US manned flight to Mars in 2039

This article is largely a summary of NASA’s MTAS (Mars Transportation Assessment Study, March 2023).

Flight paths Currently, with existing jet propulsion technologies, the most important step in planning a manned expedition into deep space is the choice of flight path. For Mars, this is expressed in the presence of two types of trajectories: “conjunction-class” (type “connection”) and “opposition-class” (type “opposition”).

“Conjunction” type trajectories require smaller reserves of characteristic velocity (the so-called ΔV) due to the use of the launch window on the way there and back, but require a lot of time 900-1000 days, of which the time of stay of people on Mars is about 400-600 days. “Opposition” type trajectories, on the contrary, require more ΔV, but reduce the expedition time to 560-700 days and the time of people’s stay on Mars to about 30-90 days.

A small increase in the costs ΔV for the flight sharply (3.1 times) increases the mass of fuel that needs to be launched into low-Earth orbit for the expedition. Therefore, initially “opposition” type trajectories were not considered promising by NASA, but in recent years the concept has changed. The new MTAS study (2023) looks exclusively at the opposition mission concept. For the first manned missions to Mars, it is extremely important to reduce the risks of equipment failure and radiation exposure to the crew, which are greater the longer the mission. And in this regard, the “opposition” class expedition benefits greatly.

Flight plan The above conclusions led NASA designers to the following conclusions: 1) the manned expedition should last 2 years, the “opposition” type trajectory in the 2030s; 2) limit the time spent on Mars to 30 days, while 2 crew members will be on the surface of the planet, and 2 at this time in Mars orbit; 3) the ship for the flight to Mars will be assembled in lunar orbit (at NRHO, to be more precise), where there should be a prepared infrastructure from the Artemis lunar program (Lunar Gateway station); 4) the crew will live in a special residential module DSH (Deep Space Habitat) during the flight, the start to Mars will be from the orbit of the Moon, the return back will also be to the orbit of the Moon

5) the super-heavy SLS rocket from the lunar program (as well as the super-heavy Starship) will be widely used to launch payloads into orbit; 6) a long SLS fairing with a diameter of 8.6 m will be used; 7) before the crew arrives in Mars orbit, 3 landing modules weighing 65,000 kg each must be delivered. Two propulsion concepts were considered: nuclear thermal engines NTP (Nuclear Themal Propulsion) and nuclear electric engines (Nuclear Electric Propulsion) in conjunction with chemical engines using liquid methane-liquid oxygen fuel (LCH4+LOX).

The effect of radiation on the crew from various sources is noted in the study as an unresolved problem. Apparently, in the future it will be necessary to seek a balance between shorter missions with a decrease in radiation from natural sources (galactic cosmic rays and solar flares) and longer missions with a decrease in radiation from the nuclear systems of the ship. 4. Option I. Nuclear thermal propulsion (NTP) flight Flight pattern: 1) departure from the Earth-Moon system – 04/26/2039 (1st day) 2) maneuver in deep space – 07.27.2039 (93rd day) 3) entry into Mars orbit – 02/16/2040 (297th day) 4) departure from Mars orbit back – 03/26/2040 (336th day)

5) flyby of Venus – 10/06/2040 (530th day) 6) entry into Earth orbit – 03/16/2041 (691st day) Total 690 days. The manned expedition is just the tip of the iceberg; it is a small part of a long launch campaign stretching over almost the entire 2030s.

In total, the mission requires the production of 7 nuclear engines: at least 1 NRE for technology development, 2 NRE for cargo ships, 2 NRE for a manned spacecraft, and another 2 NRE required for assembling the manned spacecraft structure. The engines of a manned spacecraft must have a thrust of 110 kN and a specific impulse of at least 870 s in order to get by with a reasonable amount of fuel. The study adopted a value of 900 s to take into account the inefficiency of engine starting. Each engine must be able to operate for 4 hours to be able to continue the mission if one of the engines on the ship fails. The maximum number of engine starts is expected to be about 8.

