BNTR DRM 3.0 (2001)

In October 2001, nuclear engineers at NASA's Glenn Research Center (GRC) in Cleveland, Ohio, led by Stanley K. Borowski, Advanced Concepts Manager in GRC's Space Transportation Project Office, described a variant of NASA's 1998 Mars Design Reference Mission (DRM) 3.0 based on Bimodal Nuclear Thermal Rocket (BNTR) propulsion. The BNTR DRM concept, first described publicly in 1998, evolved from nuclear-thermal rocket mission designs Borowski and his colleagues developed during the abortive Space Exploration Initiative (1989-1993).

NASA's first Mars DRM, designated DRM 1.0 in 1997, was developed in 1992-1993 (link below). It was based on Martin Marietta's 1990 Mars Direct mission plan. The demise of President George H. W. Bush's Space Exploration initiative temporarily halted DRM work in 1993. NASA resumed its DRM studies after the discovery in 1996 of possible microfossils in martian meteorite ALH 84001, and released its baseline chemical-propulsion DRM 3.0 in 1998. There was no official DRM 2.0, though a "scrubbed" (that is, mass-reduced) DRM 1.0 bears that designation in at least one NASA document.

Shortly thereafter, NASA Johnson Space Center (JSC) in Houston, Texas, which led the DRM study effort, was diverted from DRM work by the in-house COMBO lander study. In the absence of guidance from Houston, NASA GRC developed a pair of DRM 3.0 variants: a solar-electric propulsion (SEP) DRM 3.0 and the BNTR DRM 3.0 considered here.

In BNTR DRM 3.0, two unpiloted spacecraft would leave Earth for Mars during the 2011 low-energy Mars-Earth transfer opportunity, and a third, bearing the crew, would depart for Mars in 2014. Components for the three spacecraft would reach Earth orbit on six Shuttle-Derived Heavy-Lift Launch Vehicles (SDHLVs), each capable of launching 80 tons into 220-mile-high assembly orbit, and in the payload bay of a Space Shuttle orbiter which would also deliver the Mars crew.

SDHLV 1 would launch Bimodal Nuclear Thermal Rocket (BNTR) stage 1. Each BNTR DRM mission would need three 28-meter-long, 7.4-meter-diameter BNTR stages. BNTR stage 1 would carry 47 tons of liquid hydrogen (LH2) propellant. The BNTR stages would each include three 15,000-pound-thrust BNTR engines developed as part of a joint U.S./Russian project in 1992-1993.

SDHLV 2 would boost the unpiloted 62.2-ton cargo lander into assembly orbit. The lander would include a Mars aerobrake and entry shield, landing parachutes, a descent stage, a 25.8-ton Mars surface payload including an in-situ resource utilization (ISRU) propellant plant, and an unfueled Mars Ascent Vehicle (MAV) made up of a conical Earth Crew Return Vehicle (ECRV) capsule and an ascent stage.

SDHLV launch 3, identical to SDHLV launch 1, would place into assembly orbit BNTR stage 2 containing 46 tons of LH2 propellant. SDHLV launch 4 would place the unpiloted 60.5-ton Habitat lander into assembly orbit. It would include a Mars aerobrake/entry shield identical to that of the cargo lander, parachutes, a descent stage, and a 32.7-ton payload including the crew's Mars surface living quarters.

The BNTR stage forward section would include chemical thrusters for attitude control and for maneuvering capability for docking the stages with the Habitat and cargo landers in assembly orbit. The BNTR 1/cargo lander combination would have a mass of 133.7 tons, while the BNTR 2/Habitat lander combination would have a mass of 131 tons. Both combinations would measure 57.5 meters long. As the launch window for Mars opened, the BNTR stages would fire their engines to depart assembly orbit for Mars.

Following Earth-orbit departure, the BNTR engine nuclear reactors would switch to electricity-generation mode. They would heat a working fluid to drive three turbine generators providing a total of 50 kilowatts of electricity. Fifteen kilowatts of this would power a refrigeration system in the BNTR stage for minimizing LH2 boil-off.

