Benefit of Raptor Engines

Question

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}

Answer 1

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 enter 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, the engine is simply larger than the Merlin. To produce twice the thrust with 20% higher specific impulse, the propellant mass flow rate has to be ~75% higher. The propellants are less dense as well, so the turbopump volume has to increase further.

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.

Answer 2

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.