STCAEM Cryo Credit - © Mark Wade

The STCAEM cryogenic / aerobrake (CAB) concept was used as the NASA reference vehicle. It offered conceptual continuity with the mainstream Mars transportation studies performed over the previous several years. Its only major new technology development was high-energy aerobraking (HEAB) for planetary capture, but the concept also required a high-thrust cryogenic space engine. Being able to land on Mars using the CAB concept required a successful rendezvous between separately captured vehicles in Mars orbit. The vehicle consisted of three main elements: the Mars Excursion vehicle (MEV), the Mars Transfer Vehicle (MTV) and the Trans-Mars Injection Stage (TMIS).

STCAEM (Space Transfer Concepts and Analyses for Exploration Missions) was a major NASA funded study produced by Boeing in 1991. It provided an exhaustive trade analysis of mission profiles and trajectories for manned Mars missions using four different propulsion technologies (cryogenic chemical with aerobraking, nuclear thermal, nuclear electric, and solar electric). Within each study alternate mission profiles using split/sprint missions, flyby rendezvous, and additional aerobraking were examined. Only the baseline for the Cryogenic AeroBrake mission is presented here. The nominal baseline mission was as follows:

• The vehicle was assembled, checked out and boarded in LEO
• The TMI burn occurs and the TMIS was jettisoned
• MTV/MEV coast to Mars
• MTV and MEV separate 50 days prior to Mars capture
• The MEV aerocaptures robotically a day ahead of the MTV, providing last minute verification of atmospheric conditions and targeting
• The MTV captures, followed by rendezvous in the parking orbit with the MEV
• The landing crow transfers to the MEV and checks it out
• The MEV descends to the surface, jettisoning its aerobrake prior to landing
• After surface operations, the ascent vehicle (MAV) leaves its descent stage and surface payloads, ascends to orbit and docks with the MTV for crew transfer
• The MAV was jettisoned in Mars orbit, and the TEl burn occurs
• The MTV coasts back to Earth
• The crew transfers to a modified ACRV (MCRV), jettisons the MTV and performs a direct entry at Earth (optional: the entire MTV aerocaptures into a LEO parking orbit for refurbishment and reuse).

• Summary: Major NASA funded study produced by Boeing in 1991; focus on in-space propulsion
• Propulsion: LOX/LH2
• Braking at Mars: aerodynamic
• Mission Type: opposition
• Split or All-Up: all up
• ISRU: no ISRU
• Launch Year: 2016
• Crew: 4
• Mars Surface payload-metric tons: 25
• Outbound time-days: 350
• Mars Stay Time-days: 30
• Return Time-days: 200
• Total Mission Time-days: 580
• Total Payload Required in Low Earth Orbit-metric tons: 800
• Total Propellant Required-metric tons: 500
• Propellant Fraction: 0.62
• Mass per crew-metric tons: 200
• Launch Vehicle Payload to LEO-metric tons: 140
• Number of Launches Required to Assemble Payload in Low Earth Orbit: 8
• Launch Vehicle: Shuttle Z

Length: 50.00 m (164.00 ft). Span: 30.00 m (98.00 ft). Mass: 801,000 kg (1,765,000 lb). Main Engine Propellants: Lox/LH2. Main Engine Isp: 475 sec.

• STCAEM Cryogenic AeroBrake TMIS.  Class: Tug. Destination: Mars. Nation: USA. Agency: NASA. Manufacturer: Boeing.

  The Trans-Mars Injection Stage (TMIS) consisted of a core unit with four advanced space engines (ASE), avionics and cryogenic propellant tanks, and provision for up to four "strap-on" propellant tank sets. This configuration allowed propellant cross-feeding in the case of engine-out, and modular accommodation of the entire stage's performance according to the mission opportunity requirements.

  Keeping the engines close together on the core stage allowed tracking the center of mass (CM) during an engine-out condition via gimballing. This strategy avoided either opposing-shutoff (leading to long burn times and greater gravity losses), or a requirement for extra structure (a 125m truss) between the propellant tanks and engines to allow CM tracking. The TMIS accounted for about 75 % of the total initial mass in low earth orbit (IMLEO), a substantial pre-mission resupply.

  The TMIS had dry mass of 54,560 kg and a propellant load of 490,950 kg, for a total mass of 545,510 kg.

• STCAEM Cryogenic AeroBrake MTV.  Class: Manned. Type: Mars Orbiter. Destination: Mars. Nation: USA. Agency: NASA. Manufacturer: Boeing.

  The Mars Transfer Vehicle (MTV) configuration consisted of a transit habitat sized for four crew, an aerobrake, and a TEl Propulsion system. The transit habitat was located centrally in the aerobrake with an external airlock and an MCRV attached to the top (an Apollo-style ECCV was used to represent the MCRV). The airlock allowed access to the MEV crew cab and surface habitat during all phases of the transfer mission until the MEV separation 50 days prior to Mars arrival.

