Image credit: NASA. |
The Phase I nuclear stage was envisioned as a NERVA II with a single 33-foot-diameter propellant tank sized for launch from Earth atop a two-stage, 33-foot-diameter Saturn V rocket. Its main purpose would be to push piloted spacecraft out of low-Earth orbit (LEO) toward Mars.
Phase II commenced in October 1969, immediately after President Richard Nixon's Space Task Group (STG) endorsed (with reservations) NASA's aggressive Integrated Program Plan (IPP) for future U.S. spaceflight. The IPP was the brainchild of George Mueller's NASA Headquarters Office of Manned Space Flight, which supervised NASA's manned spaceflight centers, including MSFC.
In NFSD Phase II, MSFC directed its contractors to design a reusable nuclear rocket stage equipped with a 75,000-pound-thrust NERVA I engine. The stage, dubbed the Reusable Nuclear Shuttle (RNS), was intended mainly for roundtrip crew and cargo flights between space stations in LEO and lunar orbit. In January 1970, MSFC presented the contractors with an ambitious RNS traffic model calling for 157 Earth-moon flights between 1980 and 1990 by a fleet of 15 RNS vehicles, each toting 50 tons of cargo. Piloted Mars missions, though still considered a part of the IPP, were in NFSD Phase II relegated to secondary importance.
Image credit: NASA. |
A Reusable Nuclear Shuttle tanks up at the Orbital Propellant Depot using a soft-docking Refueling Adapter. Image credit: NASA. |
Soon after, MSFC directed LMSC to examine launching the RNS inside the Space Shuttle payload bay, which was expected to measure 15 feet wide by 60 feet long. LMSC's Shuttle-launched "modular" RNS would comprise a NERVA I engine and multiple hydrogen tanks launched separately into LEO and joined together through a labyrinth of pipes. NAR continued work on a single-tank RNS sized for launch on a future heavy-lift rocket, while MDAC divided its study efforts between the two launch options.
Phase II segued into Phase III in May 1970, when MSFC directed the NFSD contractors to assume a 1978 or 1979 NERVA I flight readiness date. The postponement reflected an anticipated Fiscal Year 1971 NERVA funding cut. MSFC also directed the contractors to limit to 150 tons the amount of liquid hydrogen propellant each RNS would carry.
In February 1971, with the NFSD study set to conclude in less than two months, D. J. Osias, an analyst with NASA Headquarters planning contractor Bellcomm, summarized and critiqued reports prepared by the three contractors. He began by examining the ways that the contractors had approached the problem of radiation shielding. "Nuclear propulsion," he wrote, "complicates in-space operations by introducing a radioactive environment."
All the RNS designs included a 3000-pound radiation shield on top of the NERVA I to create a conical radiation "shadow" for crew protection, but also relied on the vehicle's propellants and structure for supplemental shielding. Osias asserted that "in regard to radiation shielding. . .the most optimistic results are being accepted and attention to the problem is diminishing."
He also noted that, as liquid hydrogen was expended as propellant, it would cease to be available to serve as radiation shielding. As the RNS tank or tanks emptied, crew radiation dose would thus steadily increase. To solve this problem, NAR had developed a "stand-pipe" single-tank RNS concept, in which a cylindrical "central column" running the length of the main tank stood between the crew and the NERVA I engine. The central column would remain filled with hydrogen until the surrounding main tank was emptied. MDAC, for its part, had developed a "hybrid" RNS shielding design that included a small hydrogen tank between the bottom of the main tank and the top of the NERVA I engine.
Image credit: NASA. |
Radiation would create other operational problems, Osias wrote. Spacecraft could dock with an RNS by approaching through the cone-shaped radiation shadow that protected its crew. Docking an RNS to a large vehicle that protruded beyond the shadow — for example, a space station or a liquid hydrogen propellant depot — would, however, generate obvious problems. The large vehicle's crew might be exposed to radiation from the NERVA I; more insidious, the large vehicle's structure would reflect radiation back at the RNS, endangering its crew.
The NERVA I engine would emit radiation not only while it was in operation; it would also generate spent nuclear fuel that would emit harmful levels of radiation for decades or centuries. Osias noted that NAR had "repeatedly emphasized [that] maintainability is essential to economic operation of the RNS." A spacewalking repairman who approached to within 400 feet of the side of an RNS 10 days after its tenth (and, going by MSFC's traffic model, final) Earth-moon round-trip would, however, receive one REM per hour from the spent fuel it contained. Maintenance robots might replace the servicing capabilities of astronauts, Osias noted, but such systems would need costly development before they could become available.
Osias also reported that the "NFSD contractors. . .devoted little effort to [studying] emergency operations and malfunctions," adding that "[n]uclear systems, more than chemical propulsion vehicles, have the ability to involve the general population of the [E]arth in a space accident." A NERVA I explosion in LEO, for example, could lead to "random reentry of large pieces of radioactive material" that would probably survive reentry heating and strike Earth's surface. He urged that prevention of "return of the NERVA engine to the [E]arth's surface. . .be a basic rule of nuclear propulsion planning."
In NFSD study Phase II, LMSC estimated that, after just one Earth-moon round-trip, enough spent fuel would have accumulated within a NERVA I engine that it would need to remain in a safe high-altitude disposal orbit for 135 years. By the end of its operational life — after ten Earth-Moon flights — the "most desirable method of disposing of an engine" would, Osias wrote, be to "send the RNS on an unmanned, one-way mission to deep space."
