numbers on statistical failure rates per engine, versus the expected number of launch failures, launch success rates would start rising again as the number of engines becomes large, because the criteria changes from all engines working to some allowed percentage of engines working.

On a re-usable system, if the expected engine life is comparable to the total number of engines, this gives the huge benefit of allowing you to run the engines to the physical end of their service lives (with some swapping to make sure their ages are evenly distributed) by running each engine till it finally fails. If instead you have to make a very conservative estimate on each engine's remaining reliability, perhaps underrating the service life by a factor of two or three, you end up buying two or three times as many engines as you actually needed just to reduce the possibility of an engine failure.

Another benefit you gain is much faster accumulation of engine reliability data. With the first test launch you get a statistical dataset of 81 burns. With the second test launch the dataset is 162 burns. That's a better data set than we had on the SSME's 50 launches into the Shuttle program. Very early in such a system's life, you could probably stop doing engine tear downs and inspections between launches because you'd have a good handle on the expected failure rates, and the large numbers of engines means you expect small numbers of engine failures as a routine part of operations.

Offsetting all this, of course, is complexity. If you're using ten times as many engines, you had to perform ten times as many engine assembly operations. Of course if the assembly gets vastly more automated because of the bigger production run, that factor might go away. I think one thing that's inhibited the move to large numbers of engines (massive parallelism) is that we build engines largely by hand (though all the machining is automated), and the same crew can build a really big engine or they can build a really small engine in about the same amount of time, so the cost doesn't scale at all linearly with thrust.

ETA: I should dig up my statistics on this. It's pretty easy to get to the point on a hundred-engine system where you almost always expect one engine failure, often two, very rarely three, four is almost unheard of, five won't happen in decades of frequent flight ops, and six or seven is rarer than being struck by an asteroid. It would take eleven to hinder a launch as much as an engine-out on a Falcon-9.

Of course what eventually would get you is metal fatigue, somebody forgetting a wrench, or a guy uploading the wrong version of flight control software.

[sojourner]

gturner wrote:

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Another benefit you gain is much faster accumulation of engine reliability data. With the first test launch you get a statistical dataset of 81 burns. With the second test launch the dataset is 162 burns. That's a better data set than we had on the SSME's 50 launches into the Shuttle program. Very early in such a system's life, you could probably stop doing engine tear downs and inspections between launches because you'd have a good handle on the expected failure rates, and the large numbers of engines means you expect small numbers of engine failures as a routine part of operations.

This is not entirely true. You would get a data set of 81 initial burns. Not the same data set as firing the same engine 81 times. The former would be of limited use for reliability testing of reusables.

The Falcon 9 actually has engine out capability only after the first 30 seconds of launch. Before that the T/W prevents 8 engines from providing enough lift.

It's probably moot anyway. If NASA asked SpaceX fora rocket with that large a payload They would clean sheet it with a larger core diameter and bigger main engine just to avoid the functional nightmare of a frankesteinian 81 engine, 9 core system.

[gturner]

sojourner wrote:

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This is not entirely true. You would get a data set of 81 initial burns. Not the same data set as firing the same engine 81 times. The former would be of limited use for reliability testing of reusables.

Oops. An unstated assumption was that the 81 engine rocket wasn't built from completely new hardware, but assembled from existing re-used Falcon-9 stages or their engines. That's another benefit of the idea over a clean sheet design, because you already have a pretty good idea about the system reliability going in.

The Falcon 9 actually has engine out capability only after the first 30 seconds of launch. Before that the T/W prevents 8 engines from providing enough lift.

Yep. It's close! Just a few more engines...

It's probably moot anyway. If NASA asked SpaceX fora rocket with that large a payload They would clean sheet it with a larger core diameter and bigger main engine just to avoid the functional nightmare of a frankesteinian 81 engine, 9 core system.

Very true, and even Elon is doing that with his Merlin II design, with a thrust of 1.9 million lbf. It's like nobody will get to the other side of that statistical hump in the failure rate.

