We will review nuclear thermal gaseous core reactor rockets again in light of the new Bifrost project for nuclear thermal rockets. We had an interview with Tabitha Smith who is leading that project
A brief recap of the Liberty ship and then some information on Russian work on nuclear thermal rockets and then an embedded document that compares several advanced rocket concepts.
The liberty ship is a Gaseous Core Nuclear Reactor design, of the Nuclear Lightbulb subvariant. This design is from nuclearspace.com
It has an ISP of 3,060 and leverages existing technology to conservatively deliver 1000 tons to low earth orbit, 33% of its takeoff weight.
Development Nuclear Gas Core Reactor in Russia (by Koroteev in 2007)
After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions. While Russia has used over 30 fission reactors in space, the USA has flown only one - the SNAP-10A (System for Nuclear Auxiliary Power) in 1965. Research and development Nuclear Gas Core Reactor (NGCR) for rockets in Russia (USSR) started at Keldysh Center since 1954. The NGCR consists of the ¯ssile fuel with highly enriched uranium (U-235 or U-233) in a gas phase jet at pressure up to 1000 atm (100 MPa) and temperature up to 70000 K. For high specific impulse hydrogen is used as propellant and for radiation energy transfer to gas hydrogen is enriched by alkali metal vapors like Li. The shape of the considered fissile gas core is cylindrical. NGCR can provide much higher specific impulse than solid core nuclear rockets because their temperature limitations are in the nozzle and core wall structural temperatures, which are distanced from the hottest regions of the gas core. Consequently, nuclear gas core reactors can provide much higher temperatures to the propellant. Development of the NGCR project could be realized with the basic research in the field of high energy density matter as the nonideal plasma where Coulomb interaction more than thermal particle energy, radiation in dense Uranium plasma, turbulent mixing at high Re numbers between Nuclear Core and surrounding propellant, influence of magnetic field to mixing and Rayleigh-Taylor instability of gravity and inertial forces, etc. These problems have been investigated in Keldysh Center and MIPT, partially published by Ievlev (1964),2 Ievlev (1975),3 Martishin (1975),4 Handbook on Thermophysical Properties of Working Media Nuclear Gas Reactor (1980),5 Ievlev, Son (1985) ,8 Son (1979),9 comprehensive books edited by Koroteev (2002),76 and presented in the report. Due to the inability to perform live testing on earth, research is focused on experiments with solid state reactors IGR developed by Kurchatov Institute in Russia. Results of these investigations in basic science are reported.
The NASA spaceflight forum discusses nuclear thermal rockets
Kirk Sorenson comment -
The fused silica is fragile and any darkening would lead to a hot spot and failure of the "lightbulb" and its containment. This is no system that would pass muster for a frequently-used earth-to-orbit launch vehicle, even if the T/W values were there, which for a system reliant on a single-surface heat transfer is hard to imagine.
Robotbeat comment -
It should be noted that darkening of the fused silica is unavoidable through any chemical treatment, since there will be neutrons flying everywhere, transmuting the once-pure silica into stuff contaminated with phosphorus compounds (for instance).
Paul March -
As to stronger nuclear light bulb materials there are several other materials that might be useable such as a-axis sapphire, see attached transmittance chart, but it needs improvements in its neutron damage capabilities to be competitive with fused quartz.
In regards to enlarging the nuclear light bulb's surface area to maximize heat transfer, a modular light bulb approach could supply a solution such as that shown in the attached sketch.
Bottom line is that if we look for solutions to the gaseous reactor rocket's problems, we might find them. If we don't look, we will never do so.
The Nuclear DC-X was reviewed here
103 page pdf, advanced propulsion study for the US Air Force made in 2004 prepared by Eric Davis of Warp Drive Metrics. Eric W Davis is an advisor to the Lifeboat Foundation. On pages 48-57, the recent nuclear thermal rocket variants are described. They are estimated to require 5 years of technological development and could have launch costs of $85-150/kg for a single stage to orbit vehicle.
