Raptor, Methane and FFSC

[MathiLpHD]

Well, as far as i know, they use active cathodic protection on cars. That should normally do it because active protection is better than passive protection. And if you have a look at https://en.wikipedia.org/wiki/Corrosion#High-temperature_corrosion: "The products of high-temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperatures in hostile conditions. Such high-temperature corrosion products, in the form of compacted oxide layer glazes, prevent or reduce wear during high-temperature sliding contact of metallic (or metallic and ceramic) surfaces."

[RizzoTheRat]

@shynung, Well the materials and techniques are there to deal with it.  I'm assuming the main benefit of being very rich or very lean is to keep the temperatures down, but then as stated you potentially have a coking or oxidation problem.  Fuels that don't coke when burned rich sound ideal, but on the overall cost of the rocket the use of some expensive superalloys (some Nickel/ based "superalloys" keep a significant proportion of their tensile strength at around 80% of their melting temperature, compared to steels where it's usually all over by about 50%) who's performance is well understood doesn't seem like a huge cost.

Running these things at "low temperatures" still means running them pretty hot, the power requirements are insane.  The fuel pump on a Saturn V's F1 engine was about 55,000bhp, which is about the same power as the pair of WR-21's in a Type 45 destroyer!

 

I don't really see the benefit of using the twin turbine technique shown in your third diagram above though.  Rich potentially means coking problems, lean means oxidation problems, what benefit is there in having 2 problems to solve?

Edited March 8, 2016 by RizzoTheRat

[shynung]

10 minutes ago, RizzoTheRat said:

I don't really see the benefit of using the twin turbine technique shown in your third diagram above though.  Rich potentially means coking problems, lean means oxidation problems, what benefit is there in having 2 problems to solve?

It's Wikipedia's diagrams, not mine. But I'll answer anyway.:)

It's not much of a benefit as it is a necessity. Different propellant densities require different pump designs, which may not be easily coupled together (i.e. needing different RPM). Certain propellant mixes, like RP-1/LOX, can use a shared-turbine pump like the first and second diagrams because their densities are pretty close, while ultra-low density propellants like LH2 would necessitate it's own turbopump.

[MathiLpHD]

Because you get higher thrust, lower stress on each turbine, higher isp and eventually higher thrust-to-weight-ratio. The russian solved the oxidation problem in the 60s or 70s and with using methane you won't have to deal with coking. Compared to a gas generator you should be able to get ~40s more impulse with FFSC and compared to a ORSC it gains ~10s. So it should be a pretty big improvement.

Edited March 8, 2016 by MathiLpHD

[MathiLpHD]

Now that we know most stats of the Raptor engine, what do you think?

Do you think there will be multiple versions of the Raptor like the Merlin engine? So that they increase the stats over time?

And what are your estimations for the weight of the engine? I mean, the core has the same size as the Merlin engines core but the Raptor has a higher nozzle ratio so it shouldn't have MUCH more weight. The burning chamber will be heavier due to the 3 times higher pressure compared to the Merlin engine...

I would estimate the weight to about 1,5* to 2*Merlin Engine so ~750kg to ~1000kg, maybe even more. But i think it should get over a TWR of 200 and i think it's highly possible that it has a TWR of 300 or even 400 (3050kN/(0,750t*10m/s^2) > 400).

[K^2]

15 hours ago, MathiLpHD said:

Do you think there will be multiple versions of the Raptor like the Merlin engine? So that they increase the stats over time?

And what are your estimations for the weight of the engine? I mean, the core has the same size as the Merlin engines core but the Raptor has a higher nozzle ratio so it shouldn't have MUCH more weight. The burning chamber will be heavier due to the 3 times higher pressure compared to the Merlin engine...

The turbo pump unit is also going to be heavier, as well as all of the plumbing. And we know that at very least there will be the short bell and full bell versions of Raptor. But I think they are going to try to keep variety to a minimum and try to leverage economy of scale for these. That's why it's Raptors all the way down.

Scot Manley did some videos that are relevant to the Raptor and some of the discussions that took place earlier in the thread. It won't be anything new for people who already did their homework on liquid fuel rockets, but for anyone wondering about the different sizes of nozzle bells or turbo pump configurations, this could be a very simple way to learn about it.

Nozzles

Full Flow Staged Combustion

[Superluminaut]

I have an unrelated related question. Why use methane for the booster? It never leaves earth. As I understood it the driving factor for methane propellant was the reduced earth-mars transit mass due to it's production on mars. What advantage does methane have in a suborbital booster?