As Mars loomed large ahead, the power generators would charge the lander batteries. The BNTR stages would then separate and fire their engines to miss Mars and enter disposal orbit around the Sun. The landers would aerobrake in Mars's upper atmosphere. The Habitat lander would capture into Mars orbit and extend twin solar arrays to generate electricity. The cargo lander would capture into orbit, then fire its deorbit engines to enter the atmosphere a second time. After casting off its heatshield, it would deploy three parachutes. Descent engines would fire, then landing legs would deploy just before touchdown. The GRC engineers opted for a horizontal landing configuration; this would, they explained, prevent tipping and provide the astronauts with easy access to the lander's cargo.

After cargo lander touchdown, a teleoperated cart bearing a nuclear power source would lower to the ground and trundle away to a safe distance trailing a power cable. This would power the lander's ISRU propellant plant, which over several months would react four tons of "seed" hydrogen brought from Earth with martian atmospheric carbon dioxide to produce 39.5 tons of liquid methane and liquid oxygen propellants for the MAV's ascent engines.

SDHLV launch 5, identical to SDHLV launches 1 and 3, would mark the start of launches for the 2014 Earth-Mars transfer opportunity. It would place BNTR stage 3 into assembly orbit. Because it would propel a piloted spacecraft, it would include three 3.24-ton radiation shields (one per engine). BNTR stage 3 would carry about 48 tons of LH2.

Thirty days after SDHLV launch 5, SDHLV launch 6 would place into assembly orbit a 5.1-ton spare Earth Crew Return Vehicle (ECRV) attached to the front of an 11.6-ton truss. A 17-meter-long tank with 43 tons of LH2 and a two-meter-long drum-shaped logistics module containing 6.9 tons of contingency supplies would nest along the truss's length. BNTR stage 3 and the truss assembly would rendezvous and dock, then propellant lines would automatically link the truss tank to BNTR stage 3.

A Shuttle orbiter carrying the Mars crew and a 20.5-ton deflated Transhab module would rendezvous with the BNTR stage 3/truss combination one week before the crew's planned departure for Mars. After rendezvous, the spare ECRV would undock from the truss and fly automatically to a docking port in the Space Shuttle payload bay. Astronauts would use the Shuttle's robot arm to hoist the Transhab from the payload bay and dock it to the front of the truss in the spare ECRV's place.

The Mars astronauts would enter the spare ECRV and pilot it to a docking at a port on the Transhab's front, then enter the cylindrical Transhab's solid core and inflate its fabric-walled outer volume. The inflated Transhab would measure 9.4 meters in diameter. Unstowing floor panels and furnishings from the core and installing them in the inflated volume would complete assembly. Transhab, truss, and BNTR stage 3 would make up the 64.2-meter-long, 166.4-ton Crew Transfer Vehicle (CTV).

The truss-mounted tank and BNTR stage 3 would hold 90.8 tons of LH2 at the start of CTV Earth departure on January 21, 2014. The truss tank would provide 70% of the propellant needed for departure. In the most demanding scenario, the BNTR engines would fire twice for 22.7 minutes each time to push the CTV out of Earth orbit toward Mars.

Following Earth-orbit departure, the crew would jettison the empty truss tank and use small chemical-propellant thrusters to start the CTV rotating end over end at a rate of 3.7 rotations per minute. This would create acceleration equal to one Mars gravity (38% of Earth gravity) in the Transhab module. Artificial gravity was a late addition to BNTR DRM 3.0; it made its first appearance in a 1999 paper. In artificial gravity mode, "down" would be toward the spare ECRV on the CTV's nose; this would make the Transhab's forward half its lower deck. Halfway to Mars, about 105 days out from Earth, the astronauts would stop rotation and perform a course correction burn using the thrusters. They would then resume rotation for the remainder of the trans-Mars trip.

The CTV would arrive in Mars orbit on August 19, 2014. The three BNTR engines would fire for 12.3 minutes to slow the spacecraft for Mars orbit capture. The spacecraft would complete one Mars orbit each 24.6-hour martian day.

