Author Topic: MiniRaptor / SC Merlin  (Read 12630 times)

Offline Nilof

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MiniRaptor / SC Merlin
« on: 04/16/2017 10:53 am »
Assuming that the Raptor prototype that was test-fired last September was one-third scale, i.e. around 1000 kN, is there a possibility that we could see a smaller version of the Raptor as a replacement to the Merlin? I remember seeing a lot of discussion about this around 2014 back when we still didn't have a lot of data on Raptor.

Or more generally, is there a chance that the Merlin could be replaced by a staged combustion engine eventually, as SpaceX gains experience with staged combustion? While total liftoff thrust is still more important for first stages than Isp, and available footprint at the bottom of the rocket for nozzles could be a bottleneck, a potential 50 seconds of extra specific impulse from staged combustion/high chamber pressure is pretty significant.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Hanno

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Re: MiniRaptor / SC Merlin
« Reply #1 on: 04/16/2017 11:18 am »
Unlikely.

All technical considerations aside, the Merlin is a proven, working design with all the logistical, manufacturing and quality control infrastructure in place. The performance bonus of an SC engine is just not big enough to throw it all away and start from scratch.

You need something that current engine just can't do in order to justify the development on a new engine. Like ITS/Mars for the Raptor or the need to get rid of the RD-180 for the BE-4/AR-1.

Offline Nilof

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Re: MiniRaptor / SC Merlin
« Reply #2 on: 04/16/2017 11:28 am »
True. But if you've gotten to the point where you have a prototype engine that has completed a significant number of test firings, most of the R&D expenses for developing the engine should be behind you. As far as manufacturing goes, I believe that EM mentioned that Raptor would have a lot in common with the Merlin.

With that said, you'd have to essentially design a new rocket around the new engine. That would probably not be as desirable. Though the idea of a Raptor US still seems to be fairly widespread.
« Last Edit: 04/16/2017 11:47 am by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline macpacheco

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Re: MiniRaptor / SC Merlin
« Reply #3 on: 04/16/2017 07:50 pm »
F9 / FH design was grounded by the requirements of expendable missions + road transportability (optimize for construction cost and ease of ground transportation).
SpaceX now has a way to fully reuse boosters and upper stage reuse is mostly a matter of having lots of spare performance (for deorbit+landing burns plus a lot of TPS for re-entry).
When we add to this the fact that despite of its higher ISP and higher T:W, raptor rockets need to be thicker so road transport need to be given up.
This suggests that even a F9/FH Raptor 2nd stage is a kludge with limited benefit vs a brand new rocket.
The more I think about this, the more I think the best way is go Raptor all the way, to a mini ITS (meaning a big upper stage with lots of engines) or a Raptor Falcon 9 on steroids (single engine upper stage + 7-9 raptors on the booster). Either way we should end up with a boosters that might be reusable 100x without refurb and upper stages that could be reflown dozens of times without refurb too.
A new rocket that's unconstrained by road transport can achieve FH expendable performance with full reuse. In fact it should be designed without any contingency for any expendable usage. Its reuse all the way. It doesn't matter if it costs ~5x as much as a FH.
Trying to squeeze limited raptor usage in between probably doesn't save enough expended upper stages for the redesign cost. Better aim for a rocket that could be a workhorse for several decades instead.
Remember that it costed SpaceX one billion (so far) to figure out reuse. F9 Block I design costed US$ 400 million. Above that, it ties down lots of engineers that could be better used for more permanent ideas. Better to "waste" a few hundred million in 50 or even 100 more 2nd stages being wasted than divert engineers from getting a mini ITS flying sooner.
From a 1/3 thrust Raptor to 75+% thrust its probably a matter of 6-12 months more in testing/development. Better go all the way.
Besides the new workhorse rocket doesn't need to wait until Raptor testing is complete to be designed. ITS design apparently will go into high gear soon.
« Last Edit: 04/16/2017 07:56 pm by macpacheco »
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Offline MikeAtkinson

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Re: MiniRaptor / SC Merlin
« Reply #4 on: 04/16/2017 09:07 pm »
There are multiple paths towards what is probably the medium term aim, ITS + methalox replacement for F9/FH. SpaceX have all the facts to make a rational choice between them, we just argue from ignorance. We will know more about SpaceX's decision in a few weeks when the updated ITS plan is announced.