[blowfish]

36 minutes ago, Superluminaut said:

I have an unrelated related question. Why use methane for the booster? It never leaves earth. As I understood it the driving factor for methane propellant was the reduced earth-mars transit mass due to it's production on mars. What advantage does methane have in a suborbital booster?

There are a few reasons they might have in mind

- Methalox has a slightly higher specific impulse for the same changer pressure

- You can't do FFSC with kerosene

- They want to use as much of the same technology for the lower and upper stages as possible.  The two engine variants will have different nozzles but all the same tuebomachinery.

- Methane might be somewhat less taxing on the engine in the long term compared to kerosene (or hydrogen), however methane hasn't been studied much in engines.

[Rakaydos]

Also, methane is cheap. Fuel might be pennies to the dollar for current rockets, but it's also the 1 price he cant reduce with fast cadence reusability, so the cheaper the rocket, the more the fuel costs matter.

[DerekL1963]

10 minutes ago, Rakaydos said:

Also, methane is cheap. Fuel might be pennies to the dollar for current rockets, but it's also the 1 price he cant reduce with fast cadence reusability, so the cheaper the rocket, the more the fuel costs matter.

None of the rockets SpaceX has on the drawing board is within a couple orders of magnitude cheap enough for fuel costs to rise significantly out of the noise.

[K^2]

50 minutes ago, DerekL1963 said:

None of the rockets SpaceX has on the drawing board is within a couple orders of magnitude cheap enough for fuel costs to rise significantly out of the noise.

Well, no, not with current fuel prices. But you have to also keep in mind that Elon Musk is a founder and CEO of yet another high-tech company, Tesla Motors. A company founded on a principle that fossil fuels have had their time, and we can and should be moving on to better things. The future that Tesla Motors envisions has no need for massive global scale oil extraction and refining. A world where kerosene is actually a very expensive fuel. Whether you think we can get there within the time frame set for ITS or not is beside the point. Fact remains that SpaceX acting as if kerlox is a good idea in the long run would be undermining that. So SpaceX is building the ITS as if kerosene is something that might need to be synthesized whether you are on Mars or Earth. And if you are going to synthesize your fuel, methane is by far the cheaper of the two.

If SpaceX ends up using the same production pipeline for methane on Earth as it's planning to use on Mars, it can also be killing two more birds with that stone. First, they get to test all of their systems extensively, and see first hand all the little things that can go wrong so that they can make sure this doesn't cause problems on Mars. Second, provided that they manage to source all their energy from renewables or nuclear, they get to claim to have environmentally friendly rockets. After all, net carbon footprint of an ITS launch is actually going to be negative in that case. All the carbon for fuel is extracted from atmospheric CO2, and only some of it is burned in the atmosphere, so they'd be ferrying CO2 off the planet in a way.

Of course, for the time being, it's way cheaper to extract methane from natural gas, so they might not bother for the boosters. But I bet the fact that they can claim they could was at least a small factor.

[DerekL1963]

8 hours ago, K^2 said:

Well, no, not with current fuel prices.

Not with fuel prices ten times as high as currently.  Maybe with fuel prices a hundred times as much as currently.  Fuel is cheap in industrial quantities.

8 hours ago, K^2 said:

If SpaceX ends up using the same production pipeline for methane on Earth as it's planning to use on Mars

They would be insansely stupid.  Why take H2, and add energy (thereby increasing the cost of the fuel) to turn it into methane rather than burning it directly?  Not to mention, currently H2 production is from fossil fuels - if they manage to make one of the processes work that don't use fossil fuels...  well, see the first two sentences of this paragraph.

Elon Musk is many things, but stupid is not one of those.

[Rakaydos]

Actually, my point was that if the booster really can be reused 100+ times, the fuel cost (relative to the booster cost) just went up by 2 orders of magnatude. It's not that the fuel isnt cheap, it's that the rocket is cheap too and goes through a LOT of fuel over it;s lifetime.

[DerekL1963]

1 hour ago, Rakaydos said:

Actually, my point was that if the booster really can be reused 100+ times, the fuel cost (relative to the booster cost) just went up by 2 orders of magnatude. It's not that the fuel isnt cheap, it's that the rocket is cheap too and goes through a LOT of fuel over it;s lifetime.

Well, no.  It doesn't work quite that way.  What matters is the end cost to the customer - which includes the amortized cost of the booster, the amortized cost of major maintenance on the booster, the amortized cost of fixed overhead, non reusable hardware, variable overhead directly chargeable to the flight, refurbishments costs directly chargeable to the flight, handling and launch operations, oh, and fuel plus a bit of profit.

