Space rocket engines performance and technology: SSME vs. RD-0120

Question

Between this two LH2/LOX rocket engines the SSME(RS-25 different) of the Space Shuttle rocket and RD-0120 of the Energia rocket, which one has higher performance and better technology to launch space rockets in low earth orbit?

Answer 1

The two big performance considerations for a liquid fueled engine are thrust and specific impulse. On these criteria, the two engines are very comparable.

The SSME has a bit more thrust: 1860kN at sea level, increasing to 2279kN in vacuum, versus RD-0120's 1526kN to 1961kN -- a 22% difference at sea level and 16% in vacuum.

The other major factor is specific impulse, which is a measure of fuel efficiency. The SSME's specific impulse is 366 seconds at sea level, increasing to 452 seconds in vacuum; the RD-0120's goes from 353 seconds to 455 seconds. RD-0120 is spending 3% more fuel per unit of thrust during takeoff.

So, once out of atmosphere, the Russian engine is almost 1% more efficient; in all other cases, the SSME outperforms it.

If you swapped the engines between Energia and the shuttle -- keeping the number of engines the same, so 3 RD-0120 on the shuttle and 4 SSME on the Energia -- the re-engined Energia would be able to lift a little more payload, and the shuttle could only make it to orbit with a reduced payload -- about 15 tons instead of 25.

The shuttle would still be able to take off despite the reduction in thrust, because at liftoff the solid rocket boosters are doing most of the work. It would ascend a little more slowly at the start, which would cost a little more fuel due to gravity losses, but it turns out that the shuttle throttles its engines down substantially over the course of the ascent to reduce aerodynamic loads and peak g-forces, so the RD-0120s would have more than enough power during much of the ascent.

Its slight edge in performance doesn't necessarily make the SSME "better technology". It's one of the most complicated and expensive rocket engines ever flown. RD-0120 gives you more bang for the buck (or maybe "more rumble for the ruble").

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Here's a method to roughly estimate the payload hit. Ignore the SRBs for the moment and consider a 68 ton orbiter, 25 ton payload, 30 ton external tank, and 730 tons of propellant: 853 tons total, 123 tons with propellant expended. Apply the rocket equation with the 366 second sea level SSME figure:

$$∆v = 366s \cdot 9.81m/s^2 \cdot \ln \frac{853}{123} = 6953 m/s$$

Now determine the change in payload mass (∆m) needed to get the same result from the RD-0120's specific impulse:

$$6953 m/s = 353s \cdot 9.81m/s^2 \cdot \ln \frac{853+∆m}{123+∆m}$$

$$ 7.447 = \frac{853+∆m}{123+∆m} $$

Rearranging and solving, we get:

$$ ∆m = -9.77 $$

So if the sea level specific impulse held throughout the flight, and the SRBs weren't involved, shaving 9.77 tons off the payload would produce the same ∆v. This is a worst case because we expect the specific impulse of the RD-0120 to "catch up" with and even exceed that of the SSME over the course of the flight, so the actual payload hit should be smaller -- but we'll also lose some ∆v to gravity during the slightly slower ascent, so 10 tons is a reasonable estimate.

You can run similar calculations on the Energia side to work out the additional payload mass it could lift if equipped with SSMEs, but I can't find clear numbers for its dry, wet, and payload masses; probably it's a fairly similar delta.