Designing Merlin M

[Okie_Steve]

Why pick propane? It can be stored at lox temperatures. [...]

When I take my propane tank for my gas grill down the the refill station, they don't fill it at LOX temperatures. propane (a.k.a. LPG or liquefied petroleum gas) is generally stored and transported at room temperature. Are we talking about something else?

Propane is a liquid under pressure at room temperature. Down to a little below -40C (or F  ) it boils to a gas for your grill. Below that it stays liquid. This is commonly but incorrectly referred to as propane tanks "freezing up" in places where it gets that cold. In reality pure propane does not freezes and turn solid until nearly -190C. FYI, what is marketed as LPG is a mixture of propane and butane with somewhat different characteristics.

As a gas propane has about twice the energy of methane for an equivalent volume at the same pressure. As a liquid propane has about 10% more energy than methane for an equivalent volume. I do not recall the liquid density comparison.

[TomH]

As a liquid propane has about 10% more energy than methane for an equivalent volume. I do not recall the liquid density comparison.

Energy per mass unit/ISP are about the same. Dry mass would be slightly less as propane's fuel tankage would be slightly smaller.

https://www.pioneerlng.com/lng-vs-lpg/

[Robotbeat]

As a liquid propane has about 10% more energy than methane for an equivalent volume. I do not recall the liquid density comparison.

Energy per mass unit/ISP are about the same. Dry mass would be slightly less as propane's fuel tankage would be slightly smaller.

https://www.pioneerlng.com/lng-vs-lpg/

Isp and specific energy are slightly lower for propane. Propane has a specific energy of 50MJ/kg whereas methane has 56MJ/kg. Additionally, average atomic mass is lower for methane meaning you get a higher exhaust velocity for a given temperature.

[hkultala]

But in reality, the direct isp advantage of methane over kerosine is very small. Raptor has much better isp mostly because merlin has much pressure and closed cycle. Look at isp of high-pressure kerosene-based closed cycle engine(RD-170/180/190) and see the differences. (*)

I really, **really** do not understand what you are saying here.

Maybe you should educate yourself about the difference between atmospheric nozzle and vacuum nozzle and you might understand.

The RD-180 (kerosine)has a vacuum ISP of 338

with atmospheric-optimized nozzle.

It has 311 s isp at on sea level, where we can make apples-to-apples comparison

The  Raptor (Methane) has a vacuum ISP of 380

With vacuum optimized nozzle.

At sea level (where we can make apples-to-apples comparison), the sea level version of raptor has only about 330s isp

So, the apples-to-oranges comparison between RD-180 and Raptor is 311s vs 330s, difference of only 6%

Yet you say this makes for a " isp advantage of methane over kerosine is very small"
380, compared to 338, is not a "very small" improvement.

No, it's not, because you are comparing sea level nozzle vs vacuum nozzle at vacuum. Totally apples-to-oranges comparing. Correct comparison is 311 vs 330.

It means that to get 10km/s, you need 40% more fuel.   That's about whats needed to get to orbit.
But to carry the 40% more fuel at liftoff, you need more and/or bigger engines. About the same tankage size, due to better kerosine density. But your liftoff mass ends up being 60% more than with the methane-fuelled comparible rocket. Also more expensive, because more engine needed.

That does not make for a "very small" difference.

... all this is calculations based on totally wrong numbers, and then you take totally unrealistic/irrelevant 10km/s delta-v target from your hat. And then you totally ignore the increased tank, engine and piping weight.

garbage in => garbage out.

Boosters job is to give something slightly less than to 4km/s, not something like 10km/s.

That <4 km/s with 6% isp difference gives quite different results.

[xvel]

There is ISP of sea level optimized (small nozzle) engine at sea level and
there is ISP of sea level optimized (small nozzle) engine at vacuum and
there is ISP of vacuum optimized (big nozzle) engine at sea level and
there is ISP of vacuum optimized (big nozzle) engine at vacuum

4 different numbers

[TomH]

There is ISP of sea level optimized (small nozzle) engine at sea level and
there is ISP of sea level optimized (small nozzle) engine at vacuum and
there is ISP of vacuum optimized (big nozzle) engine at sea level and
there is ISP of vacuum optimized (big nozzle) engine at vacuum

4 different numbers

Then you have ISP of a sustainer engine (RS-25 on STS/SLS and center sustainer on original Atlas: SM-65) at sea level and
ISP of the same sustainer at vacuum.

[livingjw]

I have done the RP1/LOx vs CH4/LOx trade. The small increase in CH4/LOx Isp Over RP1/LOx is just about canceled out by its decreased bulk density. CH4/LOx requires larger tanks and hence greater empty mass which reduces its mass ratio and delta V. CH4 shines in two areas, cost and reduced coking, neither of which matter much for expendable vehicles, but both matter a lot for reusable vehicles.

