12 August 2017

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

Cutaway of a reentering Apollo Command Module showing the position of its crew. Image credit: NASA
On 14 April 1964, a NASA Atlas-D rocket lifted off from Cape Kennedy, Florida, bearing the first Flight Investigation Reentry Environment (FIRE) payload. Project FIRE aimed to gather data on atmosphere reentry at lunar-return speed - about 36,000 feet per second (fps) - to enable Apollo engineers to develop the heat shield for the conical Apollo Command Module (CM).

Initiated in 1961 and managed by NASA's Langley Research Center (LaRC) under direction of the NASA Headquarters Office of Advanced Research and Technology, FIRE focused mainly on testing instrumented sub-scale model CM capsules in wind tunnels and thermal chambers at LaRC. Engineers realized, however, that there could be no substitute for data gathered in the actual spaceflight environment.

NASA rolls back the gantry structure surrounding the Atlas-D rocket bearing the first Project FIRE spacecraft, April 1964. Image credit: NASA
The Atlas-D rocket lobbed the Project FIRE payload, the 14-foot-long, 4150-pound Velocity Package (VP), onto an arcing course toward remote Ascension Island in the South Atlantic Ocean, a British possession that since 1957 had been home to U.S. missile tracking facilities. The VP cast off its two-part aerodynamic shroud and separated from the spent Atlas-D a little more than five minutes after liftoff. Attitude control motors mounted in its roughly cylindrical support shell then ignited to adjust its pitch so that it pointed its nose at Earth at a shallow angle.

About 21 minutes after separation from the Atlas-D and 800 kilometers above Earth, three rockets on the support shell ignited to spin the VP, giving it gyroscopic stability. Three seconds later, the VP cast off the support shell, revealing the engine bell of its solid-propellant Antares II-A5 rocket motor. Three seconds after support shell separation, the 24,000-pound-thrust motor ignited, driving the VP toward Earth's atmosphere.

The Antares motor burned out 33 seconds later, with the VP moving at nearly 37,000 fps. About 26 seconds later, the Apollo CM-shaped Reentry Rackage (RP) separated. Seven seconds after that, the 200-pound capsule fell past 400,000 feet, where the aerodynamic effects of reentry began to become obvious.

Image credit: NASA
Project FIRE Reentry Package. Image credit: NASA
The FIRE RP's heat shield heated rapidly as the falling capsule compressed and heated the atmosphere in its path. More than 300 sensors gathered data on the high-speed reentry environment. As the RP achieved a maximum speed of about 38,000 fps, the shockwave in front of the heat shield reached about 20,000° Fahrenheit (that is, about twice as hot as the Sun's surface).

Reentry heating formed a sheath of ionized gas around the FIRE RP, blocking radio signals. During the "blackout" period, which lasted for about 40 seconds, the RP stored data on magnetic tape. It transmitted the data after blackout ended.

Observers on Ascension Island - where the Sun had set - were able to track the FIRE RP visually as it automatically threw off two layers of heat shield material. They also observed the destructive reentry of the spent Antares II motor.

Thirty-two minutes after launch, the RP splashed into the Atlantic southeast of Ascension, about 5200 miles from Cape Kennedy. It was not designed for recovery.

NASA carried out the Project FIRE II test 13 months later, on 22 May 1965. The FIRE II RP was nicknamed the "flying thermometer" because it transmitted more than 100,000 temperature readings before ocean impact 5130 miles from Cape Kennedy. After FIRE II, engineers felt confident that they understood the atmosphere reentry effects the Apollo CM would experience as it returned from the moon.

The unmanned Apollo 4 (November 1967) and Apollo 6 (April 1968) Saturn V test missions carried out full-scale Apollo CM reentry tests. Astronauts first put the CM heat shield to the test at lunar-return speed during the Apollo 8 mission, which saw the second manned Apollo Command and Service Module (CSM) spacecraft orbit the moon 10 times on Christmas Eve 1968. Frank Borman, James Lovell, and William Anders reentered Earth's atmosphere in the Apollo 8 CM at nearly 36,000 fps on 27 December and splashed down safely in the Pacific southwest of Hawaii.

The FIRE flight tests were fresh in the minds of D. Cassidy, H. London, and R. Sehgal, engineers with Bellcomm, when they wrote a 14 April 1966 memorandum that proposed heat shield tests ahead of piloted Mars and Venus missions. Bellcomm was formed in 1962 to serve as the NASA Headquarters Apollo planning contractor, but almost immediately had extended its bailiwick to include planning beyond Apollo.

A piloted flyby spacecraft of the 1970s dispenses automated probes near Mars while a radar dish and a telescopic camera scrutinize the planet. Image credit: NASA
The three engineers wrote that Mars has a noticeably elliptical orbit around the Sun. Because of this, a piloted Mars flyby mission with a duration of 1.5 years would return to Earth at speeds ranging between 45,000 and 60,000 fps depending on where Mars was in its orbit when the flyby took place. A two-year Mars flyby mission would reenter Earth's atmosphere at between 45,000 and 52,000 fps. An opposition-class (short-stay) Mars "stopover" (orbiter or landing) mission would reenter at between 50,000 and 70,000 fps.

Venus, by contrast, has a nearly circular orbit around the Sun, so all flyby missions would return to Earth moving at about 45,000 fps. All Venus stopovers would reach Earth moving at between 45,000 and 50,000 fps. An opposition-class Mars stopover mission that flew past Venus before reaching Mars to speed up so that it could use a slow Earth-return path or flew past Venus during return from Mars to slow its approach to Earth would also reenter at between 45,000 and 50,000 fps.

