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Lots of articles out about how Orbital Sciences is talking with three vendors for a replacement for the NK-33/AJ26 engine pair currently used on the Antares booster.

An article on SpaceNews notes three vendors and says:

Thompson told investors on April 17 that Orbital is weighing offers from three companies for the Antares first-stage engine. Two are from Russia, including the current supplier, and one is from the United States.

and also:

Orbital Sciences Corp.’s contract with startup Stratolaunch Systems on a rocket to be launched from beneath a large aircraft will produce an enhanced version of Orbital’s Antares rocket capable of handling 15-25 percent more payload, depending on the orbit, Orbital Chief Executive David W. Thompson said April 23.

So, current vendor seems to be Kuznetsov and it looks like they are considering reopening NK-33 production.

I assume the other Russian company would have to be whoever makes the RD-180. Asked in a question " How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares? it seems that the RD-180, with slightly higher thrust would likely be used to increase the payload, per my second quote snippet.

Who could be the US company? What RP1-LOX engine is out there, from a US manufacturer, since Orbital does not have time for a new engine development project, if they intend to bid on the second Cargo Resupply contract for NASA. They need engines sooner than that. Knowing Orbital, maybe it is ATK offering a solid booster (Minotaur writ large?)

So what could the three engines be? I guess new build NK-33's made in Russia, RD-180, and some unknown US engine.

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In Dec 2014, Orbital selected the Russian Energomash RD-181, which is basically an export version of the RD-191 used in the Angara booster.

This is also a LOX/kerosene engine, derived from the RD-170. The RD-170 is a 1.6 MLbs thrust engine used on Zenit boosters (which were initially strap on boosters for Energia) that has 4 thrust chambers. The two thrust chamber and about 800 Klbs thrust version is known as the RD-180 of political fame and Atlas V usage.

The single thrust chamber version cuts thrust by half again to about 420 Klbs thrust and is being used on the Angara common core booster as the RD-191. The export version of this, RD-181 will be used in a set of two on the upgraded Antares.

They will get a payload boost out of the upgrade, and access to an in-production engine that should be manufactured at a reasonably high rate. The Russians are testing an Angara 5 configuration in Dec 2014 that is meant to replace the Proton booster. The Angara family is meant to replace almost all Russian boosters, so the engine should be in production, and reasonably high volume production, for the forseeable future.

The choice of a Russian booster in a time of political turmoil with Russian behavior and congressional disapproval of the use of RD-180 engines on Atlas V from the same vendor makes this either a very brave or very foolish choice. Time will tell.

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I was looking for possible answers for an US-sourced engine, and was going to float the RS-27A as a possibility (though it's been out of production for over a decade) -- you'd probably need a cluster of 4 to substitute for a pair of AJ-26.

Then I saw this reference to the Aerojet Rocketdyne AR-1 proposal. At 2200 kN, a pair of them would be up in the RD-180 class, and it looks like they're expecting to use them in pairs. And lo! One article mentions it in the Antares context!

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The Falcon 9 uses an RP1-LOX engine developed by SpaceX called the Merlin. The thrust is significantly lower (using a Merlin-1D) than a RD-180 (155,000 lb*f vac. vs 933,400 lb*f vac.), but they are smaller (2.92m high x 1.67m diameter x 630Kg vs 3.56m high x 3.15m diameter x 5,480Kg) and the Falcon 9 uses nine of them.

Eye balling the diameters you could fit about 4 Merlins per RD-180, still significantly less thrust, but 10,000Kg less.

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    $\begingroup$ I will not accept this, since Orbital and SpaceX are literally direct competitiors, and it seems utterly improbable that SpaceX would bail out their direct competitor. $\endgroup$ – geoffc Apr 24 '14 at 22:04
  • $\begingroup$ While you would think this never happens, it occasionally does. There are only 3 major manufactures of LCD panels (Samsung, LG, Sharp), but countless TV brands. Their competitors literally buy the panels from them. Other industries have similar things that go on. That said, I'm all for hearing alternatives and my answer may not be the best when all is said and done, but it is an answer :) General Dynamics is the only other company I can think of with experience in the RP1-LOX field, but as you know I'm sure, they use RD-180's. $\endgroup$ – Jack Apr 25 '14 at 13:09
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    $\begingroup$ Fair enough. I think your analogy to LCD panels is a bit strained, but I concede stranger things have happened. $\endgroup$ – geoffc Apr 25 '14 at 13:36
  • $\begingroup$ It probably isn't correct, but it's worth noting :) Let's see what others add, maybe someone knows first hand. Oh, the curious note about the LCDs is that it is almost directly applicable. The reason other TV companies can make money is because there are 3 major manufactures they have to keep prices reasonable... In this case there are 3 vendors and SpaceX could price Orbital out or make a little profit off of it, either way Orbital will buy the engines. $\endgroup$ – Jack Apr 25 '14 at 13:37

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