The Aerojet RS-68A

[JoeFromRIUSA]

Were there any proposals to use the RS-64 in clustered or single form to propel an American heavy lift vehicle?

[edkyle99]

Were there any proposals to use the RS-64 in clustered or single form to propel an American heavy lift vehicle?

If you mean RS-68, the answer is yes.  There were proposals to cluster RS-68 engines on the Ares V core stage.

 - Ed Kyle

[russianhalo117]

Were there any proposals to use the RS-64 in clustered or single form to propel an American heavy lift vehicle?

If you mean RS-68, the answer is yes.  There were proposals to cluster RS-68 engines on the Ares V core stage.

 - Ed Kyle

More specifically the ablative cooled baseline version RS-68B using 6 engines cancelled when regenerative cooling upgrade path was needed to counter Vehicle base heating.

RS-68 family regenerative cooling upgrade:
http://www.astronautix.com/r/rs-68regen.html

THRUST: 3,180KN
ISP SL: 371S

The alternate was the RS-800 the upclassed version of the RS-68 (i.e. RL60 is the upclassed version of the RL10). RS-800 like RS-68A and RS-68B came from the Delta IV Heavy growth path.

[sdsds]

AIUI, USAF got what they needed with RS-68A. DIV-H gets all their payloads to the necessary orbits powered by that engine.

"The Road Not Taken" (aka Ares V) might have used an engine similar to RS-68 on the top but with a different nozzle on the bottom. It would have required that because the thermal environment of engines packed tightly under the Ares V core would make ablative cooling inadequate.

Out on astronautix.com Mark Wade mentions RS-800K as an upgrade path for Delta IV. Can anyone find independent mention of this engine?

Here on the NSF forums, discussing Isp but long, long ago:

Vacuum nozzle and regen cooling would probably get you to about 440, but you wouldn't be able to run at sea level.  To get 450 and vacuum performance you'd have to get off gas-generator and go to staged-combustion.

But then you've just got an SSME.

RS-68 is just right for what it does.  That's why it's cheap.  But it's not a high-performance engine, nor is it intended for extended vacuum performance.

[spacenut]

Isn't Aerojet going to quit making the RS-68 after Delta IV heavy retires?

[ZachS09]

Isn't Aerojet going to quit making the RS-68 after Delta IV heavy retires?

Seems like it to me. What else could the RS-68A be used for?

[russianhalo117]

Isn't Aerojet going to quit making the RS-68 after Delta IV heavy retires?

The production line has long been inactivated and closed out. Testing has either completed or soon to be completed at NASA SSC.

[spacenut]

Nothing really.  Hydrogen has proven to be a hard and expensive booster fuel.  Rockets have to be larger to get a certain mass to orbit.  Compare the smaller Atlas V to the Delta IV.  Atlas is a smaller rocket with cheaper fuel.  Same with F9.  Metholox is a compromise between hydrolox and kerolox for boosters. Methane is better for a booster than hydrogen, but not as good as kerosene.  However is will be better for a reusable booster as three companies are now pursuing.  RS-68 was good at the time, but has proven to be too expensive. 

[Sam Ho]

Isn't Aerojet going to quit making the RS-68 after Delta IV heavy retires?

The production line has long been inactivated and closed out. Testing has either completed or soon to be completed at NASA SSC.

Press release and photo of the final hot-fire acceptance test April 12, 2021:

Today, the world’s most powerful hydrogen-fueled rocket engine built by Aerojet Rocketdyne, the RS-68A, completed its final hot-fire acceptance test for use on the United Launch Alliance (ULA) Delta IV Heavy launch vehicle on the B-1 Test Stand at NASA’s Stennis Space Center in Mississippi.

https://www.rocket.com/article/worlds-most-powerful-hydrogen-fueled-rocket-engine-completes-final-acceptance-test-ula-delta

[Vahe231991]

Isn't Aerojet going to quit making the RS-68 after Delta IV heavy retires?

Seems like it to me. What else could the RS-68A be used for?