NTP technology poses the following serious problems: 1) The only fuel capable of giving such a high specific impulse can only be hydrogen, which for this must be heated to a temperature of 2700 K. In this case, the peak temperatures of the fuel can be several hundred degrees higher than the temperature of hydrogen at the outlet of the reactor. Fuel rods must be made of heat-resistant materials and withstand the corrosion of hot hydrogen. CerMet (ceramic-metallic) fuel rods can give a specific impulse of 870 s, and CerCer (all-ceramic) can give 900 s with a margin. However, CerCer fuel elements have never been tested under nuclear conditions and have yet to be tested.

2) Bench testing of a new engine is very capital-intensive and labor-intensive. Due to the fact that open-air experiments with radioactive materials are currently prohibited in the United States (as was the case in the Rover/NERVA project), the development of a specialized stand becomes a serious problem. Such a stand must completely capture and process exhaust gases from the engine being tested and must itself undergo a radiation safety examination.

3) It is necessary to take the technology for managing cryogenic liquids to a new level, because It will be necessary to store liquid hydrogen in space for several years. Hydrogen is a small molecule and can easily leak through small gaps in valve seats and even through the crystal lattice of the material. Reducing hydrogen leakage to negligible levels requires the development of zero-leakage valves and couplings, advanced thermal insulation, and high-capacity, high-efficiency cryocoolers.

5. Option II. Flight on a nuclear electric engine+LPRE (NEP+CCP) Flight pattern: 1) departure from the Earth-Moon system – 02/28/2039 (1st day) 2) entry into Mars orbit – 12/21/2039 (297th day) 3) departure from Mars orbit – 01/30/2040 (337th day) 4) flyby of Venus – 10/09/2040 (590th day) 5) entry into Earth orbit – 03/29/2041 (761st day) Only 760 days.

This variation utilizes the strengths of each movement type. First, upon departure, the high-thrust engine (LRE) quickly changes the speed of the ship, then the low-thrust ion engine (LTE) is turned on, which runs for months. When approaching the target, the high-thrust engine is turned on again to perform the orbital entry maneuver.

The ship’s reactor is located on a 50 m telescopic rod to isolate it from radiators, electronics and the habitation module. On one of the walls of the reactor there is a shield that creates a cone of space facing the rest of the ship, protected from radiation. 20 Hall effect engines are mounted on separate rods. The stage with the liquid rocket engine and the stage with liquid methane and liquid oxygen are located on the right side of the ship. Both the nuclear power plant and the chemical stage were designed to be the largest that could fit under the long SLS fairing with a diameter of 8.6 m.

The study suggests that the Mars craft’s Hall effect thrusters will be broadly similar to the 12.5 kW solar-powered thrusters developed for the Lunar Gateway. In addition, NASA has demonstrated the scaling of Hall effect thruster technology to 100 kW power (and 4.6 N thrust). Based on these data, the study included the specific impulse of ion engines 2600 s. The specific impulse of the LCH4+LOX chemical stage is taken to be 350 s.

The main factor in the design of a nuclear electric propulsion system are radiators, the size of which determines the layout of the ship and its assembly in space. The size of the radiator depends on the required efficiency of converting thermal energy into electrical energy and the permissible temperature of the coolant being removed. To minimize the risk of coolant leaks, it is now recommended that radiators be assembled and filled before they are launched into space. Thus, it is advisable to place the radiators in the same launch vehicle with the reactor and energy converter.

NASA management has intentions to achieve the required NTP characteristics using low-enriched uranium fuel with moderators based on zirconium hydrides ZrH and yttrium YH. The study notes that this will greatly complicate the design. Existing concepts are based on the use of highly enriched uranium (NERVA 1960s and SNTP 1990s). HEU (highly enriched uranium, containing more than 20% uranium-235, typically 93%) reactors have been developed since the 1950s. Such reactors usually operate on fast neutrons and are characterized by a simple and reliable design. Fast neutrons can support a fission reaction, although less efficiently than thermal neutrons, but this can be done with an excess of uranium-235.