guest blogger john hare
The multiple problems of solid rocket first stages would lead one to believe that the people that specified them would change their minds after they sobered up. That not being the case, it is somewhat interesting to think of ways to make it work anyway. The primary problems seem to be, excessive vibration, catastrophic failure modes requiring robust escape systems, control on all 3 axis, and poor scaling.
Apparently the upper stages and interstages are going to be tasked to handle most of these shortcomings. This Orion II idea is in response to people commenting on the Aries II idea. I agree that Direct, EELV, Falcon 9, depots, and a hundred other approaches are better than der Griffenschaft, but it is possible that the country will remain stuck with it. The context here is not supposed to be a better vehicle than the competition, but rather a way to get some lemon juice if even lemonade is not available.
Place an over sized H2/O2 tank on top of Aries with low pressure gas generator driven pumps pushing both propellants into the small 3rd stage tanks on Orion. The Orion rests on shock absorbers between the large tank and its’ RL10 clusters. Outboard of the stack are two or more large H2/O2 engines that light on the ground before Aries ignition. The large engines have sufficient thrust to keep the shock absorbers in tension throughout the Aries burn. The secondary function is enough thrust to make up the performance shortfalls of the solid first stage.
Three axis control is also supplied by these engines both during Aries first stage and their independent second stage burn after Aries drop off. In case of abort, the second stage tank becomes the shrapnel absorber while the Orion accelerates away on the large engines using the propellant that would have been used for the RL10s in a nominal mission. On a nominal mission, the large H2/O2 engines are dropped when the second stage tank is depleted, leaving the third stage with a full propellant load for its’ onboard RL10s.
The large propellant tank provides one layer of shock absorption from the solid rocket shaking. The large H2/O2 engines have enough thrust to keep the Orion clear of solid contact with the tank during the solids burn for a second layer of protection. The tensile shock absorbers between the tank and the Orion are the third layer of protection and should be capable of eliminating all but the most violent shaking.
The large H2O2 engines suffer considerable performance losses due to being canted outboard. These Isp losses on the order of 5% may be not too high a price to pay if it is necessary to save the concept. Gimbaling these engines can control the whole stack without much problem considering the relative sizes. During the first stage burn, they make up for the possible under performance of the Aries by providing several hundred thousand pounds of higher Isp thrust that would not normally be available to the concept. After Aries separation, they have enough thrust to carry the partially depleted second stage tank and the Orion spacecraft to near orbital velocity.
In case of abort, the second stage tank remains attached to the Aries to absorb shrapnel, while the large engines accelerate the bare spacecraft away using the onboard propellant that would have been used for the RL10s during a nominal mission. The large engines have enough thrust to escape at 4+G because they are sized to keep Orion clear of hard contact with the lower stages and carry the partially full second stage tank during a nominal mission.
On a nominal mission, the large engines are dropped with the second stage tank leaving the spacecraft to continue with the reliable vacuum rated RL10s. Internal tankage and number of RL10s would depend on final design details. One possibility is that the whole stack could be optimised for a lunar mission with abort to orbit on RL10s possible.
With the extra performance and escape routes available, this upper stage/escape system/spacecraft could be placed on the Atlas or Delta launchers with sufficient confidence to guarantee that either of them could perform the missions. With the safety aspects of the inherent escape system, relatively early flights of the Falcon 9 could do the job. Up and coming launch providers could be used as available and desirable for unmanned resupply at fairly early stages in development.