Today I think they will do something like: ITS (full size, but derated capability), mini-ITS replacement to FH (refuel in LEO), full capability ITS.

Tomorrow I will probably think something different, I've regularly changed my mind on this as there do not seem to be any conclusive arguments based on the known facts.

It is interesting that with refueling in LEO something with a payload a bit smaller than the current F9 could carry out all existing missions, and most future ones (although fairing size would be a problem for some).

Offline su27k

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Re: MiniRaptor / SC Merlin
« Reply #5 on: 04/17/2017 05:01 am »
When we add to this the fact that despite of its higher ISP and higher T:W, raptor rockets need to be thicker so road transport need to be given up.

I think it has been explained on this forum many times that for SpaceX methane will have better performance than RP-1 even when using the same tank volume, mainly due to different mixture ratio and the big Isp improvement from Raptor over Merlin.

Quote
This suggests that even a F9/FH Raptor 2nd stage is a kludge with limited benefit vs a brand new rocket.

I don't think so, seems to me a reusable and refuelable F9/FH Raptor 2nd stage is the 2nd best option for going forward, right after BFS SSTO. The only reason they want to switch first stage to methane is they find out RP-1 makes it impossible to reach their reusability goal, which I think it's unlikely.

Offline CorvusCorax

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Re: MiniRaptor / SC Merlin
« Reply #6 on: 04/17/2017 11:41 am »
A design for a Methane based fully reuseable "bigger version" of Falcon 9 is already in development.

Over at Blue Origin, they call it "New Glenn".

I'd say either SpaceX already has a Raptor based Falcon successor design (other than ITS) in the drawing boards to bring to the market when F9 doesn't kick it anymore (because, if, eventually, New Glenn enters service and B/O becomes competitive with it - lots of ifs) Or they skip it altogether assuming that ITS could and would serve any LEO and GEO launches that would ever be needed. easily - once in service.

If ITS does what it's designed for, it could easily haul any number of GEO sat where they need to be. All at once if needed (They could do a launch once a quarter, and stuff any sat on board that happens to be payload ready by then - unlike Ariane where dual launches have to be carefully mached, you could just bulk load com-sats, kick them out in actual GEO so all they'l ever need to do on their own is a slot change, and then deorbit and land the carrier)

The one thing ITS would possibly be less than optimal is LEO and vLEO orbits with high inclination - especially if you need to launch many small sats into many different planes.

That's where F9 would shine however, all they need to do is get the upper stage reuseable.

So I'd say, considering the market situation and potential competition, there's likely no need for SpaceX to develop a (Mini-?)raptor based F9 successor anytime soon - unless upper stage reuse on F9 current architecture fails.


That being said, SpaceX does IMHO need a fully reusable launcher for their mega-constellation. I'm curious what's the plan there...

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #7 on: 04/17/2017 01:03 pm »
A staged combustion Merlin would be something like ORSC RP-1/LOX. That's not a direction SpaceX is going long-term, and the performance increase isn't nearly enough to warrant the effort.

A scaled-down Raptor might also be unnecessary long term. A single Raptor with a mid-length nozzle (something like 120:1 expansion) would be ideal for a large FH upper stage, or for an all-methalox FH class single stick upper stage, much like New Glenn's single BE-4U 2nd stage.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #8 on: 04/18/2017 07:50 pm »
When we add to this the fact that despite of its higher ISP and higher T:W, raptor rockets need to be thicker so road transport need to be given up.

