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I know that the Raptor engines are significantly different from the Merlin 1-D's since they use liquid methane and are cryogenic. However, what exactly enables the Raptor engines to produce a lot more thrust than the Merlin's and how could they be compared with other engines such as the RD-180?

Also does SpaceX save money by manufacturing their engines themselves as compared from buying them elsewhere?

\begin{array}{lrrrr} &\text{Thrust}&\text{(kN)}&\text{Isp}&\text{(sec)}\\ & Vac. & SL & Vac. & SL\\ Merlin & 914 & 845 & 311 & 282\\ Raptor & 1900 &1700 & 375 & 356 \end{array}

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  • $\begingroup$ @NathanTuggy that's quite snazzy! How did you do that? $\endgroup$ – uhoh Jul 26 '18 at 8:44
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    $\begingroup$ @uhoh: A distressing amount of time in the editor window and a lot of cross-referencing with Math SE's MathJax tutorial and links from there. But basically, & to separate columns, \` for new rows, \begin{array}{lrrrr}` for five columns, one left-aligned, four right-aligned, and the rest is just details. $\endgroup$ – Nathan Tuggy Jul 26 '18 at 9:13
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    $\begingroup$ It's hard to say what's cheaper. When you're buying on free market, you can sometimes strike a deal as good as Aerojet who bought 36 mothballed NK-33 for $1.1 million each, which as a bargain price in the 1990s. On the other hand, it ultimately cost them money when the original supply dried out and they had to retool to RD-181 and then finally Russia declined to sell more. $\endgroup$ – Agent_L Jul 26 '18 at 10:34
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A few different factors contribute to the Raptor's higher thrust:

  • The specific impulse -- force delivered per mass of propellant consumed -- of methane-LOX combustion is generally higher than kerosene-LOX, because the exhaust is composed of simpler and lighter molecules;

  • The Raptor uses the staged combustion cycle, where the hot partially-combusted gases which drive the engine's turbopump then enters the main combustion chamber, which is more efficient than the gas generator cycle used by the Merlin, but more complicated to engineer;

  • The stated specific impulse figures for the Raptor are extremely high, even for staged combustion, so the chamber pressure is likely to be significantly higher —- 250 bar or higher, as compared with about 100 bar for the Merlin.

  • Finally, I think the engine must simply be larger than the Merlin to produce twice the thrust with 20% more specific impulse; the propellant mass flow rate has to be ~75% higher for the math to work out, but the propellants are slightly less dense to start with.

The performance of the Raptor is not too dissimilar to the RD-191 -- that is, half of an RD-180. That engine burns kerosene/LOX in a staged combustion cycle at a 250-bar chamber pressure.

Developing the engine in-house decreases per-unit costs by an enormous amount, of course, but it introduces also-enormous development costs. If development goes smoothly and a large number of engines are produced, the gamble will be worthwhile. However, the Raptor is a much more ambitious design than the Merlin, and it remains to be seen if it can achieve its targets. In-house development also generally makes it quicker and easier to make incremental changes to the engine design.

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    $\begingroup$ Chamber pressure is given on wikipedia as 250 bar initially, with an expectation of getting up to 300 later. It's not much larger than a Merlin because they want to use the same factories, transport routes, etc., so the chamber pressure seems to be the main consideration. $\endgroup$ – Steve Linton Jul 26 '18 at 9:41
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I can answer the second question — engines are by and large fuel specific. There's plenty of complicated stuff that goes on in the combustion and the transition to supersonic flow that means you can't just exchange one fuel for another and expect it to be high-performing.

As far as I know, there are no pre-existing methalox-burning engines that SpaceX could have bought.

Methalox was chosen for a number of reasons, one of which is that you can reclaim methane while on Mars through the Sabatier Process.

Engine throttleability may also be an issue. Unlike what you might think from playing Kerbal Space Program, rocket engines are by and large not designed to be throttleable. The throttleability of the Merlin engines (down to about $67\%$ off the top of my head) is key for the landing and reuse of the Falcon $9$ booster. It may be less of an issue for the BFR stage $1$ booster, as it will have $31$ Raptor engines (as opposed to $9$ Merlin engines) that will allow it to throttle more deeply by virtue to having more engines to cut off.

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    $\begingroup$ Might be more accurate to say that rocket engines don't throttle much. Most liquid-fuel engines do throttle at least a little, with the notable exception of at least the F-1 (because of the difficulty making combustion stable in an engine that size). But a range much beyond maybe 85-100% isn't terribly common. $\endgroup$ – Nathan Tuggy Jul 26 '18 at 9:18

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