Author Topic: Precursor to Starship  (Read 7682 times)

Offline redneck

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Re: Precursor to Starship
« Reply #20 on: 06/03/2023 07:48 pm »

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.

that is wrong. Need to make up for a difference of 50% or more in less volume and not 10% less mass.
Density
RP-1  801–1020  g/L
Methane 422.8 g/L

and it is more like 45 seconds ISP difference but the 15 is not going to matter

Merlin 282 sec
Raptor 327 sec

10 kg of RP-1 is the same as 8.6 kg of Methane ISP wise

But 10 kg of RP-1 requires a tank of  9.8 liters but 8.6 kg of methane requires 20.3 liter tank

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

Offline Jim

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Re: Precursor to Starship
« Reply #21 on: 06/04/2023 01:57 pm »

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

Ah, no.  LOX does not "dominate" when paired with LOX (16% difference is not "dominate") The volumetric ratio for LOX to Methane on Vulcan is 1.16 to 1 (87k to 75k gallons) for Falcon 9 it is 1.67 to 1 (65k to 39k gallons of LOX to RP-1) (634 klb LOX, 263klb RP-1)

There is no way of maintaining the same Falcon 9 performance when switching to Methane and keeping the first stage dimensions fixed.

Per your 10% less propellant mass due to ISP increase, 10% less LOX mass Falcon 9 is 571Klbs, which is about 60k gallons.  60k of LOX pars with 52k gallons of Methane (1.16 to 1 ratio).   This is a total of 112k gallons
Total propellant load of Falcon 9 first stage is 104k gallons.  Where are the 8k gallons going to fly?

You are completely off base and wrong here.

Offline redneck

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Re: Precursor to Starship
« Reply #22 on: 06/04/2023 05:09 pm »
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.

Offline Jim

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Re: Precursor to Starship
« Reply #23 on: 06/04/2023 07:43 pm »
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.

Your numbers are wrong.  Methane needs more tank volume for equivalent impulse

Offline redneck

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Re: Precursor to Starship
« Reply #24 on: 06/04/2023 08:08 pm »
For momentary impulse true. For DeltaV over roughly 2,500 m/sec no. By the time you get over 3,000 m/sec, methane is a clear win. And gets better as mass ratio increases such that by the time 3,800 m/sec, methalox needs 6.4% less tank volume for the same velocity.

Offline Jim

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Re: Precursor to Starship
« Reply #25 on: 06/04/2023 10:10 pm »
Still doesn't fix the increased costs due to mixed propellants and the inability to land

Offline deltaV

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Re: Precursor to Starship
« Reply #26 on: 06/04/2023 11:00 pm »
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.

Offline deltaV

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Re: Precursor to Starship
« Reply #27 on: 06/04/2023 11:20 pm »
10. 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.

Offline redneck

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Re: Precursor to Starship
« Reply #28 on: 06/05/2023 06:19 am »
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.

It's SpaceX so sub cooling should be expected. Your last paragraph is the question to be investigated. People assume both directions, often without checking details. The details, and the integrity with which they are checked, matter enormously. 

Offline redneck

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Re: Precursor to Starship
« Reply #29 on: 06/05/2023 06:35 am »
10. 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.

Not sure of your definition of spam or poor quality, though some of the posts I responded to were not thought out. Many people that were not blinded by the successes of SpaceX noticed the potential problems years ago. I have been going back over the lists of posts from as much as 7 years ago on the Starship forums and finding them. (after my searches didn't work) Many of the arguments discussed the issue of going too big too fast. To me, getting your flight and infrastructure  experience from the largest launch vehicle in history was a questionable decision.

Not looking at what has been done, and what might should have been done, often leads to similar avoidable problems in the future. Can't live in the past, but ignoring it is also a problem.


Online edzieba

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Re: Precursor to Starship
« Reply #30 on: 06/05/2023 12:34 pm »
10. 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.
Except Starship development pace is faster than SpaceX expected. Which is why they scrapped the idea of Falcon 9 upper-stage recover, and scrapped the idea specifically of a subscale Starship upper stage for Falcon 9.

Developing a methalox Falcon 9 (or Falcon 9 first stage) would be sinking money into a developmental dead end, delaying work on Starship, hobbling performance (Falcon 9 is dimension limited by needing to be road-transportable), and for the solitary gain of the slight price/kg difference between RP-1 and LCH4.