I think it has been explained on this forum many times that for SpaceX methane will have better performance than RP-1 even when using the same tank volume, mainly due to different mixture ratio and the big Isp improvement from Raptor over Merlin.
Yep, definitely. The increase in specific impulse and in TWR means that even with the fluffier propellant, putting Raptors on a Falcon 9 body (assuming you could move the common bulkhead appropriately) would get you a 40% increase in payload.
« Last Edit: 04/19/2017 05:40 pm by sevenperforce »

Offline TheKutKu

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Re: MiniRaptor / SC Merlin
« Reply #9 on: 04/19/2017 04:29 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #10 on: 04/19/2017 04:54 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.
It's not really a myth. Subcooled kerolox is ~1115 kg/m3. Subcooled methalox at 3.8:1 is ~930 kg/m3. There's roughly a 20% bulk density difference.

And the RP-1 in Falcon 9 is also subcooled, just not the extent that the LOX is... the RP-1 is loaded at several degrees below 0 C.

Offline livingjw

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Re: MiniRaptor / SC Merlin
« Reply #11 on: 04/19/2017 05:54 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.

Agree. The overall performance of RP-LOX and CH4-LOX engines are very close. CH4-LOX performance is a little higher but has lower propellant density so the two almost cancel each other out. The performance improvement between the Merlin 1D and Raptor is due mostly to its staged combustion cycle and its high pressure. The real reasons for choosing CH4-LOX are:
- minimal coking,
- autogenous pressurization,
- in situ propellant production,
- more cooling capacity,
- lower cost

Offline TheKutKu

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Re: MiniRaptor / SC Merlin
« Reply #12 on: 04/19/2017 06:07 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.
It's not really a myth. Subcooled kerolox is ~1115 kg/m3. Subcooled methalox at 3.8:1 is ~930 kg/m3. There's roughly a 20% bulk density difference.

And the RP-1 in Falcon 9 is also subcooled, just not the extent that the LOX is... the RP-1 is loaded at several degrees below 0 C.

Do you have a source for theses density numbers? Astronautix links give a higher Density for (non subcooled) Methalox.

Offline acsawdey

Re: MiniRaptor / SC Merlin
« Reply #13 on: 04/19/2017 06:32 pm »
http://unitrove.com/engineering/tools/gas/liquefied-natural-gas-density gives 0.4484 for CH4 at 93K.

https://www.gpo.gov/fdsys/pkg/GOVPUB-C13-26d428ad4ca587866a90da5f71b4a727/pdf/GOVPUB-C13-26d428ad4ca587866a90da5f71b4a727.pdf gives 1.2817 for O2 at 60K.

(3.6*1.2817+0.4484)/(1+3.6) = 1.1005

Using the astronautix number for un-densified RP-1 with the densified LOX number:
(2.56*1.2817+0.806)/(1+2.56) = 1.1481

Gives 0.9586 as the ratio.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #14 on: 04/19/2017 06:38 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.
It's not really a myth. Subcooled kerolox is ~1115 kg/m3. Subcooled methalox at 3.8:1 is ~930 kg/m3. There's roughly a 20% bulk density difference.

And the RP-1 in Falcon 9 is also subcooled, just not the extent that the LOX is... the RP-1 is loaded at several degrees below 0 C.

Do you have a source for theses density numbers? Astronautix links give a higher Density for (non subcooled) Methalox.

Astronautix give densities for CH4 and LOX, not bulk density for methalox. At 3.8:1 O/F, 424 kg/m3 fuel density, and 1140 kg/m3 oxidizer density, the calculation is:

(424*(1+3.8))/(424*3.8/1140+1) = 843 kg/m3

If you do this: (424+1140*3.8)/(1+3.8) = 990 kg/m3 you are really calculating the bulk density of a VOLUMETRIC O/F ratio of 3.8:1, which corresponds to a MASS O/F ratio of 10.2:1. Fuel ratios in rocketry are always mass ratios.