  The MCRV was used for mission scenarios featuring direct-entry crew return; these scenarios expended the entire MTV upon return to Earth. In a reusable mode, the entire MTV would be aerocaptured back at Earth for refurbishment and re-use; a second airlock would be located in place of the MCRV. The aerobrake was of identical geometry and construction as the MEV aerobrake, but was stronger and heavier due to its larger payload mass, and did not require any engine doors. The propulsion system (TEI) was divided symmetrically into two tank-stacks swaddling the transit habitat), like the MAV tank set configuration.

  The propulsion system was oriented at an angle relative to the aerobrake axis, with the two engines aimed out the rear of the aerobrake, to avoid TPS penetrations while still permitting mass-balanced operation during the burn.

  Mass breakdown was as follows:

  • Aerobrake, 23.8 metric tons
  • Habitat module, 36.6 metric ton, consisting of empty mass, 28.5 metric tons; 7.1 metric tons consumables and 1.0 metric ton experimental equipment
  • Propulsion stage, 18.2 metric tons
  • Propellant, 85.1 metric tons.

  Crew Size: 4. Span: 30.00 m (98.00 ft). Mass: 163,700 kg (360,800 lb).

• STCAEM MEV.  Class: Manned. Type: Mars Lander. Destination: Mars. Nation: USA. Agency: NASA. Manufacturer: Boeing.

  The reference Mars Excursion vehicle (MEV) was a manned lander that could transport a crew of four to the surface. It consisted of a surface-stay habitat module (roughly Space Station Freedom-module size), an airlock, 5 metric tons of surface-science payload, a cryogenic ascent propulsion system with four engines and bus structure, and the ascent vehicle (MAV).

  The MAV ascent vehicle consisted of a short-duration crew cab and a cryogenic ascent propulsion system with two engines. All propellant tanks were mass-balanced around their maneuver CMs so that no lateral CM shifting occurs. The entire MEV was packaged in a rigid, truncated-hyperboloidal aerobrake with L/D = 0.5, to which it was attached at eight points (four bus-frame corners and four landing-gear footpads). The aerobrake was fitted with doors, which opened to allow the descent engines to extend and ignite prior to aerobrake separation (allowing full benefit of the brake's drag). The brake was then jettisoned as the landing gear extends prior to terminal approach and hovering touchdown.

  Dominant configuration constraints for the MEV were as follows:

  • Payload manifesting
  • Surface access
  • Contiguous crew volumes
  • Short vehicle stack
  • Engine-out capabilities
  • On-orbit assembly

  Payload manifesting was mainly a proximity and mass balance issue. The surface habitat and airlock, which were the bulk (80%) of the payload, required access to the ascent crew cab and the surface, as well as being mass balanced for proper flight. The science payload required surface access for ease of unloading. Docking was facilitated by placing the crew cab high in the vehicle stack. The flight deck window was located to provide viewing of the surface for landing as well as to the upper hatch for docking. Keeping crew volumes contiguous allowed access during flight for check-out procedures and simulation training. The vehicle stuck was kept as short as possible for aerobrake wake protection, which tended to conflict with having the centre of mass (CM) as high as possible, desirable for a small engine gimbal-angle to provide minimal steering loss in an engine-out scenario. A high CM within a short stack was accomplished by placing the dense ascent LOX high in the configuration. Finally, although the dominant constraints for the MEV derived from its performance at Mars, consideration had been given to its earth-to-orbit launch. It was configured to be launched in a few, large, pre-integrated systems for on-orbit assembly. For example, the ascent vehicle could be launched intact in a 10 m diameter shroud, while the descent structure could be launched in 2 sections for fairly simple on-orbit assembly and integration.

  The MEV for the cryogenic/aerobrake mission had a total mass of 84,349 kg with the breakdown was as follows:

  • Mars capture and descent aerobrake, 15,138 kg
  • MEV Ascent stage, 22,754 kg
  • MEV Descent stage, 21,457 kg
  • Surface cargo, 25,000 kg

  The MEV for the NTR, NEP, and SEP missions (no aerobraking into Mars orbit required) had a total mass of 73,118 kg with the breakdown was as follows:

  • Mars descent aerobrake, 7,000 kg
  • MEV Ascent stage, 22,464 kg
  • MEV Descent stage, 18,659 kg
  • Surface cargo, 25,000 kg

• NASA Report, STCAEM Cryogenic Propulsion / Aerobrake Variant, . Accessed at: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930014062_1993014062.pdf. -right click to download pdfs!

• Boeing Aerospace and Electronics, Space Transfer Concepts and Analyses for Exploration Missions, NASA Contract NAS8-37857.