With all the talk of re-engining Antares, many possibilities arise. Solids, new liquid engine, existing Russian engines (RD-180, RD-190), etc.

Focussing on the RD-1xx family though, an RD-180 is a bit overpowered for Antares, but more thrust is rarely a problem, and you need a stage redesign anyway, so roll with it, and boost your performance.

But there was some unsourced speculation that maybe two RD-190's were possible, if ULA managed to lock them out of the RD-180 source.

The RD-170 was a four chamber, but one turbopump engine. Is it one or 4 engines? 1 because although it has four 'bells' it only has one set of turbo-machinery. (Used on Zenit as Sea Launch, and as strap on boosters for Energia)

Split in half, is the RD-180 with 2 bells, and one set of turbomachinery. Used on Atlas V (and Atlas III (now retired)).

Split in half again, is the RD-190 family, with a single bell and single set of turbomachinery. Used on Angara family in Russia.

The thrust scales mostly linerally with the RD-190 having about 1/4 the thrust of the RD-170.

All in all, an immenseley successful family of engines, well done to the designers!

So on a re-engined Antares, would there be a benefit or cost, to using two RD-190s vs a single RD-180.

  • $\begingroup$ Your question, if I understand correctly, is what the benefit of using a single set of turbomachinery for several thrust chambers is. Correct? $\endgroup$ Nov 3 '14 at 15:32
  • $\begingroup$ @Rikki-Tikki-Tavi Nope. It is much more specific. There are two possible configurations 1 RD-180 vs 2 RD-190's that are vaguely equal otherwise. WHat are the benefits/costs to each choice? One real one is, if ULA has legally locked up rights to the RD-180, then the RD-190 pair makes much more sense. $\endgroup$
    – geoffc
    Nov 3 '14 at 18:16
  • $\begingroup$ Russian design philosophy sees the turbomachinery as the main part of the rocket, with the combustion chambers as more of an "auxiliary component". I would therefore look for a reason in the design of the turbomachinery, and not so much on the system level. My money is on the specific speed of the pumps and theoretical spouting velocity of the turbine(s), but I don't have time to run the numbers right now. $\endgroup$ Nov 4 '14 at 1:22

Generally speaking, you want as few moving parts as possible, unless the extra parts add a useful redundancy. More moving parts = more things that can go wrong.
Some examples: intercontinental aircraft used to require 4 engines, because that meant they could lose one and still make it to the other side of the ocean. These days jet engines are reliable enough that this requirement has been abandoned. In rocketry, the Saturn V and Falcon 9 have engine-out capability: they can lose an engine and still get to orbit.
So for Antares the big question would be: if one engine fails, would the remaining engine have enough thrust to get the rocket to orbit? If not, then there's no point in having two engines, and you might as well go with the less complex and lighter single-engine option.
I suspect the answer to that question is 'yes, for the second part of the trajectory'. Initially the rocket will be too heavy, but after some fuel has been burned there comes a point where one engine is enough.


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