The crew would pilot the CTV to rendezvous with the Habitat lander in Mars orbit. If the Habitat lander proved unable to land on Mars, the crew would remain in the CTV in Mars orbit until Mars and Earth aligned for the flight home (a wait time of about 500 days). They would survive by drawing upon the contingency supplies in the drum-shaped logistics module attached to the truss. If the Habitat checked out as healthy, however, the crew would fly the spare ECRV to a docking port on its side. After discarding the spare ECRV and Habitat solar arrays, they would fire the Habitat's deorbit engines, enter Mars's atmosphere, and land near the cargo lander.

The Habitat lander's horizontal configuration would provide the astronauts on board with easy access to the martian surface. After the historic first footsteps on Mars, the astronauts would inflate a Transhab surface habitat and commence a program of exploration lasting about 500 days.

The CTV would briefly fire its engines to align its orbit for the crew's return. The MAV bearing the crew and about 90 kilograms of Mars samples would then lift off burning methane and oxygen propellants made from martian air. It would dock at the front of the Transhab, then the astronauts would transfer to the CTV. They would cast off the spent MAV ascent stage, but would retain the MAV ECRV for Earth reentry.

The CTV would leave Mars orbit on January 3, 2016, after 502 days at Mars. The astronauts would first abandon the contingency supply module to reduce their spacecraft's mass so that the propellant remaining in BNTR stage 3 would be sufficient to launch them home to Earth. They would then fire the NTR engines for 2.9 minutes to change the CTV's orbital plane, then fire them again for 5.2 minutes to place it on course for Earth. The crew would spin the CTV end over end to create acceleration equal to one Mars gravity in the Transhab. The authors advocated increasing the spin rate gradually during the flight home to prepare the crew for return to Earth's gravity. About halfway home they would stop rotation, perform a course correction, then resume rotation. Flight to Earth would last 190 days.

Near Earth, the crew would stop CTV rotation for the final time, enter the MAV ECRV with their Mars samples, and undock from the CTV. The abandoned CTV would fly past Earth and enter solar orbit. The MAV ECRV would reenter Earth's atmosphere on July 11, 2016.

The authors compared their Mars plan with the baseline chemical-propulsion DRM 3.0 and with the NASA GRC SEP DRM 3.0. They found that their plan would need eight vehicle elements, of which four would be designs unique to the BNTR DRM 3.0. The baseline DRM 3.0, by contrast, would need 14 vehicle elements, 10 of which would be unique, and the SEP DRM would need 13.5 vehicle elements, 9.5 of which would be unique. BNTR DRM 3.0 would require that 431 tons of hardware and propellants be placed into Earth orbit; the baseline DRM 3.0 would need 657 tons and SEP DRM 3.0, 478 tons. Borowski and his colleagues argued that fewer vehicle designs and reduced mass would add up to reduced cost and mission complexity.

The BNTR DRM 3.0 variant became the basis for DRM 4.0, which was developed during NASA-wide studies in 2001-2002 (though NASA documents occasionally back-date DRM 4.0 to 1998, when BNTR DRM 3.0 was first proposed). DRM 4.0 differed from BNTR DRM 3.0 mainly in that it adopted a "Dual Lander" design concept developed in response to JSC's 1998-1999 COMBO lander study. In 2008, a decade after BNTR DRM 3.0 first became public, NASA released a version of DRM 4.0 modified to use planned Constellation Program hardware (for example, the Ares V heavy-lift rocket and the Orion Crew Exploration Vehicle). It dubbed the new DRM Design Reference Architecture (DRA) 5.0.

"Bimodal Nuclear Thermal Rocket (NTR) Propulsion for Power-Rich, Artificial Gravity Human Exploration Missions to Mars," IAA-01-IAA.13.3.05, Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 52nd International Astronautical Congress in Toulouse, France, October 1-5, 2001.

"Artificial Gravity Vehicle design Option for NASA's Human Mars Mission Using 'Bimodal' NTR Propulsion," AIAA-99-2545,Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit in Los Angeles, California, June 20-24, 1999.

"Vehicle and Mission Design Options for the Human Exploration of Mars/Phobos Using 'Bimodal' NTR and LANTR Propulsion," AIAA-98-3883, Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit in Cleveland, Ohio, July 13-15, 1998.

http://beyondapollo.blogspot.com/2011/09/design-reference-mission-10-1993.html