RS-68

[Aeneas]

Is someone out there actually ashamed for the RS-68?

TWR of ~ 50
vacuum Isp of ~412 s
and still ~ 14 m USD?

[MATTBLAK]

The RS-68 is a 'dead engine walking'. When Delta IV-Heavy goes, so will the engine. It's unlikely to be used on another booster ever again. Yes, it's relatively cheap, yes it's relatively powerful for a LH2 fueled engine. But it can't be air-started and it weighs more than 14,000 pounds (6.3 metric tons) so it would make a lousy upper stage engine. It weighs so much because of it's huge ablative nozzle.

The only real use I could see for it again would be if funding were allocated to turn that nozzle into a regeneratively cooled one like the Shuttle/SLS's RS-25 engines. The specific impulse would increase and then there would be no problem clustering them closely together as was first proposed in Constellation. The Isp increase on a big thrusting engine like RS-68 would give the SLS Corestage a modest payload increase to orbit. Five, maybe 7 metric tons.

But such an upgrade to RS-68 would not come cheap and the SLS would not have a high enough flight rate to justify the multi-billion dollar upgrade so...

[ugordan]

Ashamed? Not really. It was good enough for what it was meant to be at the time: a low(er) cost booster engine alternative to the SSME. As an engine, I think it's an indicator on where the US went wrong - focusing pretty much all of the development on either LH2 engines or solid rockets after Apollo, neglecting hydrocarbon engines.

Unfortunately, the choice of a LH2 booster engine and the low impulse density of that propellant made it uncompetitive with the more traditional approach of a hydrocarbon engine for boost stage, LH2 for upper stage. The large tankage and associated difficulties with cryogenic hydrogen were apparently enough to drown out any Isp advantage over hydrocarbon booster engines.* That's not to say a U.S. made staged combustion RP-1 engine would have been any cheaper, but the overall system cost probably would have been lower.

* Atlas V has entered the chat

[Aeneas]

The RS-58 is a 'dead engine walking'. When Delta IV-Heavy goes, so will the engine. It's unlikely to be used on another booster ever again. Yes, it's relatively cheap, yes it's relatively powerful for a LH2 fueled engine. But it can't be air-started and it weighs more than 14,000 pounds (6.3 metric tons) so it would make a lousy upper stage engine. It weighs so much because of it's huge ablative nozzle.

The only real use I could see for it again would be if funding were allocated to turn that nozzle into a regeneratively cooled one like the Shuttle/SLS's RS-25 engines. The specific impulse would increase and there would be no problem clustering them closely together as was first proposed in Constellation. The Isp increase on a big thrusting engine like RS-68 would give the SLS Corestage a modest payload increase to orbit. Five, maybe 7 metric tons.

But such an upgrade to RS-68 would not come cheap and the SLS would not have a high enough flight rate to justify the multi-billion dollar upgrade so...

True, true... But maybe it should not go through a re-design but new approach using Hydrolox for the 1. stage. RS-68 has about 120ish bar. Apparently there are alloys allowing for more than doubling this value. Going beyond 250 bar would drastically reduce size, probably increasing the TWR and adding FFSC probably would add some amazing SL Isp. I played around with RPA and adjusting SSME to 262 bar chamber pressure and an expansion ratio of ~35, it'll generate >404 s of SL Isp, going >430 s in vacuum. This would actually make the first stage much lighter than hydrocarbon riding rockets.

[baldusi]

So you are saying that if you use a different cycle, with 80s technology, for a different use, for a not needed requirement, hitting never achieved specs on an hydrolox engine, and without doing the numbers, you wish it to be amazing?

[dglow]

Can anyone recommend good articles or books examining the development of Delta IV – what led McDonnell to choose hydrolox for their boosters?

[GreenShrike]

This would actually make the first stage much lighter than hydrocarbon riding rockets.

...and why does this matter? Kerosene isn't exactly expensive, methane is even cheaper and, combined with lox (also very low cost), either would comprise the vast majority of the stage's mass.

You're effectively suggesting a very expensive redesign, to end up producing expensive* engines, just so you could save the cost of a bunch of cheap fuel and maybe a barrel section or two of tankage?

*When your competition is mass producing hydrocarbon engines for well under $1M (and has a target under $250K for its twice-as-powerful replacement), any hydrolox engine is more than likely going to be expensive.**

$14M  for an RS-68, you say? Consider that if Raptor only gets down to $500K, you could outfit a Superheavy with engines for about the same cost as powering a Delta IV Medium. If Raptors were $1M each, then a Superheavy would still cost less to power than a Delta IV Heavy.

