Methane RS-68

[JEF_300]

Preface: Yes, I absolutely understand that rocket engines are fundamentally designed around their propellant of choice, and that that cannot be changed without at least a partial redesign of the entire engine. That said, converting engines to run on other fuels has been done before (LH2 LR87, RL-10 on various fuels, various Russian programs), so it is a reasonable question to ask if it's possible with a specific engine and fuel, and whether or not that would be worthwhile.

I was thinking about the troubles ULA is having with Blue Origin recently, and found myself thinking, as I have before, about whether or not ULA has or had any other options for propelling Vulcan. The answer, as always, was that aside from the AR1, or perhaps building an entirely different RS-25 based monstrosity, they really didn't have another option.

The closest thing I have ever been able to come up with is an RS-68 that burns LOX and a hydrocarbon instead of hydrogen. But such an engine would be about 1.5x (7/5ths) the thrust of the BE-4 and AR1; small enough that one isn't enough, but big enough that two is too much. Not to mention that the RS-68 is huge, so just fitting two under a 5.4m core couldn't be easy.

Usually I just dismiss the idea outright at that point, but I decided today, for whatever reason, that I was curious if it was ever even feasible to begin with. So, converting the RS-68 to use methane as the fuel instead of hydrogen; plausible or not? And if it is plausible, would it be a cheaper development program than developing a brand new engine of the same size?

[Vahe231991]

Preface: Yes, I absolutely understand that rocket engines are fundamentally designed around their propellant of choice, and that that cannot be changed without at least a partial redesign of the entire engine. That said, converting engines to run on other fuels has been done before (LH2 LR87, RL-10 on various fuels, various Russian programs), so it is a reasonable question to ask if it's possible with a specific engine and fuel, and whether or not that would be worthwhile.

I was thinking about the troubles ULA is having with Blue Origin recently, and found myself thinking, as I have before, about whether or not ULA has or had any other options for propelling Vulcan. The answer, as always, was that aside from the AR1, or perhaps building an entirely different RS-25 based monstrosity, they really didn't have another option.

The closest thing I have ever been able to come up with is an RS-68 that burns LOX and a hydrocarbon instead of hydrogen. But such an engine would be about 1.5x (7/5ths) the thrust of the BE-4 and AR1; small enough that one isn't enough, but big enough that two is too much. Not to mention that the RS-68 is huge, so just fitting two under a 5.4m core couldn't be easy.

Usually I just dismiss the idea outright at that point, but I decided today, for whatever reason, that I was curious if it was ever even feasible to begin with. So, converting the RS-68 to use methane as the fuel instead of hydrogen; plausible or not? And if it is plausible, would it be a cheaper development program than developing a brand new engine of the same size?

The RS-68 is only used on the Delta IV Heavy's boosters and first stage, so with the Delta IV Heavy approaching retirement, SpaceX or Blue Origin might consider using the RS-68 as a potential engine for any advanced versions of the New Glenn and Starship. Also note that the RS-68 was also planned for the canceled Ares V rocket.

[Redclaws]

The RS-68 is only used on the Delta IV Heavy's boosters and first stage, so with the Delta IV Heavy approaching retirement, SpaceX or Blue Origin might consider using the RS-68 as a potential engine for any advanced versions of the New Glenn and Starship. Also note that the RS-68 was also planned for the canceled Ares V rocket.

Where on earth do you get the idea that SpaceX would use the RS-68 for anything?

[Vahe231991]

The RS-68 is only used on the Delta IV Heavy's boosters and first stage, so with the Delta IV Heavy approaching retirement, SpaceX or Blue Origin might consider using the RS-68 as a potential engine for any advanced versions of the New Glenn and Starship. Also note that the RS-68 was also planned for the canceled Ares V rocket.

Where on earth do you get the idea that SpaceX would use the RS-68 for anything?

I brought it up because the RS-68 is currently used in the Delta IV only, and someone on this thread thought about an RS-68 fueled by both methane and liquid oxygen. The RS-68 is a simple engine cost-effective when used for a single launch, yet has more thrust than either the BE-4 or Raptor. Therefore, if SpaceX wants to build a multiplanetary version of the Starship, that variant would need to use the RS-68 for both the Super Heavy booster and Starship second stage to achieve enough thrust to reach the gas giants.

[joek]

...
Usually I just dismiss the idea outright at that point, but I decided today, for whatever reason, that I was curious if it was ever even feasible to begin with. So, converting the RS-68 to use methane as the fuel instead of hydrogen; plausible or not? And if it is plausible, would it be a cheaper development program than developing a brand new engine of the same size?

No. Many reasons.. Look at the $ to move from RS-68 to RS-68A. Converting to new propellant mix would undoubtedly require more $$. Not to mention human rating requirements if there are aspirations to launching crew (one reason for dropping it as an Ares option). And if reusability is in the picture, forget it; these are single use engines.

[joek]

...
Therefore, if SpaceX wants to build a multiplanetary version of the Starship, that variant would need to use the RS-68 for both the Super Heavy booster and Starship second stage to achieve enough thrust to reach the gas giants.

No it would not "need to use the RS-68". Something like it, maybe. In any case, what is needed to reach gas giants has been discussed in other threads. And no, "achieve enough thrust" is not the issue for such outer-planet missions, unless your focus is time-of-travel. In any case, OT for this thread; take it elsewhere please.