Offline redneck

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Re: Precursor to Starship
« Reply #31 on: 06/05/2023 02:32 pm »


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. 

Online edzieba

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Re: Precursor to Starship
« Reply #32 on: 06/05/2023 03:47 pm »
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.
Falcon 9 is an operational launcher, it cannot tolerate a return to being a test programme with new unproven engines. It could only even be considered once Raptor would be reliable enough for use on Falcon 9 without disrupting operations... at which point it is also ready for use on Starship. Developing a second F9 variant in parallel to the operational Kerolox F9 would just wasting time and money developing a dead-end that could be used to develop Starship instead.

Offline pathfinder_01

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Re: Precursor to Starship
« Reply #33 on: 06/07/2023 09:59 pm »


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.

The problem is that by switching engines you are creating a totally new rocket and making profit harder. Certain Government launches require that a rocket be certified. This cerftifation requires a certain number of launches.  Basically a new launch vehicle can not compete for certain types of payloads. In addition the CCREW program contract requires that the Spacecraft be human rated and Falcon 9 is human rated to carry the Dragon capsule.  In addition launch insurance rates are based on flight history and the few flights a rocket has the higher the insurance.  There would be little point of replacing F9 with an new rocket until that new rocket can compete for all payloads that the F9 and FH do today.


Quote
Only have to find one glitch that eases Starship development to make it a strong win.   

There would far more than one glitch to find if something goes wrong. Rockets are more than just engines.

Quote
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.

Having F9 as it is now allows Starship to hit it's goals with less risk to the company.
« Last Edit: 06/07/2023 10:02 pm by pathfinder_01 »

Offline redneck

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Re: Precursor to Starship
« Reply #34 on: 06/07/2023 11:02 pm »
I asked originally for pointers to where this had been discussed before. Wasn't intending to get into a heavy discussion of the merits of the idea until finding out what had already been discussed.    Got no answers here, but did eventually find fruitful discussion of similar ideas back to 2015.

Offline spacenut

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Re: Precursor to Starship
« Reply #35 on: 06/08/2023 01:41 am »
Simple.  More methane is needed per volume of liquid oxygen, thus to achieve the same results as F9, you would need 3 raptor engines for the same or similar thrust, thus you would need a 5 or 5.5 meter diameter rocket.  Kerosene is very dense fuel vs methane, so you can build a smaller rocket using kerosene.  For instance, Atlas V is a 12' diameter rocket using kerosene.  Vulcan is a 5.5m diameter rocket using methane for similar payload results. 

A methane rocket with a given payload has to be larger than a kerosene rocket.  A hydrogen rocket has to be even larger than a methane rocket.  Sure the ISP can burn longer, but with less thrust per unit of fuel.  That is why in the past NASA used kerosene boosters in rockets like Saturn V with hydrogen upper stages since they could burn longer get get better orbital results. 

Compare Falcon 9 with all kerosene with Atlas V with kerosene booster and hydrogen upper.  Both get similar results with payloads.  Kerosene makes for a cheaper to build rocket, especially when lower stage and upper stage use the same basic engine.  SpaceX is doing this with Starship/Superheavy.  Same basic engine with both stages, thus making the rocket cheaper.  Hydrogen is great for upper stage engines but is more costly to build and operate. 

Methane was and is a compromise fuel.  Clean burning with no coking for reusable engines, and has a liquid temperature similar to liquid oxygen so equipment is cheaper.  Liquid hydrogen is way colder and requires more specialized equipment to maintain and operate as well as engines cost more. 

Methane is not a direct replacement for kerosene without having larger tanks.  There was a few years ago, a proposal to make a 5.5m upper stage for F9 or FH especially using a subscale Raptor at about the same thrust as a Merlin upper stage engine.  This would have improved payload somewhat, from what I remember about 10-15 tons.  Falcon Heavy has had no payloads large enough to justify it.  70-75 tons would have been the payload maximum.  Then if you added cross feed on the boosters maybe another 5-10 tons.  SpaceX determined it was not worth the effort as there was no payloads planned or in the near future to justify the expense. 

Offline redneck

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Re: Precursor to Starship
« Reply #36 on: 06/08/2023 09:11 am »
https://forum.nasaspaceflight.com/index.php?topic=35533.msg1249705#msg1249705

This is the best so far from 2014.  Chasing links there is some discussion of the concept from 2011.
Still working my way through previous discussions. When doing new hardware, I look hard for prior art, trying to do similar here.

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