Still, the idea is a pretty trivial one. To multiply the payload by X, use X times as many of everything - in parallel. And the same system covers all configurations of (X - n), so if X is large, you've got a good granuarity to match launchers to payloads without designing anything new. Of course the logic only pays off if all those stages fly back for re-use, otherwise you're just throwing away X times as many stages.

[sojourner]

gturner wrote:

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Very true, and even Elon is doing that with his Merlin II design, with a thrust of 1.9 million lbf.

SpaceX is not working on such an engine.

[gturner]

sojourner wrote:

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SpaceX is not working on such an engine.

Did they shelve it? They had such good presentations about it, including budget, cost, and timelines.

[sojourner]

Actually, no, SpaceX had none of those. Merlin 2 was nothing more than speculation.

[gturner]

I'm pretty sure they had presentations on it. Here's one of them.

http://images.spaceref.com/news/2010...Propulsion.pdf

Elon Musk much later said that the Merlin 2 engine was a key element of any effort they make toward super-heavy launch vehicles, but that the particular layouts in the briefing above were just ideas being tossed around. I haven't heard much about it since 2010, but they've been very busy lately on their Falcon 9 missions and the Dragon.

[sojourner]

Yeah, it never went beyond a couple slide shows like that. That was basically a sales pitch to NASA to build an HLV (the Falcon X). Elon later pretty much disavowed it.

[gturner]

Putting that off was a good decision. It will be a while before the Falcon 9 program pays off its development costs, and spending an extra billion on R&D for an engine with no immediate market demand is risky and unnecessary. It could also put them too obviously in competition with the SLS, with potential repercussions to their CCDev program. If private industry was actively building an Apollo+ class rocket, some Senators would demand to know why NASA is duplicating the effort at vastly higher costs, and that wouldn't be a bit pleasant.

I'm sure some Merlin 2 designing still goes on unofficially, because rocket engineers can't stop playing around with their own ideas, even if they have to limit their efforts to break times, and because Elon knows he'll eventually need an engine that big if he's going to move to Mars.

[sojourner]

Oh, SpaceX still has the development team, and now that Merlin 1-D is pretty much in the bag, I am sure they are keeping busy. The rumor now is that they are working on a methane engine.

[gturner]

I'd read they were interested in running a Merlin on methane for ISRU on the moon or Mars. Other than the ease of procurement, I don't think methane is all that much better than RP-1 (about 3% better ISP?) while presenting the cooling and leak hazards of LH2 (though without the horrible density penalty).

It's possible they want an excuse to say "fracking fuel" over a deep space radio network.

I wonder what misplaced name they'll give to the new engine? Their rocket is already confusing enough.

Merlin would be the guy in launch control in a funny vest who waves a wand at T minus zero, not the thing he commands to breathe fire and fly. A falcon is much smaller than a dragon, and a dragon belches smoke and fire, so logically the little capsule should be the Falcon and the booster should be the Dragon, because a little bird can ride on a dragon by a dragon can't ride on the little bird. The crewed capsule is called Dragon Rider, which should be the term for the people who ride in the capsule, not the capsule itself. And the second stage LH2 engine is called Raptor, but a falcon is species of raptor, piling on more confusion.

Perhaps all the nonsense started with PayPal, where the people you're paying are probably not your pals. If they were, it would let you pay with beer. If SpaceX was trying to baffle the Russians it might make some sense, but then the Air Force. who should've tried baffling the Soviets, ran all the X plane projects in perfect numeric sequence without a gap. If they'd have left gaps it would've caused most KGB agents to dig through trash looking for non-existent fighter prototypes. (I used to think the sequence jumped from the mid-90's (XF-94) directly to the Century Series, but sure enough, the sequence continued with upgrade projects that were dropped or renamed back to their parent designs, the XF-98 and XF-99 were missiles (Falcon and Bomarc), and XF-110 was reserved for the F-4 Phantom.).