Boeing and the Marshall Space Flight Centre brought to pass many studies of improved Saturn V launch vehicles. The rockets proposed under the MLV and ELV projects would feature longer stages for more propellant capacity, and would be fitted with new, uprated engines.

https://archive.org/stream/NASA_NTRS_Archive_19650020081#page/n13/mode/2up

http://www.astronautix.com/h/hg-3.html

The HG-3 was one such engine. Some basic information on the engine is as follows.

• Total length = 3.38 metres
• Nozzle exit diameter = 2.03 metres
• Vacuum Isp = 451 seconds
• Vacuum thrust = 314,900lbf (to be increased to 375klbf and 400klbf)
• Sea level Isp = 280 seconds
• Chamber pressure = 3,000psia
• LOX:fuel mixture ratio = 5.1:1 to 5.3:1
• Operating cycle = Staged combustion

Unfortunately, there is very little other information available on the engine, as it was never built – no test data or proper geometry. However, there are certain traits it shares with other engines that may permit close estimates. The HG-3 ‘formed the basis’ for the design of the SSME, that engine also being a hydrogen/oxygen staged combustion unit with a vacuum Isp of 453 seconds and a very similar chamber pressure. The most powerful 400klbf HG-3s would have been roughly 80,000 pounds short on vacuum thrust.

Interestingly, the HG-3 designs show a very short nozzle that expands quickly. The engine also features a much larger power head and turbopump assembly than the J-2S.

So, given this similar performance and close heritage to the RS-25:

• Could the nozzle area ratio of the HG-3 have been similar to that of the SSME’s 69:1?
• What causes the large disparity of sea level specific impulse (280s vs 366s) even with the nearly identical chamber pressures?

A sea level-optimised variant is also featured on Astronautix. Vacuum Isp barely changes while sea level Isp rises dramatically to 360 seconds. The only change listed is a lower expansion ratio nozzle.

http://www.astronautix.com/h/hg-3-sl.html

• Why is the vacuum Isp change so small while the sea level change is so large? (Changing area ratio often does this, but here it seems disproportionally large)

Al.

Boeing and the Marshall Space Flight Centre brought to pass many studies of improved Saturn V launch vehicles. The rockets proposed under the MLV and ELV projects would feature longer stages for more propellant capacity, and would be fitted with new, uprated engines.

https://archive.org/stream/NASA_NTRS_Archive_19650020081#page/n13/mode/2up

http://www.astronautix.com/h/hg-3.html

The HG-3 was one such engine. Some basic information on the engine is as follows.

• Total length = 3.38 metres
• Nozzle exit diameter = 2.03 metres
• Vacuum Isp = 451 seconds
• Vacuum thrust = 314,900lbf (to be increased to 375klbf and 400klbf)
• Sea level Isp = 280 seconds
• Chamber pressure = 3,000psia
• LOX:fuel mixture ratio = 5.1:1 to 5.3:1
• Operating cycle = Staged combustion

Unfortunately, there is very little other information available on the engine, as it was never built – no test data or proper geometry. However, there are certain traits it shares with other engines that may permit close estimates. The HG-3 ‘formed the basis’ for the design of the SSME, that engine also being a hydrogen/oxygen staged combustion unit with a vacuum Isp of 453 seconds and a very similar chamber pressure. The most powerful 400klbf HG-3s would have been roughly 80,000 pounds short on vacuum thrust.

Interestingly, the HG-3 designs show a very short nozzle that expands quickly. The engine also features a much larger power head and turbopump assembly than the J-2S.

So, given this similar performance and close heritage to the RS-25:

• Could the nozzle area ratio of the HG-3 have been similar to that of the SSME’s 69:1?
• What causes the large disparity of sea level specific impulse (280s vs 366s) even with the nearly identical chamber pressures?

A sea level-optimised variant is also featured on Astronautix. Vacuum Isp barely changes while sea level Isp rises dramatically to 360 seconds. The only change listed is a lower expansion ratio nozzle.

http://www.astronautix.com/h/hg-3-sl.html

• Why is the vacuum Isp change so small while the sea level change is so large? (Changing area ratio often does this, but here it seems disproportionally large)

Al.