To check this, think: are you really going to burn 1140*3.8 = 4332 kg of LOX with only 424 kg of CH4? Don't try to do this, you'll end up with a lot of hot leftover oxygen, which is really nasty stuff.
« Last Edit: 04/19/2017 06:40 pm by envy887 »

Offline livingjw

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Re: MiniRaptor / SC Merlin
« Reply #15 on: 04/19/2017 06:39 pm »
The higher density propellant is preferred to minimize tank weight which is proportional to volume.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #16 on: 04/19/2017 06:45 pm »
http://unitrove.com/engineering/tools/gas/liquefied-natural-gas-density gives 0.4484 for CH4 at 93K.

https://www.gpo.gov/fdsys/pkg/GOVPUB-C13-26d428ad4ca587866a90da5f71b4a727/pdf/GOVPUB-C13-26d428ad4ca587866a90da5f71b4a727.pdf gives 1.2817 for O2 at 60K.

(3.6*1.2817+0.4484)/(1+3.6) = 1.1005

Using the astronautix number for un-densified RP-1 with the densified LOX number:
(2.56*1.2817+0.806)/(1+2.56) = 1.1481

Gives 0.9586 as the ratio.

That's a volumetric ratio of 3.6:1 bulk density, not mass ratio of 3.6:1 bulk density.

At 3.6:1 mass ratios with the cooled CH4/LOX densities above, the bulk density is: (448.4*4.6)/(448.4*3.6/1281.7+1) = 912.9 kg/m3

The numbers here are correct - use them:
https://forum.nasaspaceflight.com/index.php?topic=42302.msg1642537#msg1642537

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #17 on: 04/19/2017 07:04 pm »
The higher density propellant is preferred to minimize tank weight which is proportional to volume.
Yes, but higher specific energy can more than make up the difference. If you shifted the bulkhead on the Falcon 9 first stage and filled it up with densified methalox, and slapped three Raptors underneath, it would have a 40% performance increase.

Offline TheKutKu

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Re: MiniRaptor / SC Merlin
« Reply #18 on: 04/19/2017 07:23 pm »
Could we stop spreading the myth that LCH4/LOX is much more lighter than RP-1/LOX? According to the astronautix link http://www.astronautix.com/l/loxlch4.html http://www.astronautix.com/l/loxkerosene.html

At Raptor's 3.8 O/F ratio the propellant's density is 0.991 g/cc, that's 97.1% of RP-1's density, with a more common 3.6 O/F ratio it has 96.5% of RP-1's density. Also that's not taking into account that both Methane and Oxygen are subcooled for the Raptor whereas only the Oxygen is for the Merlin.
It's not really a myth. Subcooled kerolox is ~1115 kg/m3. Subcooled methalox at 3.8:1 is ~930 kg/m3. There's roughly a 20% bulk density difference.

And the RP-1 in Falcon 9 is also subcooled, just not the extent that the LOX is... the RP-1 is loaded at several degrees below 0 C.

Do you have a source for theses density numbers? Astronautix links give a higher Density for (non subcooled) Methalox.

Astronautix give densities for CH4 and LOX, not bulk density for methalox. At 3.8:1 O/F, 424 kg/m3 fuel density, and 1140 kg/m3 oxidizer density, the calculation is:

(424*(1+3.8))/(424*3.8/1140+1) = 843 kg/m3

If you do this: (424+1140*3.8)/(1+3.8) = 990 kg/m3 you are really calculating the bulk density of a VOLUMETRIC O/F ratio of 3.8:1, which corresponds to a MASS O/F ratio of 10.2:1. Fuel ratios in rocketry are always mass ratios.

To check this, think: are you really going to burn 1140*3.8 = 4332 kg of LOX with only 424 kg of CH4? Don't try to do this, you'll end up with a lot of hot leftover oxygen, which is really nasty stuff.