While the disadvantage of an expensive first stage might be mitigated via reusability, that just ends up driving the cost per flight down to the cost of your propellant, and LH2 would remain expensive (and a pain to work with). And you'll less easily be able to expend an expensive booster stage if you happen to need more performance on a given mission.


Overall, I'd say admiring the RS-68 (and, even more so, the RS-25) is like admiring a fine Maserati, with its wonderful craftsmanship and marvelous engineering. Except if you just care about getting the job done, then a basic Ford F-150 pickup truck will do just as well. And if we want to truly expand into space, a hundred F-150 equivalents will serve us a hell of a lot better than a few showpieces, no matter how pretty and fast the showpieces might be.

So, no, there's nothing shameful about the RS-68 -- it just belongs in the past. The world has moved on, and before any redesign of old technology is warranted, there needs to be sufficient justification of the result against the current state-of-the-art -- and not in terms of ISP, but rather in terms of cost effectiveness.


** And if you really want to stick with hydrolox, I'd ignore the RS-68 and take a closer look at the LE-9. If JAXA and Mitsubishi can hit the H3's price targets, then its a hydrolox booster engine that might actually be cost effective, at least vis-a-vis the Merlin (absent F9 reusability, at any rate).

[Aeneas]

So you are saying that if you use a different cycle, with 80s technology, for a different use, for a not needed requirement, hitting never achieved specs on an hydrolox engine, and without doing the numbers, you wish it to be amazing?

Yes. Isn't it obvious? ^^

I'm well aware, that this is highly speculative, I just tend to dream a bit about possible rocket futures.

But regarding requirements, I wouldn't go that way. It's risky but having a technology in place offering cheap and large unit access to space will probably provoke the requirement once it's there.

[Aeneas]

This would actually make the first stage much lighter than hydrocarbon riding rockets.

...and why does this matter? Kerosene isn't exactly expensive, methane is even cheaper and, combined with lox (also very low cost), either would comprise the vast majority of the stage's mass.

You're effectively suggesting a very expensive redesign, to end up producing expensive* engines, just so you could save the cost of a bunch of cheap fuel and maybe a barrel section or two of tankage?

*When your competition is mass producing hydrocarbon engines for well under $1M (and has a target under $250K for its twice-as-powerful replacement), any hydrolox engine is more than likely going to be expensive.**

$14M  for an RS-68, you say? Consider that if Raptor only gets down to $500K, you could outfit a Superheavy with engines for about the same cost as powering a Delta IV Medium. If Raptors were $1M each, then a Superheavy would still cost less to power than a Delta IV Heavy.

While the disadvantage of an expensive first stage might be mitigated via reusability, that just ends up driving the cost per flight down to the cost of your propellant, and LH2 would remain expensive (and a pain to work with). And you'll less easily be able to expend an expensive booster stage if you happen to need more performance on a given mission.


Overall, I'd say admiring the RS-68 (and, even more so, the RS-25) is like admiring a fine Maserati, with its wonderful craftsmanship and marvelous engineering. Except if you just care about getting the job done, then a basic Ford F-150 pickup truck will do just as well. And if we want to truly expand into space, a hundred F-150 equivalents will serve us a hell of a lot better than a few showpieces, no matter how pretty and fast the showpieces might be.

So, no, there's nothing shameful about the RS-68 -- it just belongs in the past. The world has moved on, and before any redesign of old technology is warranted, there needs to be sufficient justification of the result against the current state-of-the-art -- and not in terms of ISP, but rather in terms of cost effectiveness.


** And if you really want to stick with hydrolox, I'd ignore the RS-68 and take a closer look at the LE-9. If JAXA and Mitsubishi can hit the H3's price targets, then its a hydrolox booster engine that might actually be cost effective, at least vis-a-vis the Merlin (absent F9 reusability, at any rate).

Fuel cost will likely not play a role for at least the next decade - though I'd be happy to be wrong (looking at you Starship!). And maybe, if there's something like a space trucking established, H2 production on site might reduce costs considerably. The one driver I have in mind, is the thrust needed at launch. If I want to go big, this is actually something that needs considering. Otherwise, it's a path into the Sea Dragon ocean which might be something yet it's a whole bunch of new considerations that would be needed to be taken into account.

If one can drive down costs with Raptor, one could do the same with a hydrolox equivalent. And yes, Delta IV is something of the past, which is said by Tory Bruno, too. I just open this thread to rant about the RS-68.

What's so good about LE-9 accept the possible price tag?