Edit: CH4 also is a better coolant.

John

[Redclaws]

I have done the RP1/LOx vs CH4/LOx trade. The small increase in CH4/LOx Isp Over RP1/LOx is just about canceled out by its decreased bulk density. CH4/LOx requires larger tanks and hence greater empty mass which reduces its mass ratio and delta V. CH4 shines in two areas, cost and reduced coking, neither of which matter much for expendable vehicles, but both matter a lot for reusable vehicles.

Edit: CH4 also is a better coolant.

John

Cost as in cost of the actual propellant?  I thought that was pretty irrelevant for both of them?

[livingjw]

I have done the RP1/LOx vs CH4/LOx trade. The small increase in CH4/LOx Isp Over RP1/LOx is just about canceled out by its decreased bulk density. CH4/LOx requires larger tanks and hence greater empty mass which reduces its mass ratio and delta V. CH4 shines in two areas, cost and reduced coking, neither of which matter much for expendable vehicles, but both matter a lot for reusable vehicles.

Edit: CH4 also is a better coolant.

John

Cost as in cost of the actual propellant?  I thought that was pretty irrelevant for both of them?

As the booster's lifetime number of launches increase from tens, to hundreds the cost of propellant will become significant.

John

[Joris]

I'd say the ideal number of engines depends on how many you need to land. Given the high ratio between the weight of the full rocket and the weight of the empty stage, you don't want to have a single engine because that is impossible to throttle down to.

If you have around 10 engines you can land on 1, the problem is that it doesn't give you engine out capability, but you don't want much fewer engines than that.

Having more engines gives you things like roll control and engine-out capability, my guess is that it doesn't matter and you should just use whatever is best and already developed for another purpose.

[edzieba]

you don't want to have a single engine because that is impossible to throttle down to.

In theory, you can land (using a hoverslam) with a completely non-throttling engine. Doing so requires exquisite timing for startup and shutdown (and excellent measurement of altitude and local atmospheric conditions, and excellent modelling of vehicle aerodynamic behaviour) to deliver just the right thrust duration exactly when you need it, but landing with a single 'oversized' non-throttleable motor is entirely possible. It's certainly more difficult and has more avenues for failure than one or more throttleable engines though.

[Welsh Dragon]

you don't want to have a single engine because that is impossible to throttle down to.

In theory, you can land (using a hoverslam) with a completely non-throttling engine. Doing so requires exquisite timing for startup and shutdown (and excellent measurement of altitude and local atmospheric conditions, and excellent modelling of vehicle aerodynamic behaviour) to deliver just the right thrust duration exactly when you need it, but landing with a single 'oversized' non-throttleable motor is entirely possible. It's certainly more difficult and has more avenues for failure than one or more throttleable engines though.

And require the structure to be resistant to very significant g-loads repeatedly (read: heavy).

[edzieba]

you don't want to have a single engine because that is impossible to throttle down to.

In theory, you can land (using a hoverslam) with a completely non-throttling engine. Doing so requires exquisite timing for startup and shutdown (and excellent measurement of altitude and local atmospheric conditions, and excellent modelling of vehicle aerodynamic behaviour) to deliver just the right thrust duration exactly when you need it, but landing with a single 'oversized' non-throttleable motor is entirely possible. It's certainly more difficult and has more avenues for failure than one or more throttleable engines though.

And require the structure to be resistant to very significant g-loads repeatedly (read: heavy).

You get some of that for 'free' due to the need to not crumple before MECO. e.g. If you have 9x engines, if the wet upper-stage is 8x or less the mass of the almost-dry (with just landing prop) first stage, acceleration with 1x engine at landing is equal to or less than acceleration at MECO, without a big mass sat on top of your structure. IIRC for Falcon 9 at least, peak deceleration is during the early stages of EDL (entry burn) rather than at landing.

[Okie_Steve]

Just to be sure, is there anything fundamental about a Merlin-Like GG engine that would have trouble with liquid Methane/Oxygen propellants?

[livingjw]

Just to be sure, is there anything fundamental about a Merlin-Like GG engine that would have trouble with liquid Methane/Oxygen propellants?

None that I know of, but why? A GG engine needs the same number of pumps. FFSC requires one extra burner and turbine and a couple of bearings. FFSC technology has already been demonstrated by SpaceX. I don't see them giving up the FFSC performance improvement.

John

[Okie_Steve]

Just to be sure, is there anything fundamental about a Merlin-Like GG engine that would have trouble with liquid Methane/Oxygen propellants?

None that I know of, but why? A GG engine needs the same number of pumps. FFSC requires one extra burner and turbine and a couple of bearings. FFSC technology has already been demonstrated by SpaceX. I don't see them giving up the FFSC performance improvement.

