# Tag Info

81

Methane has the benefit of being easier to store than hydrogen. Mostly passive cooling can suffice to keep it cryogenic, whereas hydrogen needs active cooling, and will still vent over time. Which makes Methane much closer to 'storable' than hydrogen can be. This would make it useful for deep space missions, with long mission durations. Methane is less ...

50

Indeed it is due to contaminants — dust! — in the plume. Did you notice all the dust being kicked up around the landing site? Some of that dust cloud, a very small and low-dust-density part of it (so it's really hard to see), flows upward, then back toward the rocket, and then is entrained in the rocket's exhaust plume. It's just like the toroidal flow ...

49

While an ideal engine would just ingest fuel and oxidizer and produce exhaust gas real world engines will have some combination of regenerative cooling, film cooling, turbine exhaust, hydraulic power, ignition systems, pressure sensing, tank pressurization systems, drain/purge/test lines, and electrical connections that just look like pipes for heat ...

45

Methane (CH4) and RP-1 are roughly equivalent in realizable performance. As previously mentioned by other posters, CH4 has slightly higher impulse – about 370 s in vacuum vs the 360 s – at the same chamber pressure of 7 MPa. But, this is counterbalanced by its lower bulk density of about 830 kg/m3 vs about 1030 kg/m3. Bulk Density is the density of the ...

40

Or am I wrong and have there been attempts to build a methane rocket in the past? Well, if there were, I figured that John D. Clark's famous book Ignition! (1972, free online copy) would be the place to find it. And indeed, the index at the end of the book has a convenient entry for "methane, usefulness of" pointing to pages 8 and 191. On page 8, Clark ...

38

The three main competitors for liquid fuel choices to date have been: Hypergolics - easiest to get started with Kerosene/LOX - Good thrust, low performance, but dense LH/LOX - Best performance, hardest to do So if you were starting a new space program with a clean sheet design, LOX/LH is out of the question, too hard. Hypergolics are well understood, ...

29

Elon Musk posted a tweet on this topic: The Raptors were well below max thrust or the ship would have blown through the altitude limit. As we hit min throttle point, an engine would shut off. The issue was that three Raptors at full thrust would have gone too high, too fast. They needed to control speed and altitude to stay inside their allowed flight ...

26

A couple days ago I watched this video which does a pretty good job of explaining the whole test. Basically: Take-off needed all three engines to reach enough altitude, so all three engines were initially fired up. As the ship was climbing up, higher altitude and lower weight due to fuel consumption meant less thrust was needed to reach the apogee and turn ...

23

Logistically, methane can be easier to work with than hydrogen. Methane's boiling point is about 110K, compared to hydrogen's 20K. This means that both fuel and oxidizer lines can be purged with gaseous nitrogen. Liquid hydrogen lines can only be purged with helium, as hydrogen's boiling point is below the melting point of other inert gases.

20

The Raptor is a full-flow staged combustion methalox engine. There is a lot of new technology in that sentence. Staged Combustion Fuel and/or oxidizer is ignited to run a turbine that spins a turbopump, increasing the feed pressure into the combustion chamber. The gases in the turbine are then fed into the combustion chamber instead of being discarded. ...

19

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 ...

18

The bright yellow light from RP-1 burning engines is from carbon soot built up from already-complex carbon molecules in kerosene. However the single-carbon methane molecules quickly oxidize to CO2 and H2O and are not conducive to soot growth. When we cook using natural gas (methane) we rarely see soot produced, and so the flame is blue mostly due to diatomic ...

17

I wouldn't draw any conclusions based on diagrams alone, SSME is very well-known to the public, so we have a more detailed diagram. That doesn't mean that the actual engine is more or less complex, because many things are omitted from diagrams. To prove my point, here is a newer Raptor diagram drawn by a propulsion engineer Elisei Maslov, who's ...

16

Another disadvantage to hydrogen is that it requires advanced metallurgy to prevent hydrogen embrittlement, where more common alloys tend to become prone to fracture and fatigue in high hydrogen environments.

15

This question was actually asked and answered in the announcement itself. Elon Musk: In order to minimize the development risk and cost we decided to commonize the engine between the booster and the ship. A future upgrade path for BFS would be to have a vacuum-optimized nozzle. […] Where you see that cargo around the perimeter, you can actually switch out ...

