The DRM-1 interplanetary transportation system consists of a trans-Mars injection (TMI) stage, a biconic aeroshell for Mars orbit capture and Mars entry, a descent stage for surface delivery, an ascent stage for crew return to Mars orbit, an Earth return stage for departure from the Mars system, and a crew capsule for Earth entry and landing.
The transportation strategy adopted in DRM-1 eliminates the need for assembly or rendezvous of vehicle elements in LEO, but it does require a rendezvous in Mars orbit for the crew leaving Mars.
DRM-1 employed a trans-Mars injection (TMI) stage (used to propel a payload from LEO onto a trans-Mars trajectory) based on nuclear thermal propulsion (NTP). However, as discussed in Section 3.4.3, the advantages of NTP are minimized if it is required to transport it to a higher Earth orbit (>1,000 km) prior to start-up for safety reasons. DRM-1 (and DRM-3) made the unlikely assumption that the NTP
164 They assumed that the capsule and propulsion stage were 2.8 and 2.6 mT, respectively.
can be fired up in LEO. The TMI stage uses four 15,000 lb thrust NERVA (Nuclear Engine for Rocket Vehicle Application) derivative reactor engines (ISP = 900 seconds). This stage is assumed to have a maximum diameter of 10 meters and an overall length of 25.3 meters. Much of this volume is taken up by stored hydrogen. Each TMI stage utilizes 86 mT of hydrogen. If the hydrogen is stored as liquid at 1 bar the required volume is about 1,300 m3. For a 10 m diameter, the required length of hydrogen storage is 16.5 m. The dry mass of the TMI stage was estimated to be about 30 mT, or 35% of the propellant mass. That may prove to be optimistic.
After completion of its role, the TMI stage is inserted into a trajectory that will not reencounter Earth or Mars over the course of one million years. The TMI stage used for the crew incorporates a shadow shield between the engine assembly and the LH2 tank to protect the crew from radiation that builds up in the engines during TMI burns. The same type of TMI stage is used in all cargo missions, which allows the transportation system to deliver up to approximately 65 mT of useful cargo to the surface of Mars or nearly 115 mT to Mars orbit on a single launch from Earth. As stated previously, this is based on the assumption that NTP can be fired up in LEO, coupled with an optimistic estimate of the propulsion system dry mass.
Mars orbit capture and the majority of the Mars descent maneuver is performed using a single biconic aeroshell. The decision to perform the Mars orbit capture maneuver was based on the facts that (1) an aeroshell will be required to perform the Mars descent maneuver no matter what method is used to capture into Mars orbit, (2) the additional demands on a descent aeroshell to meet the Mars capture requirements were claimed to be modest, and (3) a single aeroshell eliminated one staging event, and thus one more potential failure mode, prior to landing on the surface. However, aerocapture technology for very large vehicles will require very challenging and expensive development, as discussed in Section 4.6, and this was not acknowledged by DRM-1 (or DRM-3). The JSC DRMs made very optimistic projections for aero-entry system masses that are much lower than those estimated by Braun et al. (see Section 4.6).
The crew is transported to Mars in a Habitat that is fundamentally identical to the Surface Habitat deployed robotically on a previous cargo mission. By designing the Habitat so that it can be used during transit and on the surface, a number of advantages to the overall mission are obtained:
• Two Habitats provide redundancy on the surface during the longest phase of the mission.
• By landing in a fully functional Habitat, the crew does not need to transfer from a "space-only" Habitat to the Surface Habitat immediately after landing, which allows the crew to readapt to a gravity environment at their own pace.
• The program is required to develop only one Habitat system. The Habitat design is based on its requirement for surface utilization. Modifications needed to adapt it to a zero-g environment must be minimized.
A common descent stage was assumed for the delivery of the Transit/Surface Habitats, the Ascent Vehicle, and other surface cargo. The Descent Vehicle is capable of landing up to approximately 65 mT of cargo on the Mars surface. The Landing Vehicle was somewhat oversized to deliver the crew; however, design of a scaled-down lander and the additional associated costs were avoided. To perform the post-aerocapture circularization burn and the final approximately 500 m/s of descent prior to landing on the Mars surface, the common descent stage employed four RL6-class engines modified to burn LOX/CH4. The use of parachutes was assumed to reduce the Descent Vehicle's speed after the aeroshell has ceased to be effective and prior to the final propulsive maneuver. The selection of LOX/CH4 allows a common engine to be developed for use by both the descent stage and the ascent stage, the latter of which is constrained by the propellant that is manufactured on the surface using ISRU. The Ascent Vehicle is delivered to the Mars surface atop a cargo descent stage. It is composed of an ascent stage and an ascent crew capsule. The ascent stage is delivered to Mars with its propellant tanks empty. However, the descent stage delivering the MAV included several tanks (about 5mT with >70 m3 volume) of seed hydrogen for use in producing 26 mT of LOX/CH4 propellant by ISRU for ascent to orbit and rendezvous with the ERV. The Ascent Vehicle used two RL6-class engines modified to burn LOX/CH4. However, it is not clear what allowance was made for the storage and cooling of this large volume of hydrogen. No mass seems to be allocated to the hydrogen storage system. The requirement for only 26 mT of ascent propellants is based on extremely optimistic estimates of the masses of the crew capsule and the ascent propulsion stage. It seems likely that a propellant mass >40 mT might actually be required, and indeed DRM-3 increased ascent propellants to 39 mT.
The ERV is composed of the trans-Earth injection (TEI) stage, the Earth return Transit Habitat, and a capsule that the crew will use to reenter the Earth's atmosphere. The TEI stage is delivered to Mars orbit fully fueled, where it waits for nearly 4 years before the crew uses it to return to Earth. It uses two RL6-class engines modified to burn LOX/CH4. These are the same engines developed for the ascent and descent stages, thereby reducing engine development costs and improving maintainability. The return Habitat is a duplicate of the outbound transit/surface. No mention was made of the requirements or methodologies for cryogenic storage of propellants.
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