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.
Quote from: macpacheco on 04/16/2017 07:50 pmWhen 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.
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.
Quote from: TheKutKu on 04/19/2017 04:29 pmCould 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.
Quote from: envy887 on 04/19/2017 04:54 pmQuote from: TheKutKu on 04/19/2017 04:29 pmCould 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.
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) = 1.1005Using the astronautix number for un-densified RP-1 with the densified LOX number:(2.56*1.2817+0.806)/(1+2.56) = 1.1481Gives 0.9586 as the ratio.
The higher density propellant is preferred to minimize tank weight which is proportional to volume.
Quote from: TheKutKu on 04/19/2017 06:07 pmQuote from: envy887 on 04/19/2017 04:54 pmQuote from: TheKutKu on 04/19/2017 04:29 pmCould 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.)/(424*3.8/1140+1) = 843 kg/m3If you do this: (424+1140*3./(1+3. = 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.
Quote from: livingjw on 04/19/2017 06:39 pmThe 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.
Quote from: sevenperforce on 04/19/2017 07:04 pmQuote from: livingjw on 04/19/2017 06:39 pmThe 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 and equal vehicle TWR.
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.
Quote from: sevenperforce on 04/19/2017 08:11 pmWell, 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.
Quote from: envy887 on 04/19/2017 08:35 pmQuote from: sevenperforce on 04/19/2017 08:11 pmWell, 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)
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.
... 3 Raptors would be hard to fit under Falcon.
Quote from: envy887 on 04/20/2017 12:30 am... 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.
Quote from: envy887 on 04/20/2017 12:30 amBut 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.Quote from: OneSpeed on 04/20/2017 01:05 pmQuote from: envy887 on 04/20/2017 12:30 am... 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...
Quote from: sevenperforce on 04/20/2017 01:43 pmRight. 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. 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...
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.
You're right. I was thinking they were 1.7m diameter for some reason. Those must be the ship launch clamp landing Raptors.