[envy887]

The RS-58 is a 'dead engine walking'. When Delta IV-Heavy goes, so will the engine. It's unlikely to be used on another booster ever again. Yes, it's relatively cheap, yes it's relatively powerful for a LH2 fueled engine. But it can't be air-started and it weighs more than 14,000 pounds (6.3 metric tons) so it would make a lousy upper stage engine. It weighs so much because of it's huge ablative nozzle.

The only real use I could see for it again would be if funding were allocated to turn that nozzle into a regeneratively cooled one like the Shuttle/SLS's RS-25 engines. The specific impulse would increase and there would be no problem clustering them closely together as was first proposed in Constellation. The Isp increase on a big thrusting engine like RS-68 would give the SLS Corestage a modest payload increase to orbit. Five, maybe 7 metric tons.

But such an upgrade to RS-68 would not come cheap and the SLS would not have a high enough flight rate to justify the multi-billion dollar upgrade so...

True, true... But maybe it should not go through a re-design but new approach using Hydrolox for the 1. stage. RS-68 has about 120ish bar. Apparently there are alloys allowing for more than doubling this value. Going beyond 250 bar would drastically reduce size, probably increasing the TWR and adding FFSC probably would add some amazing SL Isp. I played around with RPA and adjusting SSME to 262 bar chamber pressure and an expansion ratio of ~35, it'll generate >404 s of SL Isp, going >430 s in vacuum. This would actually make the first stage much lighter than hydrocarbon riding rockets.

SSME is 4 or 5 times as expensive, mostly because of the complexity required to get that high pressure. For a vertically launched main stage, it's more cost-effective to use hydrocarbons and pursue better mass fractions than to use hydrolox and chase ISP, because there is effectively no limit on liftoff mass.

The only place I can see LH2 main stages being optimal is for a horizontally launched or air-dropped stage, where gross liftoff mass is a hard constraint.

[Hog]

The RS-58 is a 'dead engine walking'. When Delta IV-Heavy goes, so will the engine. It's unlikely to be used on another booster ever again. Yes, it's relatively cheap, yes it's relatively powerful for a LH2 fueled engine. But it can't be air-started and it weighs more than 14,000 pounds (6.3 metric tons) so it would make a lousy upper stage engine. It weighs so much because of it's huge ablative nozzle.

The only real use I could see for it again would be if funding were allocated to turn that nozzle into a regeneratively cooled one like the Shuttle/SLS's RS-25 engines. The specific impulse would increase and there would be no problem clustering them closely together as was first proposed in Constellation. The Isp increase on a big thrusting engine like RS-68 would give the SLS Corestage a modest payload increase to orbit. Five, maybe 7 metric tons.

But such an upgrade to RS-68 would not come cheap and the SLS would not have a high enough flight rate to justify the multi-billion dollar upgrade so...

True, true... But maybe it should not go through a re-design but new approach using Hydrolox for the 1. stage. RS-68 has about 120ish bar. Apparently there are alloys allowing for more than doubling this value. Going beyond 250 bar would drastically reduce size, probably increasing the TWR and adding FFSC probably would add some amazing SL Isp. I played around with RPA and adjusting SSME to 262 bar chamber pressure and an expansion ratio of ~35, it'll generate >404 s of SL Isp, going >430 s in vacuum. This would actually make the first stage much lighter than hydrocarbon riding rockets.

SSME is 4 or 5 times as expensive, mostly because of the complexity required to get that high pressure. For a vertically launched main stage, it's more cost-effective to use hydrocarbons and pursue better mass fractions than to use hydrolox and chase ISP, because there is effectively no limit on liftoff mass.

The only place I can see LH2 main stages being optimal is for a horizontally launched or air-dropped stage, where gross liftoff mass is a hard constraint.

Is Delta-IV H the only launcher with a pure HydroLOx booster/first stage?

[baldusi]

[...]
Is Delta-IV H the only launcher with a pure HydroLOx booster/first stage?

Only one I can think of. And personally think that's a good thing.

[envy887]

[...]
Is Delta-IV H the only launcher with a pure HydroLOx booster/first stage?

Only one I can think of. And personally think that's a good thing.

In addition to DIVH, there was also an all-hydrolox DIVM variant, with no SRBs. And there is a all-hydrolox variant of the H3, but it hasn't flown yet.

[edkyle99]

Is someone out there actually ashamed for the RS-68?

TWR of ~ 50
vacuum Isp of ~412 s
and still ~ 14 m USD?

Ashamed?  Not in the least.  Just three of these are needed to lift what is still the heaviest-hauling operational launch vehicle, carrying important DoD payloads.  The world's most powerful LH2/LOX engine.  Lots of thrust and decent ISP given the simple, reliable gas generator cycle.  No solids or kerosene to stain the atmosphere on ascent.  No failures in flight.  Good luck to those planning on replacing Delta 4 Heavy during NSSL.  It won't be trivial.