Thank you for correcting me! That was a large oversight, i take back what i said.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #19 on: 04/19/2017 07:28 pm »
The higher density propellant is preferred to minimize tank weight which is proportional to volume.
Yes, but higher specific energy can more than make up the difference. If you shifted the bulkhead on the Falcon 9 first stage and filled it up with densified methalox, and slapped three Raptors underneath, it would have a 40% performance increase.

What are your assumptions behind this 40% number?

I have plotted in the attached image the bulk density required to achieve the Falcon 9 booster's delta-v with payload, assuming constant volume, stage dry mass, thrust, and outer mold line. Methalox Raptor is above the required Isp to get the same performance as Merlin, but not by nearly enough to get 40% improvement. More like 4% unless they also do a methane upper stage.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #20 on: 04/19/2017 08:11 pm »
The higher density propellant is preferred to minimize tank weight which is proportional to volume.
Yes, but higher specific energy can more than make up the difference. If you shifted the bulkhead on the Falcon 9 first stage and filled it up with densified methalox, and slapped three Raptors underneath, it would have a 40% performance increase.

What are your assumptions behind this 40% number?

I have plotted in the attached image the bulk density required to achieve the Falcon 9 booster's delta-v with payload, assuming constant volume, stage dry mass, thrust, and outer mold line. Methalox Raptor is above the required Isp to get the same performance as Merlin, but not by nearly enough to get 40% improvement. More like 4% unless they also do a methane upper stage.
Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size, equal vehicle TWR, and altitude compensation.

Although when I run the numbers again, my math may have been too conservative.

For example, using the Falcon 9 first stage by itself, I get an estimated payload of 2.9 tonnes to LEO. If the same Falcon 9 first stage was filled with methalox, it would only need two Raptors to have the same vehicle TWR, due to the lower propellant mass, and it would be able to get 12.3 tonnes to LEO.
« Last Edit: 04/19/2017 08:27 pm by sevenperforce »

Offline Basto

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Re: MiniRaptor / SC Merlin
« Reply #21 on: 04/19/2017 08:31 pm »
Seems like fantasy land to me. There are so many things in the vehicle that are specifically designed around the engines /fuel type.

The moment you start swapping these you are designing a new rocket. Makes no sense not to optimize this new vehicle around the new fuel / engines.

While it may seem logical that you are taking a shortcut you are actually making things harder.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #22 on: 04/19/2017 08:35 pm »
Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The Isp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #23 on: 04/19/2017 08:57 pm »
Seems like fantasy land to me. There are so many things in the vehicle that are specifically designed around the engines /fuel type.

The moment you start swapping these you are designing a new rocket. Makes no sense not to optimize this new vehicle around the new fuel / engines.
Oh, certainly. I'm not in any sense suggesting the Falcon 9 be retrofitted for methalox; I was just pointing out that the fluffiness of methane doesn't necessarily require a much larger tank, due to the higher specific energy and the lower thrust requirements.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #24 on: 04/19/2017 09:10 pm »
Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The Isp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.
Full-flow kerolox may not be achievable, but ORSC kerolox has been done and redone by the Russians, so I'll compare that.

A pair of RD-180s can deliver the same thrust as all nine Merlin 1Ds with a vacuum specific impulse of 338 s, though dry mass will be quite a bit higher, at 29 tonnes. Using the same single-stage-vehicle comparison as before, it's a payload of 2.57 tonnes to LEO, lower than with Merlins.

But let's take the first-stage-booster-only example. My estimates put the Falcon 9 FT at 23.2 tonnes to LEO, max.

If you swap out the nine Merlins for two staged-combustion RD-180s, leaving the upper stage exactly the same, you'd get 25.1 tonnes to LEO.

If you swap out the Merlins for three Raptors and move the first-stage common bulkhead, then fill it up with methalox only on the first stage, you'd get 26.4 tonnes to LEO. So that's a 14% improvement, even still retaining the Merlin-based kerolox upper stage.