The same month Osias completed his critique of the PFSD contractor studies, veteran New Mexico Senator Clinton Anderson, a close friend of former President Lyndon B. Johnson and a long-time nuclear rocket supporter, called a hearing to highlight the Nixon Administration's plan to slash NERVA funding from $110 million in Fiscal Year 1972 to only $30 million. At the hearing, Acting NASA Administrator Robert Seamans, an STG member, explained that Space Shuttle development had priority over NERVA development because the Space Shuttle was the essential transportation element that would launch into space all other IPP elements, including the RNS. He told Anderson that "NERVA needs the Shuttle, but the Shuttle does not need NERVA."
Six months after the NFSD contractors completed their reports, the Nixon White House unveiled its Fiscal Year 1973 budget request. As many had feared, it contained no funding for continued NERVA development. Anderson was ill and no longer able to adequately defend NERVA. A group of more than 30 pro-NERVA congressmen sought to sway the Nixon Administration, but to no effect. The final NERVA ground tests occurred in June and July 1972, after which the program was terminated, ending nearly 20 years of U.S. nuclear propulsion development.
Sources
"Status of Nuclear Flight System Definition Studies — Case 237," B71 02018 (NASA Contractor Report 116601), D. J. Osias, Bellcomm, Inc., 9 February 1971.
Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA SP-2001-4521, David S. F. Portree, NASA, February 2001.
More Information
Think Big: A 1970 Flight Schedule for NASA's 1969 Integrated Program Plan
Series Development: A 1969 Plan to Merge Shuttle and Saturn V to Spread Out Space Program Cost
Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1969)
Humans on Mars in 1995! (1980-1981)
The U.S. nuclear rocket program was always an odd beast, but during Apollo it was a way for LBJ to reward his friend and long-time supporter, Clinton Anderson. Though JFK in his May 1961 Urgent National Needs speech called for nuclear rockets in addition to a man on the moon, he seems to have not fully understood the implications and cut back on the program as soon as he did. LBJ acquiesced, going so far as to confirm JFK's 1963 decision to ground the Reactor In-Flight Test (RIFT), would would have seen one of the first Saturn Vs launch a nuclear rocket as early as 1967. With that early 1964 decision, U.S. nuclear rocketry became ground-based.
ReplyDeleteI go into this in some detail in my NASA monograph HUMANS TO MARS. Interestingly, one of the more bombastic nuclear rocket fans cited my account as an example of how NASA hates nuclear rockets. Or something. I wasn't quite sure what he didn't like about it, truth be told. Possibly that it spoke of challenges the technology faced in the 1960s and 1970s. Mentioning what people in the 1960s and 1970s saw as challenges is not a rejection of the technology.
We've come a long way, and I think that nuclear rockets could contribute meaningfully to a range of missions, should we ever decide to undertake some of those missions. There'd still be public concern over launching nuclear reactors, though maybe not as much as all that: concern over nuclear power sources (not reactors) in space declined from Galileo to Cassini to New Horizons to Curiosity. Did anyone protest Curiosity's launch? I confess that I don't recall.
dsfp
This comment has been removed by the author.
ReplyDeleteThe odd thing with nuclear thermal rocket is that the Soviets went much farther than NERVA ever did. The RD-0410 was much smaller and hass little thrust, but it lasted until the fall of the Soviet Union - in 1991 !
DeleteThe Soviets planned their own RIFT by placing a RD-0410 on a Proton but it seems that Chernobyl killed that idea.
https://books.google.fr/books?id=jKmIclMIwPAC&pg=PA299&dq=%22RD-0410%22%22Proton%22&hl=fr&sa=X&ved=0ahUKEwjyzYTn3p3LAhUClg4KHQaJCVUQ6AEIIzAB#v=onepage&q=%22RD-0410%22%22Proton%22&f=false
The rationale for NERVA- reducing the mass to be boosted from Earth- vanishes if LO2 is available from in-situ sources like the Moon. Easier just to combust the hydrogen then to use all LH2 for reaction mass, plus the considerable weight of the reactor itself.
ReplyDeleteThat's an interesting point. I wonder whether establishing a LOX facility on the moon would be more costly than a series of (for example) NERVA Mars missions? Of course, if the LOX facility is intended to supply other space infrastructure, then it seems like a no-brainer.
Deletedsfp
Looks like Planetary Resources gave up mining rare metals (the Earth market would crash) and realized that the best asteroid resource is water.
DeleteUsing an hydrolisis system powered by cheap and abundant solar power, asteroid water would be split into liquid hydrogen and liquid oxygen. Then Planetary Resources would sold those propellant to NASA (or SpaceX, or someone else) planning a manned trip to Mars.
As an alternative you could produce hydrogen peroxide (H2O2) which is a very good oxidizer.
The reference to a NERVA explosion reminds me of the Apollo-N accident in Stephen Baxter's novel Voyage.
ReplyDeleteShould the orbital propellant depot have sunshades to help prevent boiloff of the liquid hydrogen fuel?
Would boiloff made carrying liquid hydrogen containers inside a shuttle cargo bay problematic? I wonder if a better approach would have involve an unmanned booster and automated rendezvous and docking.
I sometimes wonder whether Baxter had access to some of the documents I write about. I don't consider the circumstances of VOYAGE wholly plausible, but I like the book.
ReplyDeleteNASA expected that "super-insulation" and abundant electricity for active cooling from the twin reactors would keep the liquid hydrogen liquid. As for hydrogen gas buildup in the payload bay, this was a time when NASA believed that it just might be able to build an Orbiter with reusable internal liquid hydrogen/liquid oxygen tanks. Had enough money been thrown at the problem, perhaps this would have been possible.
dsfp