The ETO performance capability of Nuclear DC-X (Paul March, 2001 March, P. (2001), “LANTR VTOL-SSTO Reusable Heavy Cargo Lifter Launch Vehicle,” Briefing to the Advanced Deep Space Transport Group-Propulsion and Power Subgroup, and Private Communications, Lockheed-Martin Co., Houston, TX):
* VTOL-SSTO Heavy Cargo Lifter
* Nuclear DC-X propulsion system: 5,000 MWt class LANTR engine
* Utilize Air Force Timber Wind PeBR (see below for discussion of Timber Wind) or Russian Zrhydride heterogeneous reactor design with ternary-carbide fuels, operating at power densities ≈ 20 – 40 MWt/liter with reactor temperature of 3,000 K
* LANTR segmented aerospike exhaust nozzle with variable thrust control on each engine for attitude and flight trajectory control (no gimballing): 5-throttled LANTR engines per vehicle [LANTR is LOX-Augmented Nuclear Thermal Rocket]
* Canard stabilator flight control surfaces
* Landing struts (5) perform multiple functions: provide vehicle support, aerodynamic control, heat rejection, and landing shock absorption
* X-33 type Metallic Reentry Thermal Protection System on the bottom of the vehicle, plus carboncarbon leading edges on all landing struts/stabilators
* The LANTR engines are tilted inboard to place neutron shadow-shield between ground observers and the engines after lift-off – rely on the Conda-effect for flow turning on the aerospike exhaust nozzle
* Neutron shields: graphite-Al walled tanks filled with H2O loaded with 10B
* LANTR engine Oxidizer/Fuel = 4:1
* LANTR engine run time = 200 seconds, total boost time = 500 seconds
* Nuclear DC-X is VTOL from any prepared concrete pad
* 40% GTOW can be carried to LEO (at 400 km altitude and 51° inclination) from a 45° latitude launch
* Dry vehicle mass fraction is 30%, thus giving a payload fraction of 10%, or a payload mass of 10^5 kg to orbit on each flight
* Launch cost estimate: $150/kg of payload, if commercially developed and operated
* Launch cost estimate: $85/kg of payload, if developed and operated by the U.S. government
LANTR : LOX-Augmented Nuclear Thermal Rocket benefits
Summary of LANTR performance improvements over conventional NTR’s (ISNPS, 2003):
LANTR couples a reverse scramjet LOX-afterburner nozzle to a conventional LH2-cooled NTR to achieve the following benefits:
• LANTR engines are smaller, cheaper NTR’s with “big engine” performance
• Smaller, cheaper facilities for contained ground testing
• Variable thrust and Isp capability from constant power NTR
• Shortened burn times and extended engine life
• Reduced LH2 propellant tank size, mass, and boil-off
• Reduced stage size allowing smaller launch vehicles
• Increased operational range – ability to utilize extraterrestrial sources of O2 and H2
Variants: Pebble Bed, LANTR and Fission Fragment
1. Pellet Bed Reactor (PeBR) NTR (Nuclear Thermal Rocket)
a) Performance in pure NTR mode:
* Isp ≈ 1,000 seconds
* Thrust = 1,112 kN/engine
* Thrust/Weight > 12
* vex (exhaust velocity) = 9.8 km/sec
b) Performance in LANTR mode:
* Isp ≈ 600 seconds
* Thrust = 3,336 kN/engine
* Thrust/Weight > 38
* vex = 5.9 km/sec
2. Thin-Film Fission Fragment Heated NTR
• A high-performance NTR formulated by C. Rubbia
• Reactor core consists of thin-walled porous propellant flow passages coated with a thin layer of Americium-242m
• Propellant is injected radially into the flow passages and heated directly by fission fragments from the Am-242m liner
• This approach allow for much higher bulk temperatures in the propellant than in a
conventional NTR while keeping the propellant in contact with the walls (within the
material temperature limits)
• Theoretical Isp = 2,000 – 4,000 seconds
• Thrust is comparable to a conventional NTR
• Fission Fragment LANTR mode performance is comparable to the PeBR LANTR mode
The PeBR NTR in item 1 above has nuclear fuel that is in the form of a particulate bed (fluidized-bed, dust-bed, or rotating-bed) through which the propellant is pumped (El-Genk et al., 1990; Ludewig, 1990; Horman et al., 1991; ISNPS, 2003). This permits NTR operation at a higher temperature than solid-core NTR’s by reducing the fuel strength requirements. This results in the increased engine performance noted above. The core of the reactor is rotated about its longitudinal axis at approximately 3,000 rpm so that the fuel bed is centrifuged against the inner surface of a cylindrical wall through which H2 gas is injected.
This rotating bed reactor has the advantage that the radioactive particle core can be dumped at the end of an operational cycle and recharged prior to a subsequent burn, thus eliminating the need for decay heat removal, minimizing shielding requirements, and simplifying maintenance and refurbishment operations.
Thin-film fission fragment propulsion involves allowing the energetic fragments produced in the nuclear fission process to directly escape the reactor. Thus, the fission fragments, moving at several percent of the speed of light, can be directly used as the propellant (Chapline, 1988; Wright, 1990; Ronen et al., 2000a, b). However, March (2001) prefers to use Carlo Rubbia’s modification of this concept in which the fragments are used to directly heat a conventional NTR propellant (H2) for propulsion, as described in item 2 above (Rubbia, 1999, 2000; Ronen et al., 2000a, b). In order for the fragments to escape from the nuclear fuel and reactor, a low-mass density critical reactor must be constructed. In order to design such a reactor, highly fissionable nuclear fuels such as Americium (Am) or Curium (Cm) must be used. These fairly rare fuels are produced from reprocessed spent nuclear fuel (via the extraction of Pu-241 and Am-241), which is a very expensive multistep process. However, small amounts of Am- 242m are already available. Ronen et al. (2000a, b) demonstrate that Am-242m can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000th of a millimeter thick. Am-242m requires only 1% of the mass of U-235 or Pu-239 to reach its critical state. It should be noted that obtaining fission fragments is not possible with U-235 and Pu-239 nuclear fuels because they both require large fuel rods, which absorb their fission products. The fission fragment propulsion concept is near-term technology, however it requires the development of new technology and technology risk reduction.
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