It's early 1970, and you're in charge of issuing the Shuttle Phase B design contracts. These are essentially blocks of money distributed to aerospace companies so they can refine competitive proposals. The Nixon administration has made it clear that there will be no immediate funding for moon bases or huge space stations, so the Shuttle is going to be America's presence in space until the political winds shift somewhere down the road. The only requirement that I am imposing is space for at least four crew and downmass of five tons; essentially, you're going to need a spaceplance somewhat larger than Dream Chaser. The question here is, which architectures get your design contracts?
At this point, it's become quite apparent that the Shuttle launch system will be a two-stage-to-orbit launcher composed of three main parts: the booster, the sustainer, and the orbiter. The booster could be reusable, like many of the fly-back designs that came back in Phase A, recoverable, like the OTL SRBs, or expendable, like some of the Saturn-Shuttle concepts with an expendable S-IC as the booster. Some of the Phase A designs gave the orbiter the engines and fuel tanks to act as its own sustainer, while the OTL design included the sustainer engines but not the fuel tanks in the orbiter. The other conceivable option is to have the sustainer engines and tanks in a separate, expendable stage that would boost the orbiter almost all the way to orbit.
Looking back with the full clarity of 20/20 hindsight, we can see that the OTL Shuttle design was a series of compromises driven by Air Force requirements for high downmass and long cross-range performance for missions the OTL Shuttle never actually got a chance to perform. The objective here is to find the best design for the missions that the shuttle actually flew, so the downmass and cross-range requirements become secondary to other issues. Out of the 131 operational missions that the Shuttle flew, 38 were satellite deployments that could have been conducted by expendable vehicles. The remaining 93 missions were missions that would be considered suitable for the Shuttle's capabilities as a reusable small space station with significant downmass capability. These included 8 missions to repair or retrieve satellites, including 5 to Hubble, while many more satellites were deployed and retrieved during individual missions or during later missions. A total of 37 free-flight research missions, including 19 with Spacelab, were flown, with the Shuttle acting as a temporary space station. Finally, 48 missions were flown to space stations, including 11 Shuttle-Mir flights, 25 ISS assembly flights, and 12 ISS logistics flights (with MPLMs).
An explanation of the answer choices:
1. "Expendable booster, expendable sustainer, orbiter with only on-orbit motors"
This would essentially be a fully expendable rocket with a small spaceplane carried on top as a payload. Possible launch vehicles for the era included the Saturn INT-20 concept, in which case the booster would be an expendable S-IC stage, the sustainer would be an expendable S-IVB stage, and an orbiter of up to 60 tons would be carried as payload. A modern counterpart would be the much smaller Dream Chaser launching on an Ariane 5.
2. "Expendable booster, orbiter with sustainer motors and expendable tanks"
Because I don't think its actually possible to consider rockets that have to be taken back to the factory to be rebuilt after each flight to be reusable, this is essentially the OTL Shuttle. Other vehicles in this category are the Shuttle LRB concepts with LH2/LOX or RP-1/LOX side boosters or the Saturn-Shuttle concept.
3. "Expendable booster, fully reusable orbiter with sustainer motors and tanks"
A number of orbiter concepts that carried both engines and fuel tanks to orbits appeared in both the Phase A and Phase B periods. However, all that I could find also used reusable boosters. An orbiter this big would likely require a booster at least as large as the S-IC.
4. "Reusable booster, expendable sustainer, orbiter with only on-orbit motors"
If I remember correctly, this is the architecture that the Right Side Up TL used, with a fly-back S-IC booster, S-IV sustainer, and a small lifting body orbiter.
5. "Reusable booster, orbiter with sustainer motors and expendable tanks"
The archetypal example of this would be the Saturn-Shuttle with a fly-back S-IC as a booster, which was conceptualized in 1972.
