Author Topic: Precursor to Starship  (Read 7528 times)

Offline redneck

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Precursor to Starship
« on: 06/03/2023 09:32 am »
I posted this elsewhere and am curious as to the previous discussions of this.  It is too obvious to have been missed by these forums. My search has been deficient in locating previous threads on the subject. Where has this been discussed previously??

Looking back, not forward.
1. The Falcon9 development cost is said to have been under $1B including Merlin, Falcon1, and upper stage.
2. The Raptor based Starship architecture is said to be cheaper per launch than the Falcon9.
2.5 Raptor and methane turnaround time is expected to be under one day.
3. A Raptor based drop in replacement for the Falcon9 booster should have cost less time and money than the Falcon9 to develop given the development history of SpaceX.
4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.
5. The performance advantage of the replacement would allow RTLS on every mission.
6. That leads to daily flight per booster opportunities at a booster compared cost or <1/6 of Falcon9 if point 2 is correct given <1/6 as many engines as SH.
7. The development flights of two years ago only with Falcon standard landing procedures could have had a vehicle in operational service at least a year ago.
8. Assuming most of the above is a fair take, Raptor based Falcon9 boosters could be flying daily revenue flights today at a fraction of the cost of the current Falcon9.
9. There would be a flight experienced body of knowledge right now pertaining to operational strengths and weaknesses of the methane based vehicle. This would extend to high cadence operational flight crews.
10. This concept is if these decisions had been take years ago and is likely not reasonable going forward.
11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.

Offline Jim

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Re: Precursor to Starship
« Reply #1 on: 06/03/2023 02:51 pm »

4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.


No.
A.  Raptor would be too high of thrust for F9 second stage. 
b.  Raptor thrust would be too high for landing the first stage.  (the Merlin as is can't throttle down low enough to allow hovering, hence the "hover slam".
c.  tank sizes would have to increase, changing all the infrastructure.  (first stage may no longer be road transportable)

Offline Jim

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Re: Precursor to Starship
« Reply #2 on: 06/03/2023 02:51 pm »

11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.

there is very much so

the idea fails at every point.
« Last Edit: 06/03/2023 02:53 pm by Jim »

Online mandrewa

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Re: Precursor to Starship
« Reply #3 on: 06/03/2023 03:16 pm »
I posted this elsewhere and am curious as to the previous discussions of this.  It is too obvious to have been missed by these forums. My search has been deficient in locating previous threads on the subject. Where has this been discussed previously??

Looking back, not forward.
1. The Falcon9 development cost is said to have been under $1B including Merlin, Falcon1, and upper stage.
2. The Raptor based Starship architecture is said to be cheaper per launch than the Falcon9.
2.5 Raptor and methane turnaround time is expected to be under one day.
3. A Raptor based drop in replacement for the Falcon9 booster should have cost less time and money than the Falcon9 to develop given the development history of SpaceX.
4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.
5. The performance advantage of the replacement would allow RTLS on every mission.
6. That leads to daily flight per booster opportunities at a booster compared cost or <1/6 of Falcon9 if point 2 is correct given <1/6 as many engines as SH.
7. The development flights of two years ago only with Falcon standard landing procedures could have had a vehicle in operational service at least a year ago.
8. Assuming most of the above is a fair take, Raptor based Falcon9 boosters could be flying daily revenue flights today at a fraction of the cost of the current Falcon9.
9. There would be a flight experienced body of knowledge right now pertaining to operational strengths and weaknesses of the methane based vehicle. This would extend to high cadence operational flight crews.
10. This concept is if these decisions had been take years ago and is likely not reasonable going forward.
11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.


There's a lot to discuss here, but I'll focus on your assertion that "The Raptor based Starship architecture is said to be cheaper per launch than the Falcon9."

Elon Musk is talking about the aspirational, marginal cost of a Starship launch.  First of all, marginal cost does not include development costs.  Marginal cost treats everything that came before as a sunk cost, and asks what it costs to do one more launch.  We can gradually work down the marginal cost by making every step of whatever goes into that more efficient.

And that gets us to what aspirational means.  Aspirational means, in this context, what happens if you push things to the limit.  It's not what is just around the corner.  It is not what is happening now.

It's where you can get if you spend many years pursuing a course.

Elon Musk is fond of saying something like, "If success is not of the possible outcomes, you shouldn't be pursuing it."  And that is a profound point.  People do miss this.

So if you go back to a certain point in time, Elon Musk was making aspirational, marginal cost statements about the Falcon 9.  And then he dropped that completely and switched to the Starship.

Why?  Because it had become apparent that even with the most aspirational, marginal cost assertion that Elon Musk could persuade himself might be true about an improved Falcon 9, it just wasn't going to be sufficient to get people to Mars.

So to get back to what you are saying.  You are mixing things up here.  Your confusing an assertion about the future with the present.


