Quote from: redneck on 06/03/2023 06:48 pmBecause we are discussing using an engine that SpaceX has in hand. If you will do the numbers, you will find that the 60 seconds difference in sea level Isp of Raptor against Merlin, more than makes up for having roughly 10% less propellant. that is wrong. Need to make up for a difference of 50% or more in less volume and not 10% less mass.DensityRP-1 801–1020 g/LMethane 422.8 g/Land it is more like 45 seconds ISP difference but the 15 is not going to matterMerlin 282 secRaptor 327 sec10 kg of RP-1 is the same as 8.6 kg of Methane ISP wiseBut 10 kg of RP-1 requires a tank of 9.8 liters but 8.6 kg of methane requires 20.3 liter tank
Because we are discussing using an engine that SpaceX has in hand. If you will do the numbers, you will find that the 60 seconds difference in sea level Isp of Raptor against Merlin, more than makes up for having roughly 10% less propellant.
And the LO2 tank dominates both though more so with the methane as it is a higher percentage of the total. It is the total tank volume for both propellants that matters. And that is within 10%. 16% higher Isp (using your numbers) will more than compensate for 10% less propellant mass
I started this thread asking for pointers to the previous discussions on the subject. No joy there though I did finally manage to find a 2017 thread with some bearing on it. Not intending to get into a heavy technical discussion, I didn't bother with the calculator.My thought was that a drop in replacement for the Falcon9 booster would be a relatively inexpensive route to methane vehicle and launch experience which would pay off in knowledge applied to Starship/Superheavy. With Raptor development charged off to Starship, and the standard Falcon9 upper stage used as is, the development would consist of a new booster with the new methane tehnology. The idea is that if the methane raptor with it's vehicles will be cheaper to operate than the kerosene Merlins, this lower stage could cover many of the duties currently carried out by the standard Falcon9. This would move towards lower cost than the Merlin system with experience if SpaceX is right about the advantages of Methane. This sub scale vehicle at Falcon size would turn a profit while gathering data gaining experience with the new propellant.The main argument against seems to be that the density of the methane would prevent it from use as a 12 foot diameter booster replacement. I finally went back towards first principles and checked a few things. I am working it as the Kero/LOX vehicle as a mass ratio of 3, and the methane version matching the performance of getting one ton to the same velocity at MECO.A three to one mass ratio kerosene engine (Merlin) getting 282 seconds Isp will attain 3,039 meters per second at MECO. For each ton at MECO there would have been 600 kg of kerosene and 1,400 kg of LOX. Kerpsene seems to reach a density of 0.89 when subcooled so there would be 0.674 cubic meters of tank. The 1,400 kg of LOX at 1.2 gets 1.167 cubic meters of tank for a total of 1.841 cubic meters of tanks.With the methane engine (Raptor) getting 327 seconds Isp, reaching that 3,039 meters per second requires a mass ratio of 2.58. The 348 kg of methane at a subcooled density of 0.45 would take up 0.773 cubic meters of tank. The 1,232 kg of LOX would require 1.027 cubic meters of tank. This combination would have 1.797 cubic meters of tank to reach the same velocity. For the same performance, the methalox tanks would be about 2.5% smaller than the kerolox tanks. Of course there are several more factors involved, but tank volume is not an issue.
10. This concept is if these decisions had been take years ago and is likely not reasonable going forward.
I think redneck is assuming subcooling whereas Jim is not. ULA's Vulcan also doesn't use subcooling I think. Subcooling may therefore be part of the reason why redneck is getting different results than usual. Another reason is redneck seems to have neglected to account for the lower thrust to weight ratio of Raptor compared to Merlin, which means the mass at first stage engine cutoff will be higher for the same payload mass.BTW even if redneck is right that a Raptor-engined F9 is possible that doesn't make it a good idea. Letting SpaceX phase out Merlin a few years earlier would be nice but I doubt it would be a big enough win to be worth the cost of developing a new rocket stage.
Quote from: redneck on 06/03/2023 09:32 am10. This concept is if these decisions had been take years ago and is likely not reasonable going forward.redneck spammed so many posts that were either poor quality or looked that way that I didn't read them very carefully and missed this point. If SpaceX had known a decade ago that Starship development would take a lot longer than expected something like this might have made sense. But there's no way SpaceX could have known that so I don't think it's worthwhile to discuss what SpaceX would have done if they had a time machine.
Actually, the reason was to gain a lot of Methalox experience in the safest and simplest manner. Using it as a Falcon9 booster was mainly to offset costs with the possibility of profit. Only have to find one glitch that eases Starship development to make it a strong win.
Actually, the reason was to gain a lot of Methalox experience in the safest and simplest manner. Using it as a Falcon9 booster was mainly to offset costs with the possibility of profit.
Only have to find one glitch that eases Starship development to make it a strong win.
If Starship hits a launch cadence and decent price point in the next few years, then the idea wouldn’t have merit. But significant delays and costs could prove the opposite. I finally found previous discussions on similar ideas. I want to work through a few of them to see what has already been discussed.