6. "Reusable booster, fully reusable orbiter with sustainer motors and tanks"
Concepts like these made up most of the Phase A and Phase B designs. Faget's MSFC DC-3 is one of the better-known examples. I do not consider this option to be realistic given the budgetary constraints place on NASA at the time.
Note that options 1 and 4, with separate sustainers and orbiters, would be able to fly missions without the orbiter.
At this point, it's become quite apparent that the Shuttle launch system will be a two-stage-to-orbit launcher composed of three main parts: the booster, the sustainer, and the orbiter. The booster could be reusable, like many of the fly-back designs that came back in Phase A, recoverable, like the OTL SRBs, or expendable, like some of the Saturn-Shuttle concepts with an expendable S-IC as the booster. Some of the Phase A designs gave the orbiter the engines and fuel tanks to act as its own sustainer, while the OTL design included the sustainer engines but not the fuel tanks in the orbiter. The other conceivable option is to have the sustainer engines and tanks in a separate, expendable stage that would boost the orbiter almost all the way to orbit.
Looking back with the full clarity of 20/20 hindsight, we can see that the OTL Shuttle design was a series of compromises driven by Air Force requirements for high downmass and long cross-range performance for missions the OTL Shuttle never actually got a chance to perform. The objective here is to find the best design for the missions that the shuttle actually flew, so the downmass and cross-range requirements become secondary to other issues. Out of the 131 operational missions that the Shuttle flew, 38 were satellite deployments that could have been conducted by expendable vehicles. The remaining 93 missions were missions that would be considered suitable for the Shuttle's capabilities as a reusable small space station with significant downmass capability. These included 8 missions to repair or retrieve satellites, including 5 to Hubble, while many more satellites were deployed and retrieved during individual missions or during later missions. A total of 37 free-flight research missions, including 19 with Spacelab, were flown, with the Shuttle acting as a temporary space station. Finally, 48 missions were flown to space stations, including 11 Shuttle-Mir flights, 25 ISS assembly flights, and 12 ISS logistics flights (with MPLMs).
An explanation of the answer choices:
1. "Expendable booster, expendable sustainer, orbiter with only on-orbit motors"
This would essentially be a fully expendable rocket with a small spaceplane carried on top as a payload. Possible launch vehicles for the era included the Saturn INT-20 concept, in which case the booster would be an expendable S-IC stage, the sustainer would be an expendable S-IVB stage, and an orbiter of up to 60 tons would be carried as payload. A modern counterpart would be the much smaller Dream Chaser launching on an Ariane 5.
2. "Expendable booster, orbiter with sustainer motors and expendable tanks"
Because I don't think its actually possible to consider rockets that have to be taken back to the factory to be rebuilt after each flight to be reusable, this is essentially the OTL Shuttle. Other vehicles in this category are the Shuttle LRB concepts with LH2/LOX or RP-1/LOX side boosters or the Saturn-Shuttle concept.
3. "Expendable booster, fully reusable orbiter with sustainer motors and tanks"
A number of orbiter concepts that carried both engines and fuel tanks to orbits appeared in both the Phase A and Phase B periods. However, all that I could find also used reusable boosters. An orbiter this big would likely require a booster at least as large as the S-IC.
4. "Reusable booster, expendable sustainer, orbiter with only on-orbit motors"
If I remember correctly, this is the architecture that the Right Side Up TL used, with a fly-back S-IC booster, S-IV sustainer, and a small lifting body orbiter.
5. "Reusable booster, orbiter with sustainer motors and expendable tanks"
The archetypal example of this would be the Saturn-Shuttle with a fly-back S-IC as a booster, which was conceptualized in 1972.
6. "Reusable booster, fully reusable orbiter with sustainer motors and tanks"
Concepts like these made up most of the Phase A and Phase B designs. Faget's MSFC DC-3 is one of the better-known examples. I do not consider this option to be realistic given the budgetary constraints place on NASA at the time.
Note that options 1 and 4, with separate sustainers and orbiters, would be able to fly missions without the orbiter.