Online StarshipTrooper

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Re: Precursor to Starship
« Reply #4 on: 06/03/2023 03:21 pm »
Elon thinking:
Would a starship precursor help reach Mars sooner?

No.

“I'm very confident that success is within the set of possible outcomes.”  Elon Musk

Offline redneck

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Re: Precursor to Starship
« Reply #5 on: 06/03/2023 06:19 pm »

4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.


No.
A.  Raptor would be too high of thrust for F9 second stage. 
b.  Raptor thrust would be too high for landing the first stage.  (the Merlin as is can't throttle down low enough to allow hovering, hence the "hover slam".
c.  tank sizes would have to increase, changing all the infrastructure.  (first stage may no longer be road transportable)

A. the stock Falcon9 upper stage would be used. I never suggested Raptor for the second.
B. SpaceX has perfected the hover slam and certainly has the expertise to do the same with another engine.
C. Tank sizes could remain the same as the Isp advantage would negate the requirement for larger tank. Large would be a nice to have rather than necessity. Glow would be less and acceleration higher. Still road transportable.

Offline redneck

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Re: Precursor to Starship
« Reply #6 on: 06/03/2023 06:22 pm »

11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.

there is very much so

the idea fails at every point.

Since Raptor is charged off to the Starship project, and the stock Falcon9 upper stage would be used, you will need to explain why the booster structure would be such a problem.

Offline redneck

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Re: Precursor to Starship
« Reply #7 on: 06/03/2023 06:28 pm »


""So to get back to what you are saying.  You are mixing things up here.  Your confusing an assertion about the future with the present.""


Fair enough. If Elon and company believe that Starship will be so cheap to operate, It seems that a much simpler and smaller vehicle based on know flight dynamics would also be cheaper. Not to mention much faster into revenue service.

Offline spacenut

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Re: Precursor to Starship
« Reply #8 on: 06/03/2023 06:30 pm »
Smaller rockets can't get the mass to orbit, much less Mars as efficient as larger rockets. 

Offline Jim

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Re: Precursor to Starship
« Reply #9 on: 06/03/2023 06:30 pm »

11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.

there is very much so

the idea fails at every point.

Since Raptor is charged off to the Starship project, and the stock Falcon9 upper stage would be used, you will need to explain why the booster structure would be such a problem.

It removes the commonality between the stages and that is one of largest cost reducing factors.

Offline redneck

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Re: Precursor to Starship
« Reply #10 on: 06/03/2023 06:31 pm »
Elon thinking:
Would a starship precursor help reach Mars sooner?

No.

Unless it would have walked point on some of the problems with the larger vehicle. Somewhat as the problems with Falcon1 prevented the same problems with Falcon9.

I'm sure something of this nature has been discussed before. Does anyone know where and when?

Offline redneck

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Re: Precursor to Starship
« Reply #11 on: 06/03/2023 06:33 pm »
Smaller rockets can't get the mass to orbit, much less Mars as efficient as larger rockets.

Falcon9 seems to be getting plenty of mass to orbit.

Online EspenU

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Re: Precursor to Starship
« Reply #12 on: 06/03/2023 06:36 pm »



4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.


No.
A.  Raptor would be too high of thrust for F9 second stage. 
b.  Raptor thrust would be too high for landing the first stage.  (the Merlin as is can't throttle down low enough to allow hovering, hence the "hover slam".
c.  tank sizes would have to increase, changing all the infrastructure.  (first stage may no longer be road transportable)
C. Tank sizes could remain the same as the Isp advantage would negate the requirement for larger tank. Large would be a nice to have rather than necessity. Glow would be less and acceleration higher. Still road transportable.

Don't know about the overall tank size, but the Merlin and Raptor mixture ratios are different, so the tank sizes would at least have to change internally.

In addition the fuel is now cryogenic, which would again require changes.

You also loose a lot of commonality between the stages, which will affect operations.

Offline redneck

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Re: Precursor to Starship
« Reply #13 on: 06/03/2023 06:37 pm »

11. There is very little reason to believe that this would have been a multi year and multi billion dollar detour.

there is very much so

the idea fails at every point.

Since Raptor is charged off to the Starship project, and the stock Falcon9 upper stage would be used, you will need to explain why the booster structure would be such a problem.

It removes the commonality between the stages and that is one of largest cost reducing factors.

Same engines and structural method as Starship. Same size as Falcon9. My Ford trucks have little commonality with my Chevrolet trucks and really doesn't matter for vehicles in continuous use.

Offline redneck

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Re: Precursor to Starship
« Reply #14 on: 06/03/2023 06:40 pm »



4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.