[Robotbeat]

The RS-68 is only used on the Delta IV Heavy's boosters and first stage, so with the Delta IV Heavy approaching retirement, SpaceX or Blue Origin might consider using the RS-68 as a potential engine for any advanced versions of the New Glenn and Starship. Also note that the RS-68 was also planned for the canceled Ares V rocket.

Where on earth do you get the idea that SpaceX would use the RS-68 for anything?

I brought it up because the RS-68 is currently used in the Delta IV only, and someone on this thread thought about an RS-68 fueled by both methane and liquid oxygen. The RS-68 is a simple engine cost-effective when used for a single launch, yet has more thrust than either the BE-4 or Raptor. Therefore, if SpaceX wants to build a multiplanetary version of the Starship, that variant would need to use the RS-68 for both the Super Heavy booster and Starship second stage to achieve enough thrust to reach the gas giants.

That makes no sense.

RS68 has maybe 30% more thrust than Raptor but costs 20 times as much.

Better to either upgrade Raptor’s thrust slightly (as is planned) or add a few more Raptors.

RS68 is also ablative, non-reusable. It also is lower chamber pressure thus the Isp would suffer with methane and its thrust per unit area will be lower, meaning you would actually reduce the thrust of a Super Heavy booster if you maxed out the space available with RS68 instead of Raptors.

[russianhalo117]

Preface: Yes, I absolutely understand that rocket engines are fundamentally designed around their propellant of choice, and that that cannot be changed without at least a partial redesign of the entire engine. That said, converting engines to run on other fuels has been done before (LH2 LR87, RL-10 on various fuels, various Russian programs), so it is a reasonable question to ask if it's possible with a specific engine and fuel, and whether or not that would be worthwhile.

I was thinking about the troubles ULA is having with Blue Origin recently, and found myself thinking, as I have before, about whether or not ULA has or had any other options for propelling Vulcan. The answer, as always, was that aside from the AR1, or perhaps building an entirely different RS-25 based monstrosity, they really didn't have another option.

The closest thing I have ever been able to come up with is an RS-68 that burns LOX and a hydrocarbon instead of hydrogen. But such an engine would be about 1.5x (7/5ths) the thrust of the BE-4 and AR1; small enough that one isn't enough, but big enough that two is too much. Not to mention that the RS-68 is huge, so just fitting two under a 5.4m core couldn't be easy.

Usually I just dismiss the idea outright at that point, but I decided today, for whatever reason, that I was curious if it was ever even feasible to begin with. So, converting the RS-68 to use methane as the fuel instead of hydrogen; plausible or not? And if it is plausible, would it be a cheaper development program than developing a brand new engine of the same size?

RS-68 Family production, tooling, test stands, supply chains, and production line space have already been retired and released to other programmes by AR. There are no plans to extend or resurrect the engine family. All engines needed to fly out DIVH have been built and placed in storage. I suggest picking a different engine for this thread. Plus I think I recall another thread already discussed this.

[Vahe231991]

The RS-68 is only used on the Delta IV Heavy's boosters and first stage, so with the Delta IV Heavy approaching retirement, SpaceX or Blue Origin might consider using the RS-68 as a potential engine for any advanced versions of the New Glenn and Starship. Also note that the RS-68 was also planned for the canceled Ares V rocket.

Where on earth do you get the idea that SpaceX would use the RS-68 for anything?

I brought it up because the RS-68 is currently used in the Delta IV only, and someone on this thread thought about an RS-68 fueled by both methane and liquid oxygen. The RS-68 is a simple engine cost-effective when used for a single launch, yet has more thrust than either the BE-4 or Raptor. Therefore, if SpaceX wants to build a multiplanetary version of the Starship, that variant would need to use the RS-68 for both the Super Heavy booster and Starship second stage to achieve enough thrust to reach the gas giants.

That makes no sense.

RS68 has maybe 30% more thrust than Raptor but costs 20 times as much.

Better to either upgrade Raptor’s thrust slightly (as is planned) or add a few more Raptors.

RS68 is also ablative, non-reusable. It also is lower chamber pressure thus the Isp would suffer with methane and its thrust per unit area will be lower, meaning you would actually reduce the thrust of a Super Heavy booster if you maxed out the space available with RS68 instead of Raptors.

The idea behind the RS-68 was to make a low-cost rocket engine using 80 percent fewer components than the engines of the Space Shuttle. Someday, ULA might opt to use the RS-68 for the proposed three-core variant of the Vulcan rocket, the Vulcan Heavy.

[JEF_300]

I suggest picking a different engine for this thread. Plus I think I recall another thread already discussed this.

I'm aware that the RS-68 is being retired, and even if it wasn't I seriously doubt anyone would ever bother to make a methane RS-68. Frankly, the best use case of a hydrocarbon fueled RS-68 is on the first stage of an alternate SLS... designed a decade ago. This thread was always going to be more of a "what if".

RS68 is also ablative, non-reusable.

Just taking this opportunity to note that if you were going to make this engine, you could switch it to a regenerative nozzle. Plans to do that to the RS-68 have existed since the 90s.

I suppose you could also try to increase the chamber pressure, go full Merlin... but at the scale of the RS-68, that sounds expensive.