Fuel is a major item for the airlines because they can amortize costs over thousands of flights per day and per airframe across it's lifetime - and because they've ruthlessly slashed costs overall.

At $40 million a flight (the current guesstimate of the cash price for a flight on a reflown booster)...  the fuel costs for the booster are still way down in the noise.  Costs per flight need to drop well below that figure before things change much.

[K^2]

15 hours ago, DerekL1963 said:

They would be insansely stupid.  Why take H2, and add energy (thereby increasing the cost of the fuel) to turn it into methane rather than burning it directly?  Not to mention, currently H2 production is from fossil fuels - if they manage to make one of the processes work that don't use fossil fuels...  well, see the first two sentences of this paragraph.

They actually go into quite lengthy details on why not use H2. In fact, ITS simply cannot work in its current form with H2 fuel. Neither the booster nor the interplanetary stage for slightly different reasons. I encourage you to actually watch Musk's presentation and pay attention to these parts. If it still doesn't make sense after presentation, I can expand on it by filling the gaps.

And cost becomes significant even at 10x prices, which is the minimum increase we are looking at for kerosene if space industry becomes the sole user. It's likely to be higher. Currently, Falcon 9 v1.1 takes about $300k worth of RP-1 to fuel up. That's ~$7.50/gallon, which is a bit pricey for kerosene, but you have to keep in mind that it has to be even cleaner than Jet-A, which is about $5/gallon. At 10x the price, we'll be looking at $3M of the launch cost coming from fuel, and that's at current cost of approximately $62M per launch. So we are looking at price of fuel going from 0.5% to 5% of the launch cost, and 9v1.1 is not reusable. For a reusable craft, that fraction becomes significantly higher. Musk's plan for ITS booster is to make it work like an airliner, where fuel is approximately 50% of the cost over the life time of the unit. At this point, if you can shave 10% off the cost of your fuel, you go for it. And if we really abandon fossil fuels on Earth, the difference between synthetic methane and synthetic RP-1 is going to be way higher than 10%.

Of course, that's a very big if. More likely, we'll still have significant kerosene production worldwide by the time ITS is scheduled to start flights. But again, it's not Elon Musk's stance. He is trying to push for a world where this is not the case, and everything the two companies do reflect it.

 

Of course, all of this is just an additional PR twist. The bigger reasons are purely practical. Methane is a great compromise between heavy hydrocarbons and hydrogen fuel. And FFSC design of Raptor absolutely requires methane as fuel. Hydrogen would have worked perfectly fine for Raptor, but as I've mentioned above, there are other reasons not to go with that.

As for efficiency, once you take into account the additional weight of the tank you need for hydrogen, Methane is actually almost as good. The only place where you really have to go with hydrogen is SSTOs. For everything else, hydrocarbons a good enough option. That's why Russians are still flying RP-1, and that's why even SpaceX's Falcon rockets are fueled with RP-1. And as you yourself point out, Musk is definitely not a stupid person.

[Rune]

On 2/3/2016 at 10:51 PM, MathiLpHD said:

So my question: Is that a realistic calculation i did there? Or won't they use the full pressure of an FFSC (~31MPa)? And could you please list all advantages/disadvantages of Methane and FFSC compared to other propellants/engine cycles?

Short answer? No. Now for the slightly longer answer:

This relationship are not linear. They are based on complicated thermodynamic formulae, and then the hard reality of manufacturing limits. So if SpaceX says they can get 382s of Isp out of a vacuum Raptor with a nozzle expasion ratio of 200, then you'd better trust their word, because they have the analytical tools, and you don't. And yes, they get such pressures by modelling the enigne with a chamber pressure of 300bar and all the tricks they could think of thrown in for good measure.

As to advantages/disadvantages, FFSC is basically just plain better. The single most important parameter in engine performance is chamber pressure and expansion ratio, and FFSC allow the highest possible pressure by using the full potential of the whole mass of fuel and oxidizer to power the turbopump machinery. A traditional staged combustion engine only has the oxidizer or the fuel to power itself, a gas generator wastes all of the fluid it uses throwing it overboard. The cons are, basically, complexity, and the fact that you can only run certain fuel/oxidizer combos through turbopumps. Basically, this can only ever work with H2/LOX, CH4/LOX, and a handful of other fuel combos where you can gasify and expand both propellants on turbines.

 

Rune. Of course, running an excepcionally high pressure engine like a normal one should do wonders for service life, if that's what you are after.

