How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?

Question

Orbital Sciences, which uses 2 Russian NK-33 engines on its Antares booster, has been suing Lockheed Martin/ULA over anti-trust violations, since they were not allowing Orbital to buy RD-180 engines, which power the Atlas V booster.

The NK-33 is the engine built in the 1960's for the N-1 moon rocket, with the Soyuz-2-1v and Orbital Antares booster using up the stock that remains from the 1960's. (Which is pretty interesting in itself.).

But the NK-33/AJ-26 is about a 330Klb (Sea level) thrust engine, and with two that is around 660Klbs thrust.

The RD-180 is a 860Klb thrust engine, which is quite a growth from 660Klbs, and I can imagine this will require a substantial redesign.

This is NOT a drop in replacement, and seems like it would result in a functionally new booster.

On the flip side, Yuzhnoye Design Bureau is building the first stage for Orbital, and they also build the Zenit first stage, which uses the RD-170 engine, of which the RD-180 is a half-size derivative. (RD-170 was also used on the strap-on boosters for the Energia rocket)

But how is one a replacement for the other? Sure, same fuels, RP1/LOX, but beyond that a fairly big change in design.

Answer 1

The ultimate answer depends on the structural margins used in the construction of the first stage. It's possible that it was overdesigned with this switch in mind, in which case you could simply replace the engines.

If not, as osgx points out, the RD-180 is throttleable, so it could simply be throttled down to 76% to reach 660 klbf. At it's lowest range, however, (47%) the thrust is 404 klbf, but the NK-33 is only throttleable down to 55% [1], which translates to 363 klbf from two of them.

The reason that's important is because it says here [2] that the Antares does throttle down towards the end of the flight to maintain structural limits on the rocket. However, it's unclear how far it throttles down. If it only throttled down to 61%, that matches the lower end of the RD-180, so a structural redesign of the first stage would not be necessary.

Even if it required a structural redesign of the rocket, it might still be a reasonable replacement because, as you note, the engine is no longer in production. If they kept using it, they would have to pay to restart the production line, which could be a lot more costly than a structural redesign of the rocket. The RD-180 is in production for the Atlas V, as you also mentioned.

Another question is gimbaling, which is the process of moving the nozzles slightly in order to correct the course of the rocket. This isn't really a problem, since the NK-33 can gimbal up to 6 degrees [3], and the each of the RD-180's nozzles can gimbal up to 8 degrees [4]. As osgx noted, a single RD-180 has two nozzles, and each NK-33 has one, so the nozzle count is fine. If the RD-180 had more limits on its gimbaling, Orbital would need a new solution for steering the rocket on its way up.

In any case, Orbital has time to come up with a solution. They purchased enough engines to complete their obligations to NASA under their Commercial Resupply Services agreement [5], and to date no one else has signed up to use the Antares for their payload [6]

1) http://www.spaceandtech.com/spacedata/engines/nk33_specs.shtml

2) http://www.spaceflight101.com/antares-a-one-mission-updates.html

3) http://spaceflightnow.com/antares/cots1/countdowntimeline.html

4) http://www.spaceflightnow.com/atlas/ac201/000514rd180.html

5) http://www.spaceflightnow.com/news/n1403/20ulaorbital/#.Uy-ue_ldV8E

6) http://en.wikipedia.org/wiki/Antares_(rocket)#Missions