(this assumes a Raptor TWR of roughly 200:1)
« Last Edit: 04/19/2017 09:26 pm by sevenperforce »

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #25 on: 04/20/2017 12:30 am »
Well, a methalox first stage and a kerolox upper stage certainly won't get a 40% improvement, haha. The 40% result was for a single stage vehicle assuming equal tank size and equal vehicle TWR.

That's not a realistic comparison since Falcon 9 isn't a kerolox SSTO. The F9 booster's job is to push 125 tonnes from launch pad through staging, something kerolox is much better suited for than doing SSTO.

The Isp efficiency for a vehicle is directly related to the delta-v it is required to operate through. The only reason a methalox Falcon could compete is because Raptor is FAR more advanced than Merlin. For a slow-staging booster like F9, a full-flow kerolox engine with Raptor-like pressures and would easily out-perform methalox Raptor engined booster.
Full-flow kerolox may not be achievable, but ORSC kerolox has been done and redone by the Russians, so I'll compare that.

A pair of RD-180s can deliver the same thrust as all nine Merlin 1Ds with a vacuum specific impulse of 338 s, though dry mass will be quite a bit higher, at 29 tonnes. Using the same single-stage-vehicle comparison as before, it's a payload of 2.57 tonnes to LEO, lower than with Merlins.

But let's take the first-stage-booster-only example. My estimates put the Falcon 9 FT at 23.2 tonnes to LEO, max.

If you swap out the nine Merlins for two staged-combustion RD-180s, leaving the upper stage exactly the same, you'd get 25.1 tonnes to LEO.

If you swap out the Merlins for three Raptors and move the first-stage common bulkhead, then fill it up with methalox only on the first stage, you'd get 26.4 tonnes to LEO. So that's a 14% improvement, even still retaining the Merlin-based kerolox upper stage.

(this assumes a Raptor TWR of roughly 200:1)

I get similar numbers, by the way. The 4% increase was for a dual-Raptor design; 3 Raptors would be hard to fit under Falcon.

But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

Offline TomH

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Re: MiniRaptor / SC Merlin
« Reply #26 on: 04/20/2017 01:31 am »
But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

Sometimes performance gains are from pumping and burning the prop faster. That means a shorter burn time if you don't enlarge tanks/densify, etc.. If you don't increase payload, you cut gravity losses, but wild comparisons raw thrust need to be balanced against other parts of the equation.

Online OneSpeed

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Re: MiniRaptor / SC Merlin
« Reply #27 on: 04/20/2017 01:05 pm »
... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #28 on: 04/20/2017 01:43 pm »
But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.
I can't imagine that even a cutting-edge ORSC would be able to significantly outperform the RD-180 in SL isp; it has 275% the chamber pressure of a Merlin 1D. But otherwise, yes, it's a somewhat unfair comparison. Of course, TWR really only affects dry mass.

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.
Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

Online hamerad

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Re: MiniRaptor / SC Merlin
« Reply #29 on: 04/20/2017 01:49 pm »
But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.
I can't imagine that even a cutting-edge ORSC would be able to significantly outperform the RD-180 in SL isp; it has 275% the chamber pressure of a Merlin 1D. But otherwise, yes, it's a somewhat unfair comparison. Of course, TWR really only affects dry mass.

... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.
Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

Could they perhaps use some of the thrusters they need for the BFS to land? Or are they too weak?

Offline hkultala

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Re: MiniRaptor / SC Merlin
« Reply #30 on: 04/20/2017 02:02 pm »
But Raptor has 15% higher pressure and nearly triple the TWR of RD-180, and 3 Raptors have 20% more thrust than 2 RD-180s. So RD-180 to Raptor is not exactly a level comparison either - if SpaceX did a large ORSC kerolox engine it would likely perform much better than RD-180 (in ISP, TWR, and thrust) and a bit better than Raptor as a boost engine.

