Comment by gedmark | Hacker News Reader

Comment by gedmark | Hacker News Reader

Using 50 year old Russian engines was a risky move on Orbital's part, but in fairness the NK-33's are a pretty amazing piece of technology. The US has still yet to build anything matching its performance:

http://en.wikipedia.org/wiki/NK-33

297 seconds of Isp is huge. The Saturn V's F1 engine only had 263 seconds. And the closest we've come since to that level of performance in a Lox/RP-1 engine is the Merlin 1D which will have 282.

They did it by using an oxygen-rich, fully staged combustion cycle. Which means that most of the fluid flowing around the engine and through the turbines of the pumps is super hot pure oxygen at 2000 psi. That's crazy. The US has brought over NK-33's and taken them apart and we still don't quite know how the Russians got it to work.

Of course, performance is only one consideration. All the other things you'd expect like integration costs, reliability of your suppliers, etc. made the Antares a pretty risky bet. But given Orbital's choices of developing their own engine from scratch, buying a hyper-expensive engine from Rocketdyne, or buying a wonder of engineering for cheap from the Russians, buying a bunch of surplus NK-33's wasn't totally crazy.

> but in fairness the NK-33's are a pretty amazing piece of technology

And we wouldn't have any at all if Kuznetsov hadn't ignored a direct order from Moscow to destroy all the engines and tooling, as part of the erasure of the N-1 launcher from official history.

IIRC the OKB knocked them off their inventory as them as 'surplus components' and stored them in a warehouse under plastic sheeting.

Closed cycle has another advantage over open cycle for kerosene - namely, the vibrations in the chamber are more manageable. Which paradoxically can make the engine more reliable.

A suspicion is that the engines didn't quite survive the long storage... indeed they weren't quite designed for that. But we'll wait for results of analysis of the explosion.

"The US has still yet to build anything matching its performance"

The Space Shuttle Main Engines had an Isp of 452s and 500,000 lb of thrust, easily the most efficient rocket engines ever built.

NK-33 is RP/LOX, whereas SSME is LH2/LOX. LH2/LOX cryogenic engines have higher Isp compared to other solid and liquid engines.

ISRO's CE-7.5 LH2/LOX engine[1] has an Isp of 454s.

[1] http://en.wikipedia.org/wiki/CE-7.5

Was anything about cromwellian's statement inaccurate, such that it should have been downvoted? I'm pretty sure I understand everything being discussed here but I don't understand THAT.

I didn't down vote it, but the difference between RP1 and Liquid Hydrogen engine performance is WELL known. Thus his assertion that SSME's are better, although true as comparison of rocket engines in general, is false when comparing LOX/RP1 engines specifically (since the SSME are NOT LOX/RP1), which the grandparent was clearly doing.

Similarly, citing the 850 ISP of the experimental NERVA engines is equally out of context.

I didn't see the LOX/RP1 qualifier when I replied (I was responding to the quoted sentence).

Let me qualify the SSME claim as "highest Isp/Thrust of any widely used engine". Prototypes or engines that haven't flown much IMHO aren't able to make this claim.

The NK-33 may beat a Merlin 1D, but what much do they cost to make and what is the reliability? If you make 100 NK-33s and 100 Merlin 1Ds, what's your total budget, and how many NK-33s will fail compared to the Merlin 1D?

One thing I get with SpaceX, is that their engines are not designed to push the performance envelope. They want them to be cheap and reliable.

Yes, the RS-25/SSMEs aren't "easily the most efficient rocket engines ever built". As noted below, there's NERVA.

And yesterday's launch illustrates why people are kind of nervous about using them as a way to take off from the planet's surface.

But much lower thrust.

shuttle used hydrolox, antares uses kerolox, hardly comparable.

hydrolox engines have vastly superior Isp but come with their own set of limitations.

I should have been clear on the propellant - the SSME is a hydrogen engine. For a lot of reasons, comparing engines of different propellants by using a single performance number alone doesn't make a lot of sense. But if you want to talk about hydrogen engines, the Russians built the RD-0120, which reportedly had a higher Isp and chamber pressure than the SSME...

http://en.wikipedia.org/wiki/RD-0120

Although I don't believe we ever tested it on our own test stands, so it's impossible to know for sure.

By the way you are conflating vacuum Isp with sea level Isp (what I was using). The Sea level Isp of the SSME is 366 s.

The SSME is also a fuel-rich staged combustion cycle. The point I was making above is that the Russians were able to get an ox-rich staged combustion cycle working reliably. That's the key technology breakthrough that we have never duplicated.

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