Edited October 4, 2016 by Rune

[wumpus]

On 3/4/2016 at 11:46 AM, shynung said:

Except hot oxygen is terribly corrosive. The turbine exhaust pipes are the ones needing exotic materials.

If you really think staged combustion is the way to go, why not use the standard version rather than the full-flow version? If methane indeed runs cool enough to use on regular steel components, just use a bigger fuel-rich turbine to run everything together.

You can run them together with kerolox (the densities are close enough together), but I'm guessing that you need separate turbopumps to drive the different densities of methane and oxygen (you do with hydrogen and oxygen).  Presumably it makes more sense to go fuel-lean on the oxygen side and avoid sealing issues.  Remember, a BFR can tolerate a shut down engine or two.  It is unlikely to tolerate an exploding engine.  Leaky seals could easily lead to an exploding engine.

[blowfish]

22 minutes ago, wumpus said:

You can run them together with kerolox (the densities are close enough together), but I'm guessing that you need separate turbopumps to drive the different densities of methane and oxygen (you do with hydrogen and oxygen).  Presumably it makes more sense to go fuel-lean on the oxygen side and avoid sealing issues.  Remember, a BFR can tolerate a shut down engine or two.  It is unlikely to tolerate an exploding engine.  Leaky seals could easily lead to an exploding engine.

The densities of methane and oxygen are close enough that you can do them on a single shaft (see: BE-4).  Then again, it's not impossible to do hydrogen and oxygen on the same shaft either (see: RD-0120), just a lot harder.

[RocketSquid]

The reason for methane is basically that it's cheap to make/get and cheap to store. It's very easy to produce in-situ using sabatier and electrolysis, especially on mars, where all you need to make methane+LOX is a bit of power, some water, and the martian atmosphere. It also has a higher boiling point than liquid Hydrogen, high enough that you don't have to worry quite as much about boil-off.

[DerekL1963]

22 minutes ago, RocketSquid said:

It's very easy to produce in-situ using sabatier and electrolysis, especially on mars, where all you need to make methane+LOX is a bit of power, some water, and the martian atmosphere.

Theoretically, it's very easy.  Practical though...  The devil lies in all the details hidden behind the smoke and mirrors of such a high level handwavy view.  Where does the power come from?  Where does the water come from?  Etc... etc...

[RocketSquid]

Just now, DerekL1963 said:

Theoretically, it's very easy.  Practical though...  The devil lies in all the details hidden behind the smoke and mirrors of such a high level handwavy view.  Where does the power come from?  Where does the water come from?  Etc... etc...

The water, of course, would be from ice in the soil, which is easy enough, but the power is something else altogether. I honestly have no idea where they plan to get it from, but the Sabatier reaction used to make the methane is self-sustaining given the right reaction chamber, so most of their power is going into electrolysis and refrigeration.

[monophonic]

1 hour ago, RocketSquid said:

The water, of course, would be from ice in the soil, which is easy enough, but the power is something else altogether. I honestly have no idea where they plan to get it from, but the Sabatier reaction used to make the methane is self-sustaining given the right reaction chamber, so most of their power is going into electrolysis and refrigeration.

What about the 4% of not-CO2 in the atmosphere and all the not-H2O in the ice? How do you purify your inputs so the impurities won't clog your processes? Or stuff doesn't get into the end product and ruin your ISP so you just almost got home?

These things are easy in a lab or text book, but in the field you must know everything will work 100% or you might just as well not have come at all.

[Elthy]

Purifing the water is quite easy, i guess they will mine it in solid form mixed with dirt and simply evaporate/condense it.

[Rakaydos]

12 minutes ago, Elthy said:

Purifing the water is quite easy, i guess they will mine it in solid form mixed with dirt and simply evaporate/condense it.

So...

1) mine

2) place in evaporator (heat or vacuum sublimation?)

3) pump H2O gas to condensor

-dump unused surface materials

4) Condence liquid H2O

5) Electrolize O2+H2 gasses (separated) from water

-Pump O2 into inflatable storage tanks to await spaceship

6) Sabatier H2 gas with CO2 gas (compressed external atmosphere- impure, possibly degrading?) into methane

7) Pump methane into inflatable storage tanks to await spaceship

Is there any other steps that need to be looked into to make sure they arnt hiding a secret dealbreaker?

[DerekL1963]

1 hour ago, Rakaydos said:

Is there any other steps that need to be looked into to make sure they arnt hiding a secret dealbreaker?

Nobody is saying that they are hiding a secret dealbreaker - only that folks shouldn't treat the process as a 'done deal'.  The details so often airily handwaved away (in the rare instance they're mentioned at all) hide considerable potential practical complications.