Cassidy, London, and Sehgal wrote that, at speeds beyond 50,000 fps, reentry data gathered through testing for Apollo lunar missions no longer applied. Reentry heating would occur through different mechanisms and encompass a broader swath of the electromagnetic spectrum. This would increase turbulence and decrease the effectiveness of Apollo-type ablative heat shields (that is, heat shields designed to char and erode to dissipate reentry heat). In fact, at speeds beyond 50,000 fps, shield fragments detached by ablation could contribute to turbulence and heating.

The Bellcomm engineers acknowledged that braking propulsion might be used to slow a crew capsule to a better-understood Earth-atmosphere reentry velocity. They calculated, however, that slowing a piloted crew capsule derived from the Apollo CM from 70,000 fps to 50,000 fps would double the Earth-departure mass of the entire Mars stopover spacecraft. This would occur because extra propellants would be needed to launch the Earth-reentry braking propellants from Earth orbit to Mars and back again. Doubling the mass of the Mars spacecraft would in turn double the number of expensive heavy-lift rockets required to launch its components and propellants from Earth's surface to assembly orbit about the Earth.

They acknowledged that ground tests had provided some data on the interplanetary reentry velocity regime, but warned that the problem of aerodynamic surface heating involved "a complex interaction of vehicle size, shape[,] and heat protection characteristics." There would be, they added, “no substitute for testing specific configurations and materials in the actual environment of interest."

Cassidy, London, and Sehgal proposed that up to eight reentry capsules with attached solid-propellant motors be added to an Apollo Applications Program (AAP) Saturn V flight. AAP was NASA's planned post-Apollo program of Earth-orbital and lunar missions. The program aimed to use Apollo lunar mission vehicles in new ways. In addition to keeping the Apollo industrial team intact, AAP would see astronauts perform pioneering space biomedical and technology testing in Earth and lunar orbit, paving the way for piloted interplanetary voyages in the mid-to-late 1970s and the 1980s.

Image credit: NASA
Saturn V S-IVB third stage with cutaway and section showing spin tables and reentry capsules within the aft adapter that would link the stage to the Saturn V S-II second stage. Also shown is an Apollo Lunar Excursion Module (LEM)-derived lunar laboratory within the forward adapter that would link the top of the S-IVB to the bottom of the Apollo CSM. Image credit: Bellcomm/NASA
The Bellcomm trio proposed an interplanetary reentry test during a piloted lunar-orbital mission. The eight reentry capsules might be housed in the adapter linking the bottom of the Saturn V S-IVB third stage with the top of the S-II second stage. Normally S-IVB separation would see the adapter left behind on the S-II, but for this mission it would remain attached to the S-IVB. Each reentry capsule-booster combination would be mounted on an individual spin table to spin it about its long axis for gyroscopic stability before release.

The AAP mission Cassidy, London, and Sehgal envisioned would include an Apollo CSM and a small lunar-orbital laboratory derived from the Apollo Lunar Excursion Module (LEM) lander. The S-IVB's single J-2 engine would accelerate the S-IVB stage, the S-II/S-IVB adapter, the eight reentry capsules and their associated hardware, the LEM Lab, and the CSM out of Earth parking orbit into a high elliptical Earth orbit.

After S-IVB shutdown, the crew in the CSM would detach their spacecraft from the stage, turn it end for end, and dock it with the LEM Lab. They would withdraw the LEM Lab from the front end of the S-IVB stage, then ignite the CSM's Service Propulsion System (SPS) main engine to place the CSM/LEM Lab combination on course for the moon. A few days later they would fire the SPS again to enter orbit around the moon.

The S-IVB stage would retain about 30,000 pounds of liquid hydrogen/liquid oxygen propellants after the CSM and LEM Lab went on their way. About 12 hours after departure from parking orbit, the S-IVB, with its cargo of reentry capsules and solid-propellant motors, would reach its maximum altitude above the Earth. The stage would aim at Earth, restart, and burn all of its remaining propellants, attaining a velocity of about 41,100 fps.

After J-2 engine shutdown, the spin tables would spin up the eight reentry capsules with their solid-propellant motors, then springs would push them out of the S-II/S-IVB adapter. Once clear of the S-IVB stage, the solid-propellant motors would ignite to further accelerate the reentry capsules.

Cassidy, London, and Sehgal calculated that Project FIRE's Antares II-A5 motor could increase a 10-pound reentry capsule's speed to 56,100 fps after release from the S-IVB stage. It could boost a 200-pound capsule to 48,500 fps. A TE-364 solid-propellant motor of the type used to brake unmanned Surveyor landers during descent to the lunar surface could accelerate a 10-pound capsule to nearly 60,000 fps. A 200-pound capsule with a TE-364 motor could attain 53,500 fps.

Sources

"NASA Schedules Project FIRE Launch," NASA News Release No. 64-69, April 14, 1964

Astronautics & Aeronautics, 1964: Chronology on Science, Technology, and Policy, NASA SP-4005, NASA Historical Staff, Office of Policy Planning, 1965, pp. 135, 350

"Reentry Heating Experiment on Saturn V AAP Flights or Unmanned Saturn IB Flights - Case 218," D. Cassidy, H. London, and R. Sehgal, Bellcomm, 14 April 1966

Astronautics & Aeronautics, 1965: Chronology on Science, Technology, and Policy, NASA SP-4006, NASA Historical Staff, Office of Policy Analysis, 1966, pp. 244

Project FIRE in Langley Researcher - https://crgis.ndc.nasa.gov/crgis/images/2/26/Project_Fire_Newsletters.pdf

More Information

Starfish and Apollo (1962) (Bellcomm)

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965) (piloted flybys)

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975 (AAP and piloted flybys)

"Assuming that Everything Goes Perfectly Well in the Apollo Program. . ." (1967) (AAP)

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/early 1980 (1967) (piloted flybys)

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