The RS-68A differs from the RS-68 in having slightly greater thrust, producing 705,000 lbf (3,140 kN) thrust at sea level and 800,000 lbf (3,560 kN) thrust in a vacuum. It was first flown on June 29, 2012, and all Delta IV launches following the launch of Delta flight 371 on March 25, 2015 have used the RS-68A as the first stage engine. Given that components for the last Delta IV Heavy were delivered to Cape Canaveral in May, the last RS-68A to be manufactured was fabricated in 2022. The RS-68 was designed as a simpler, less-costly, heavy-lift engine for the Delta IV, and the end of RS-68 production  along with the shelving of development of the RS-68B in 2010 mean that there are no other SLVs to which the RS-68 could be made applicable.

[Proponent]

What was the RS-68B?

[woods170]

What was the RS-68B?

RS-68B was a modified version of RS-68A, intended for use on the Ares-V launch vehicle. But the ablative nozzle that is standard on RS-68 and RS-68A, turned out to be ill-suited for the base-heating environment of Ares V. NASA briefly played with the idea of putting a regen-cooled nozzle on RS-68B. But that idea got shelved when Ares-V efforts were put on hold in the run up to cancelation of the Constellation program (CxP).


After CxP got axed, RS-68B disappeared into oblivion.

[JEF_300]

But the ablative nozzle that is standard on RS-68 and RS-68A, turned out to be ill-suited for the base-heating environment of Ares V.

Did this forum ever come to a consensus on whether the problem was specifically the big SRBs, or just clustering ablative nozzles in general?

[woods170]

But the ablative nozzle that is standard on RS-68 and RS-68A, turned out to be ill-suited for the base-heating environment of Ares V.

Did this forum ever come to a consensus on whether the problem was specifically the big SRBs, or just clustering ablative nozzles in general?

Thermal modeling showed that the combined thermal environment, both from the SRBs and the clustered RS-68 engines, was something that the ablative nozzles of RS-68 could not handle.

[edkyle99]

But the ablative nozzle that is standard on RS-68 and RS-68A, turned out to be ill-suited for the base-heating environment of Ares V.

Did this forum ever come to a consensus on whether the problem was specifically the big SRBs, or just clustering ablative nozzles in general?

Thermal modeling showed that the combined thermal environment, both from the SRBs and the clustered RS-68 engines, was something that the ablative nozzles of RS-68 could not handle.

Keeping in mind that they do just fine in a cluster of three at the base of Delta 4 Heavy.

 - Ed Kyle

[woods170]

But the ablative nozzle that is standard on RS-68 and RS-68A, turned out to be ill-suited for the base-heating environment of Ares V.

Did this forum ever come to a consensus on whether the problem was specifically the big SRBs, or just clustering ablative nozzles in general?

Thermal modeling showed that the combined thermal environment, both from the SRBs and the clustered RS-68 engines, was something that the ablative nozzles of RS-68 could not handle.

Keeping in mind that they do just fine in a cluster of three at the base of Delta 4 Heavy.

 - Ed Kyle

Remembering you of the fact that clustering on the original 5-engine Ares V design had the engines sitting much closer together. On that particular Ares V design the distance between the RS-68 nozzles was only half of what it is on Delta IV Heavy. The radiant heat load on RS-68 on Delta IV Heavy is thus only one-quarter (25%) of what it would have been on RS-68 on Ares-V.

The later switch to six RS-68 engines on the Ares V core stage actually reduced the problem a bit because the stretched-out layout provided a bit more spacing between the RS-68 nozzles. Yet, it was not enough, courtesy of the massive amount of heat coming from they 5-segment SRBs: there's a study on L2 that clearly tells the tale that it was the combined heating effects of the RS-68s and the SRBs that caused the RS-68s with ablative nozzles to be unsuitable.

[Remes]

Keeping in mind that they do just fine in a cluster of three at the base of Delta 4 Heavy.

 - Ed Kyle

3 engines in a row doesn't form closed volumes where recirculation lives on forever.

From https://ntrs.nasa.gov/api/citations/20110015765/downloads/20110015765.pdf