Back up the ESAS truck...
After rethinking the Crew Launch Vehicle's propulsion choices, NASA is now doing the same for the Cargo Launch Vehicle. With the use of "low-cost, expendible Space Shuttle Main Engines" falling into doubt, NASA is again considering the RS-68, currently used on the Delta IV.
RS-68 is cheaper than an SSME and puts out more thrust, but with significant disadvantages. Most importantly, you lose specific impulse, and thus burn fuel less efficiently. The engine is also heavier (the drawbacks to the heavier engine being offset somewhat by the superior thrust.)
Supporters of the "expendable SSME" make the case that the Cargo Launch Vehicle will need to be enlarged if the RS-68 is chosen instead. That point is certainly true--if you absolutely need to put 125 tons into orbit. If you are willing to fudge on the payload capacity, you can get away with the RS-68. The arguments for man-rating the RS-68 are bunk, too. Show me how the NASA test standards for man-rating the RS-68 are significantly more costly than the testing the RS-68 has been through. If you use the shuttle's statistical failure rate as a guide (1/57 flights will end in loss of crew,) it's apparent that NASA's definition of man-rating is a farce. Besides, a modified SSME will still need to go through the same man-rating process, as it's not the same engine that was originally man-rated.
There are other, undiscussed options available for the Cargo Launch Vehicle. When Boeing was looking at ways to enhance Delta IV's performance, a variant of RS-68 was proposed which would gain more specific impulse by adding regenerative cooling and burning densified (slush?) hydrogen. While this would cost money to develop, the same can be said about an "expendible SSME." Another option is to use stock SSME's, but mount them in a recoverable pod such as Robert Zubrin proposed for his Ares. While I think Zubrin's "Mars Direct" is a highly risky means for exploring Mars, I think that he knew what he was doing when he designed the heavy lifter.
A far-term solution is to resurrect the moribund Space Transportation Main Engine from the NLS and ALS programs. It would have been slightly heavier than the SSME, and had thrust levels and specific impulse that were about halfway between those for the SSME and RS-68. It was only designed for 10 firings, whereas the SSME was designed for 25.
It appears that NASA's attempt at an expendible SSME is an attempt to duplicate the stillborn STME. Unfortunately, the STME was killed because the rockets it was designed for (ALS, NLS, and Shuttle liquid rocket boosters) were either economically unviable or too expensive to develop. NASA now has a rocket to justify the STME, but it came 15 years too late to save the engine project from being cancelled. It's a classical chicken-and-egg dilemma, which can only be avoided by spending large amounts of money upfront to develop the rocket and its engines at the same time (as was done with the Saturn V and F1, or Falcon I and Merlin + Kestrel.)
RS-68 is cheaper than an SSME and puts out more thrust, but with significant disadvantages. Most importantly, you lose specific impulse, and thus burn fuel less efficiently. The engine is also heavier (the drawbacks to the heavier engine being offset somewhat by the superior thrust.)
Supporters of the "expendable SSME" make the case that the Cargo Launch Vehicle will need to be enlarged if the RS-68 is chosen instead. That point is certainly true--if you absolutely need to put 125 tons into orbit. If you are willing to fudge on the payload capacity, you can get away with the RS-68. The arguments for man-rating the RS-68 are bunk, too. Show me how the NASA test standards for man-rating the RS-68 are significantly more costly than the testing the RS-68 has been through. If you use the shuttle's statistical failure rate as a guide (1/57 flights will end in loss of crew,) it's apparent that NASA's definition of man-rating is a farce. Besides, a modified SSME will still need to go through the same man-rating process, as it's not the same engine that was originally man-rated.
There are other, undiscussed options available for the Cargo Launch Vehicle. When Boeing was looking at ways to enhance Delta IV's performance, a variant of RS-68 was proposed which would gain more specific impulse by adding regenerative cooling and burning densified (slush?) hydrogen. While this would cost money to develop, the same can be said about an "expendible SSME." Another option is to use stock SSME's, but mount them in a recoverable pod such as Robert Zubrin proposed for his Ares. While I think Zubrin's "Mars Direct" is a highly risky means for exploring Mars, I think that he knew what he was doing when he designed the heavy lifter.
A far-term solution is to resurrect the moribund Space Transportation Main Engine from the NLS and ALS programs. It would have been slightly heavier than the SSME, and had thrust levels and specific impulse that were about halfway between those for the SSME and RS-68. It was only designed for 10 firings, whereas the SSME was designed for 25.
It appears that NASA's attempt at an expendible SSME is an attempt to duplicate the stillborn STME. Unfortunately, the STME was killed because the rockets it was designed for (ALS, NLS, and Shuttle liquid rocket boosters) were either economically unviable or too expensive to develop. NASA now has a rocket to justify the STME, but it came 15 years too late to save the engine project from being cancelled. It's a classical chicken-and-egg dilemma, which can only be avoided by spending large amounts of money upfront to develop the rocket and its engines at the same time (as was done with the Saturn V and F1, or Falcon I and Merlin + Kestrel.)
posted by Mr. X at
4:51 PM