No.
A.  Raptor would be too high of thrust for F9 second stage. 
b.  Raptor thrust would be too high for landing the first stage.  (the Merlin as is can't throttle down low enough to allow hovering, hence the "hover slam".
c.  tank sizes would have to increase, changing all the infrastructure.  (first stage may no longer be road transportable)
C. Tank sizes could remain the same as the Isp advantage would negate the requirement for larger tank. Large would be a nice to have rather than necessity. Glow would be less and acceleration higher. Still road transportable.

Don't know about the overall tank size, but the Merlin and Raptor mixture ratios are different, so the tank sizes would at least have to change internally.

In addition the fuel is now cryogenic, which would again require changes.

You also loose a lot of commonality between the stages, which will affect operations.

Mixture ratios being different would require one tank to grow and the other to shrink. Another ring on one and lose a ring on the other. Many vehicles operate with different propellants in different stages.

Offline Jim

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Re: Precursor to Starship
« Reply #15 on: 06/03/2023 06:42 pm »

4, Raptor with much higher Isp and more than double the thrust of Merlin would have that replacement viable with 5 engines.


No.
A.  Raptor would be too high of thrust for F9 second stage. 
b.  Raptor thrust would be too high for landing the first stage.  (the Merlin as is can't throttle down low enough to allow hovering, hence the "hover slam".
c.  tank sizes would have to increase, changing all the infrastructure.  (first stage may no longer be road transportable)

A. the stock Falcon9 upper stage would be used. I never suggested Raptor for the second.
B. SpaceX has perfected the hover slam and certainly has the expertise to do the same with another engine.
C. Tank sizes could remain the same as the Isp advantage would negate the requirement for larger tank. Large would be a nice to have rather than necessity. Glow would be less and acceleration higher. Still road transportable.

A.  Stock second stage means mixed commodities at the pad and different GSE and requirements = more costs
mixed commodities mean different loading requirements = this will change the countdown sequence and cause problems with

b.  It is too high of high thrust for the hover slam. 

c.  Wrong.  If that were true, then why not go LH2 and even a greater increase ISP.  A larger tank is a necessity.   the ISP difference only affect the mass, it is not enough to make up for the difference in volume(energy density).  ISP is the difference in energy per mass unit.   Methane is about the twice the volume for the same mass of RP-1

Look at Atlas V vs Delta IV (same capability different propellant volume)
« Last Edit: 06/03/2023 06:54 pm by Jim »

Offline Jim

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Re: Precursor to Starship
« Reply #16 on: 06/03/2023 06:44 pm »

Mixture ratios being different would require one tank to grow and the other to shrink. Another ring on one and lose a ring on the other. .

wrong.

That doesn't work and still have the same lift capability.
Intuitively, that means there is less oxygen to burn and hence less total impulse.

Many vehicles operate with different propellants in different stages.

And they are not low cost

« Last Edit: 06/03/2023 06:47 pm by Jim »

Offline redneck

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Re: Precursor to Starship
« Reply #17 on: 06/03/2023 06:48 pm »


c.  Wrong.  If that were true, then why not go LH2 and even a greater increase ISP.  A larger tank is a necessity.   the ISP difference only affect the mass, it is not enough to make up for the difference in volume(energy density).

Look at Atlas V vs Delta IV (same capability different propellant volume)

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.

Online mandrewa

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Re: Precursor to Starship
« Reply #18 on: 06/03/2023 07:01 pm »
Elon thinking:
Would a starship precursor help reach Mars sooner?

No.

Unless it would have walked point on some of the problems with the larger vehicle. Somewhat as the problems with Falcon1 prevented the same problems with Falcon9.

I'm sure something of this nature has been discussed before. Does anyone know where and when?

The Raptor engine was originally intended for the second stage of the Falcon 9.  It was a much smaller engine at that point and it ran on hydrogen and oxygen, not methane and oxygen.


So it's not like SpaceX didn't realize this possibility -- for a while they were actually doing it.


A second stage engine fueled by hydrogen would have been part of the natural evolution of the Falcon 9 if they were trying to maximize mass to orbit, but that's not necessarily the case.  I'm pretty sure Elon measures things more by dollar per kilogram to orbit than maximum mass to orbit, and in that respect, as Jim points out above, it's not at all obvious that a Falcon 9 with the second stage fueled by hydrogen would be cheaper.  Or if it was eventually cheaper, it might take a long time to get there.

Look, Starship will hopefully win on both of the metrics that count in the near future:

(a) dollars per kg to LEO; and

(b) making missions to Mars practical and feasible.

The Falcon 9 has been incredibly successful, and I expect SpaceX will still be doing Falcon 9 missions five years from now.

But SpaceX does not have a limitless amount of money.  They are investing in Starship and baring disaster I don't believe they will try to improve the Falcon 9 any further.

Offline Jim

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Re: Precursor to Starship
« Reply #19 on: 06/03/2023 07:11 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
« Last Edit: 06/03/2023 07:13 pm by Jim »

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.


Offline 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. 

Offline 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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