November-December 1981:Mars in 1995?

In 1970,at the height of the Apollo programme,NASA unveiled its plans for the next thirty years in space. Not only did it include development of a re-usable Space Shuttle, it also had permanent Space Stations, a nuclear inter-orbital vehicle,bases on the Moon and a manned expedition to Mars before the end of the century. That "before the end of the century" was a conservative estimate at that - the earliest NASA would propose for such a mission was 1983. Then, in 1972, the nuclear NERVA stage was cancelled and all such plans were abandoned. At the moment even a return of men to the Moon in this century seems only a distant possiblity.

About two years ago the BIS began to wonder whether it had to be so. Viking had raised a host of questions about Mars, but to explore them with robot probes would demand a vast commitment of resources spread over decades. And back in 1952 von Braun had proposed manned missions to our neighbour planet using chemical propulsion with the very modest specific impulse of 2800 m/sec. Rocketdyne and NASA have tested the components for an Advanced Space Engine with a specific impulse in excess of 4600 m/sec and that could make nuclear propulsion unnecessary. This article is a brief description of the results of that study. It represents a conservative estimate for the requirements for a 1995 mission to Mars, using (as far as possible) off-the-shelf hardware.

Available Hardware

You cannot plan a mission for 1995 without making some assumptions about the hardware which may be available by that time. Chief among the assumptions are possible developments to the Space Shuttle. While launch costs are not an overwhelming factor in planning the mission, it is going to be necessary to launch some large boost stages (about the size of the Saturn IV-B stage) and the diameter of the Mars Landing Module is likely to be considerably bigger than the Shuttle cargo hold. Fortunately. NASA have already been looking at a Heavy Lift version of the Shuttle in which the recoverable orbiter would be replaced by a "propulsion capsule" carrying the Main Engines and avionics, and the cargo would be carried externally.

The.second assumption concerns the availability of an Orbital Transfer Vehicle. It now looks probably that NASA is going to adapt Centaur for operation with the Shuttle, but if it is to launch some of the larger, heavier payloads proposed for geosynchronous orbit in the late 1980’s, Centaur will have to get fatter and heavier. Studies have already been carried out on such a stage, and these have been used to provide propulsion for the mission.

The final assumption concerns the development of free-flying Space Stations. There is considerable pressure growing in the USA now to develop a Space Operations Center as a successor to the Shuttle, but as yet the size and mass of such a Center has not been defined.

Mission Outline

In many ways the mission planning must follow the outline indicated by the planning for nuclear missions in the late 1960's. The mission would be assembed in low Earth orbit from a number of Shuttle launches, and depart for Mars on or about 4 November 1994. Instead of the two vehicles proposed in the 1970 NASA plans, this mission would have three - two 'Orbiter' vehicles and one vehicle carrying the Lander and associated cargo on a one-way trip to Mars. The two Orbiter vehicles would provide living space for the five-man crew and give a ‘mutual lifeboat' capability in the event of a major system failure. During the long periods of ‘cruise’ the three vehicles would dock together into a cruciform configuration to allow the crew to move freely between one section and the next.

Each Orbiter vehicle consists of a single long Spacelab module fitted out to give the crew separate quarters and to provide life support and a control centre, with a half-length Pallet carrying additional equipment. One of the Orbiters would carry most of the scientific and communications equipment and a crew of three, while the second Orbiter would carry the Docking Module, which also carries the fold-out solar array providing power for the mission during the long periods of cruise.

At launch, each Orbiter vehicle has three propulsion stages. The first - a Heavy Boost stage the size of the Saturn IV-B - accelerates the vehicle onto its transfer trajectory towards Mars. The second and third stages are nearly identical, and are based around the Shuttle Orbital Transfer Vehicle. The second stage parks the expedition in a 13 hr orbit about Mars, and the third is used for departure from Mars orbit and return to Earth.

The Lander vehicle uses the same first and second stage boosters as the Orbiter vehicles, but omits the third stage. Furthermore, because the Lander assembly is lighter than the other two,the second stage booster actually has spare propulsion capability when it reaches Mars orbit, and can be used to pick up the returning surface exploration team, and carry out a side mission to Phobos. In addition to the manned Lander, the Lander vehicle will carry a communications relay satellite for the period during which the expedition is in Mars orbit, and three robot Surface Sample Return probes capable of lifting kilogramme-sized samples of soil back into orbit for sites not visited by the main expedition.

The expedition will arrive in orbit about Mars on 10 June 1995. It has 45 days in which to select a landing site, carry out a landing, field the SSR probes and carry out a side visit to Phobos. Then the two Orbiter vehicles will leave Mars orbit,not directly to Earth but to swing by Venus on the way home. Close approach to Venus would occur about 8 December 1995, and would allow the mission to launch two Pioneer-sized entry probes at that planet. Then the orbit swings away from the Sun again and returns to Earth about 16 May 1996. The Venus swingby trajectory is a plan worked out for the NASA nuclear mission, and saves both time and delta-V by shaping the trajectory closer to the ideal tangent as it encounters Earth for the second time.

Lander

The manned Mars Lander follows conservative principles by using the experience of Apollo and Viking for the capsule design. Shaped like Apollo, the capsule uses aerodynamic braking for the first part of the descent, using the minimal available lift and the capsule’s RCS system to guide the flight path towards the precise chosen touchdown point. At 10 km altitude and a speed of Mach 2.5 the capsule releases a supersonic drogue parachute to bring it to subsonic speeds before deploying the main parachute canopy. It is now 4 to 5 km above the surface and has a descent speed of 60 m/sec. With deployment of the main parachute, the Lander will jettison its heat-shield and deploy its landing legs. For the first time the crew will have a clear view of the terrain below (this may require periscopes or under-belly mounted TV cameras) and be able to select their landing site. When you consider that this may be somewhere ‘interesting’ like the Vallis Marineris it is apparent that the crew need considerable control at this point to make a safe landing. Finally, at about 600 m above the surface the landing rockets will be ignited, the parachute cut free, and the vehicle will manoeuvre for a conventional ‘helicopter’ landing.