AFAIK nothing official has been said about Raptor TWR.

People have just been speculating that it will be in the same range as Merlin, though RD-180 might be more reasonabale basis for TWR speculation.

FFSC needs three relatively high-pressure chambers and much bigger turbines than GG. These all have weight. And bigger pressure allowing relatively smaller chamber does help much when the chamber needs much stronger walls due much higher pressure.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #31 on: 04/20/2017 02:19 pm »
Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

Could they perhaps use some of the thrusters they need for the BFS to land? Or are they too weak?
Right, that's precisely what I was thinking. We don't have any specifics on the actual size of the thrusters, but pressure-fed hot-gas thrusters typically have a very good TWR. Mass penalty is very low, since they are pressurized from the existing autogenously-pressurized intertanks. SL Isp wouldn't be greater, but that's hardly a problem.

A single Raptor will be able to downthrottle to about 62 tonnes SL thrust. In contrast, the Merlin 1D can downthrottle to about 35 tonnes thrust, which is a TWR of 1.55 for the suicide burn/hoverslam. Using the same metric, a single Raptor could be used for an (unmanned) suicide burn down to a stage dry mass of 40 tonnes; below that, you'd need auxiliary thrusters.

I wonder if SpaceX would do altitude-compensating nozzles on the hot-gas thrusters.

AFAIK nothing official has been said about Raptor TWR.

People have just been speculating that it will be in the same range as Merlin, though RD-180 might be more reasonabale basis for TWR speculation.

FFSC needs three relatively high-pressure chambers and much bigger turbines than GG. These all have weight. And bigger pressure allowing relatively smaller chamber does help much when the chamber needs much stronger walls due much higher pressure.
Elon has stated categorically that the Raptor is expected to have a TWR better than the Merlin 1D.

While Elon's projections have been overoptimistic on plenty of occasions, I don't think his optimism is misplaced here. The RD-270, the only other full-flow staged-combustion engine to reach a test stand, had a TWR of 190:1, higher than the Merlin 1D. Of course, this is partly because the RD-270 was pushing hypergolics, but even with the higher isp of a methalox engine, this would translate to an expected TWR of about 171:1 on the Raptor. They'd only need about a 7% increase to beat the Merlin 1D, which is hardly aggressive considering it's been fifty-five years since the RD-270 was designed.

Offline envy887

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Re: MiniRaptor / SC Merlin
« Reply #32 on: 04/20/2017 03:40 pm »
... 3 Raptors would be hard to fit under Falcon.

Not saying it will happen, but 3 x 1.51mØ Raptor 40s would fit pretty well on a 3.66mØ Falcon.

You're right. I was thinking they were 1.7m diameter for some reason. Those must be the ship landing Raptors.

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #33 on: 04/20/2017 04:25 pm »
You're right. I was thinking they were 1.7m diameter for some reason. Those must be the ship launch clamp landing Raptors.
See, fixed that for you! ;)

Online OneSpeed

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Re: MiniRaptor / SC Merlin
« Reply #34 on: 04/21/2017 08:22 am »
Right. You can't very well fit one in the center, but a triangular cluster would be just fine. You'd even have space for dedicated landing thrusters if desired.

Again, not that they'd do it...

From the Reddit AMA, the ITS vernier thrusters are 10mT thrust each, so 6 of them would be sufficient to land a Falcon with some fuel. Then again, rather than rely on two separate thrust systems for a successful landing, 7 half scale Raptors (the current development engine) would have comparable thrust to three full scale, and be able to land on the centre engine. Not saying they'll actually do that either ...

Offline sevenperforce

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Re: MiniRaptor / SC Merlin
« Reply #35 on: 04/23/2017 04:07 am »
Partly inspired by this thread, here's a dedicated thread on second-stage reuse configurations in the future: https://forum.nasaspaceflight.com/index.php?topic=42783.0

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