14

A lot of questions here, let's tackle these two first: 4.And last but not least, what's SpaceX's solution for the oxygen-rich environment at 377bar, 748K injector and 546bar, 811K pre-burner? 2.The Raptor's oxygen pump sits directly on top of the main combustion chamber, while the SSME's two pumps are on the opposite sides of the main combustion chamber. ...

12

1) If oxidizer and fuel are both cryogenic, why is it only the fuel that gets pre-heated? I assume you are talking about the H2 that flows through the nozzle? You have it a bit backwards. This is to cool the nozzle, not heat the hydrogen (although both happen of course) and H2 is a great heat transfer agent. 2) And are there additional propellant feeds ...

11

Scott Manley comments in his video that it could be part of the engine has started to burn or erode, introducing new elements otherwise very pure rocket exhaust. Evidence for this (as opposed to entrained dust) is that the yellow colour starts suddenly, late in the flight, and it starts right up in the engine. If it was dust, you'd expect it to start at the ...

10

Methane, while a high ISP fuel that is relatively easy to work with, does have it's share of difficulties. The hardest thing about working with methane is it requires an ignition source, as methane and oxygen don't burn spontaneously. In fact, there is a paper that directly compares Methane and RP-1 as rocket fuel. The major findings include: The ISP of ...

9

In the past, the options were the optimal high tech option (Hydrogen) or the commodity option (RP1, basically kerosene.) Methane is certainly much easier to handle than hydrogen, with a boiling point slightly higher than oxygen, nowhere near the -259C (only 14K above absolute zero) of hydrogen. Its energy per unit liquid volume is better than hydrogen too. ...

9

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 ...

9

The engines are all now sea level engines, they removed the vacuum ones from version 1.0 to reduce complexity. They did arrange things such that they can add in more engines if required for the future. The bottom area near the engine nozzles now is for storage.

8

Let's turn the question on its head and see what exhaust velocity we need to if Titan's entire (mostly nitrogen) atmosphere were used as a propellant. $\Delta v = v_e log(m_i / m_f)$ Wikipedia tells us that the atmosphere of Titan is about 1.19 times as massive as that of Earth so we get about 6.13e18 kg of atmosphere (propellant) in a total mass of about ...

7

Let's do a Fermi estimate: Rockets bring about 2-5% of their start mass to orbital velocity. To cancel out Titan's orbital velocity, you're looking at two orders of magnitude more fuel and oxidizer than Titan's mass. Earth's atmosphere weighs $10^{18}$ kg, or 1/200,000 of Earth's total mass. Titan's is 1.5 times as dense, so if Titan's atmosphere were ...

7

Aside from being, essentially LNG, Methane burns Cleanly. Kerosene produces coking and polymerization within the engine. This does not matter if you use the engine only a few times. If you want to reuse it 40-50 times or more (like Spacex), you need to clean a Kerosene engine every few uses and this gets expensive if you have to dismantle it. Methane is far ...

7

SpaceX is not shooting at the moon, SpaceX is shooting at Mars. Logistically, I’m not sure there are viable choices besides methane / LOX and Hydrazine / Tetroxide. The return shot requires fuel stored for an unknown time, which means the default conditions are chilly on Mars. RP-1 is a rock hard solid requiring complex heating to liquify it and LH2 is high ...

7

A good question. In pre-EELV studies, NASA and the U.S. Air Force looked at LOX/methane. EELV resulted in the LOX/kerosene Atlas V and the LOX/hydrogen Delta IV. At the 4th International Conference on Launcher Technology in 2002, Burkhardt et al. compared a reusable LOX/kerosene launch vehicle using the RD-180 type engine from the Atlas V with a LOX/...

7

Methane would allow for around 380 second specific impulse (~3.8km/s exhaust velocity), depending on the chamber pressure, expansion ratio, and other design parameters for the engine, while LH2/LOX engines have demonstrated ~450 second specific impulse (~4.5km/s exhaust velocity). Despite this lower efficiency though, methane has a couple of major ...

6

Excellent question. But twinned to that is also the question of the thrust level which is more than three times greater than the current Merlin engine used in the second stage of the Falcon 9. I admit this is pure speculation. But based on prior SpaceX practices with the Merlin engine, I believe the Raptor might be intended for use in larger launch ...

6

The bonus cargo capacity that two different engine types would allow wasn't worth the extra development time, money, and complexity for the initial version. It's designed that in later versions vacuum-optimized engines can be added.

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