 - Ed Kyle

[Aeneas]

Is someone out there actually ashamed for the RS-68?

TWR of ~ 50
vacuum Isp of ~412 s
and still ~ 14 m USD?

Ashamed?  Not in the least.  Just three of these are needed to lift what is still the heaviest-hauling operational launch vehicle, carrying important DoD payloads.  The world's most powerful LH2/LOX engine.  Lots of thrust and decent ISP given the simple, reliable gas generator cycle.  No solids or kerosene to stain the atmosphere on ascent.  No failures in flight.  Good luck to those planning on replacing Delta 4 Heavy during NSSL.  It won't be trivial.

 - Ed Kyle

The Isp isn't decent, it's rather poor compared to RS-25. But thanks for the picture, it shows the thick ablative nozzle really well!
Delta IV is already replaceable by Falcon Heavy, isn't it?

[ugordan]

The Isp isn't decent, it's rather poor compared to RS-25.

The Isp by all means is very decent for a booster engine. You are setting the bar too high when you are comparing it to the RS-25, arguably one of if not the most complex and highest-performing engine, with a cost to accompany all that.

[Kryten]

Is someone out there actually ashamed for the RS-68?

TWR of ~ 50
vacuum Isp of ~412 s
and still ~ 14 m USD?

$14 million? Try $60 million. You're repeating stuff they were saying about the engine before it got into operation which they quickly found out they could not do; it's like asking why the space shuttle got retired when it could fly weekly.

[brickmack]

$14 million? Try $60 million. You're repeating stuff they were saying about the engine before it got into operation which they quickly found out they could not do; it's like asking why the space shuttle got retired when it could fly weekly.

Those projections were the worst case in an uncertain market. RS-68 settled around 28 million, RD-180 at around 23 million, RL10A-4-2 at 11-12 million, RL10B-2 at 6-7 million.

Anyway though, I think engine choice was the biggest problem for Delta. They spent half a billion dollars developing RS-68? At DIVs demonstrated flightrate, thats almost 10 million an engine just in amortized dev cost.

Two RS-25s in a reusable engine pod (ie, the original Boeing EELV bid) would have had a much higher ISP, more total thrust, and a slightly better TWR. And even at the most pessimistic estimates I've seen for RS-25s refurb cost, that'd still be cheaper than a single RS-68. Recent work on AR-22 (10 firings in 10 days with zero refurb needed, on an engine built from obsolete scraps) implies a much better cost. This added performance would've also made it a lot easier to achieve single-core heavy performance like Vulcan, which should drastically reduce costs (namely from fewer unique hardware designs). Plus the demonstrated reliability, plus the synergies with the still-flying Shuttle.

I'd really love to know what justified Boeing losing EELV, given from what information is available it sure looks like they had the cheapest to develop, cheapest to fly, most capable, and highest-heritage vehicle. Engine reuse could have been seen as a risk, but RS-25 reuse was already well-proven, their recovery system was pretty simple, and they did a demonstration drop test that went well

[edkyle99]

Delta IV is already replaceable by Falcon Heavy, isn't it?

It could be in theory, but not the way that SpaceX has flown it so far.  Delta 4 Heavy can lift almost 14 tonnes to GTO.  Falcon Heavy with recovery of all three boosters as flown to date can lift 8 tonnes.  Falcon Heavy could beat Delta 4 Heavy payload to GTO only by expending the center core, I think.  Falcon Heavy will also need a bigger fairing and SpaceX will need to add vertical payload integration to win NSSL, both steps the company has apparently proposed.

 - Ed Kyle

[Coastal Ron]

Delta IV is already replaceable by Falcon Heavy, isn't it?

It could be in theory, but not the way that SpaceX has flown it so far.  Delta 4 Heavy can lift almost 14 tonnes to GTO.  Falcon Heavy with recovery of all three boosters as flown to date can lift 8 tonnes.

You know quite well that it is a customer decision as to what version of Falcon Heavy they want to use, since the price is determined by the capability.

Falcon Heavy could beat Delta 4 Heavy payload to GTO only by expending the center core, I think.

SpaceX says that the Falcon Heavy has the capability to put 26.7mT to GTO, so it can certainly exceed the mass capabilities of Delta IV Heavy. It is only a matter of what price the customer is willing to pay, and Elon Musk has said a fully expendable Falcon Heavy would be about $150M.

As to the other requirements to take away payloads from Delta IV Heavy, I don't think SpaceX will want to modify Falcon Heavy very much when they plan to have Starship operational in a few years - and let's not debate whether that will happen...