John

I think you are right, but the premise of this thread is the possibility of a new GG booster engine if Raptor hits a snag. Since rapid reuse pretty much requires coking to go away, which mean methane in practical terms, I just wanted to check the assumption 

[ZachF]

But in reality, the direct isp advantage of methane over kerosine is very small. Raptor has much better isp mostly because merlin has much pressure and closed cycle. Look at isp of high-pressure kerosene-based closed cycle engine(RD-170/180/190) and see the differences. (*)

I really, **really** do not understand what you are saying here.
The RD-180 (kerosine)has a vacuum ISP of 338
The  Raptor (Methane) has a vacuum ISP of 380

Yet you say this makes for a " isp advantage of methane over kerosine is very small"

380, compared to 338, is not a "very small" improvement.
It means that to get 10km/s, you need 40% more fuel.   That's about whats needed to get to orbit.
But to carry the 40% more fuel at liftoff, you need more and/or bigger engines. About the same tankage size, due to better kerosine density. But your liftoff mass ends up being 60% more than with the methane-fuelled comparible rocket. Also more expensive, because more engine needed.

That does not make for a "very small" difference.

You aren’t comparing similar engines.

A “methamerlin” GG methane engine replacement for Raptor would probably only get an ISP of ~308 sl compared with ~330 for the FFSC Raptor.

GG engine is also going to have a problem with thrust per nozzle area, meaning you’re going to need a giant skirt or reduce ER making ISP even worse.

I’ve done the math and going GG doesn’t really make sense for Starship. The first stage ends up being considerably larger for a given equivalent throw weight. ISP is important too… especially when the rocket has to fly itself back home.

[Jimmy_C]

This is a crazy idea, but what about a three main tanks (RP-1, methane, and LOX) and 2 header tanks (methane and LOX). Use Raptor and simplified Merlin for takeoff and Raptor for landing. Merlin can delete gimbling, multiple firing, deep throttle, etc. In effect, replacing Raptor-boost engines with "Merlin-boost" engines. Merlin is quite mature, easy to produce, cheap, reliable, and has good thrust-weight ratio (edit: not better). Proven to be reusable up to 10 times. Raptor's better for sustained performance, reignition, and landing burns because of no coking and higher ISP. RP-1 is denser than methane, so equivalent volume of fuel for boost phase might be possible to mitigate Merlin's lower ISP. Thus, no changes in height needed. You'll need about twice as many Merlins to replace equivalent Raptors, though.

[baldusi]

This is a crazy idea, but what about a three main tanks (RP-1, methane, and LOX) and 2 header tanks (methane and LOX). Use Raptor and simplified Merlin for takeoff and Raptor for landing. Merlin can delete gimbling, multiple firing, deep throttle, etc. In effect, replacing Raptor-boost engines with "Merlin-boost" engines. Merlin is quite mature, easy to produce, cheap, reliable, and has good thrust-weight ratio (edit: not better). Proven to be reusable up to 10 times. Raptor's better for sustained performance, reignition, and landing burns because of no coking and higher ISP. RP-1 is denser than methane, so equivalent volume of fuel for boost phase might be possible to mitigate Merlin's lower ISP. Thus, no changes in height needed. You'll need about twice as many Merlins to replace equivalent Raptors, though.

The only way Raptor lends itself to tripropellant is by injecting RP-1/LOX straight into the nozzle to increase thrust at sea level. But I doubt you would win in the overall system. Now, you did had tripropellant engines (RD-701, RD-704), but they are not really compatible with FFSC. At least, not with RP-1. LOX/LH/CH4 may be.

[kkattula]

This is a thread to discuss the feasibility of designing a simple engine like Merlin for a 12m heavy lift booster.

Suppose there is a billionaire from an alternative universe (Mr. B) who wants to build a city on  Mars in our universe. He observes the rapid progress of SpaceX toward this goal, but is still not satisfied that the cost of travel will be low enough. He views the Raptor engine as an excellent machine that may eventually achieve Isp=380, an efficiency level that would allow the conquest of space. Still, complexity of each engine and cost to install plumbing for  many engines for each booster are outside his comfort zone. Is there a simpler engine design and a simpler plumbing plan for the booster that would bring down the total cost of building a spaceship booster?
...

How expensive do you think Raptors will be?   My understanding is they're already on a path to be cheaper per unit of thrust than Merlin. (Current production issues, not withstanding).

Sure they could probably 'dumb them down' a bit to be even cheaper, but at a cost of reduced performance, hence more (or bigger) engines and bigger tanks. Given SpaceX have been thinking very hard about this for quite a while, it seems highly improbable that would trade better.

The casual mention of a 12m booster is a huge red flag.  SpaceX have just spent several years developing the factory and techniques to build 9m diamter tanks.  AIUI, tank stretches are usually no big deal, but diameter increases are like starting all over again.