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Electric Propulsion ContinuedPowerPoint Presentation

Electric Propulsion Continued

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Nuclear Rocket Engines

Nuclear Rocket Engines

- Nuclear Thermal Rockets : Propellant gets heated by conduction/
- convection from fuel.
- Nuclear Electric Propulsion: Electric power generated by
- heat engine or thermo-electric effects is used to drive electric
- Propulsion system.

- Specific Impulse more than twice that of chemical - AND
- Large-thrust possible because of high thrust/weight ratio.
- Shorter mission times:
- Proponents point out that for long-duration space missions, this leads to LOWER total radiation exposure per mission. (Less time exposed to the Big Nuclear Furnaces in the Sky)

Nuclear Thermal Rockets (NTR)

- Originally researched by the U.S. Air Force and NASA from the late 1940s to the 1960s, the use of nuclear energy to power a rocket engine has many advantages:
- High thrust (as much as chemical rockets – temperature and pressure limits are similar)
- High Specific Impulse
- Multiple re-starts
- Disadvantages:
- Heavy (reactor and shielding )
- Radiation concerns
- Testing / development / political issues

“Thermal” reactor: neutrons slowed down below 1eV, moderator of

light elements used.

“Fast”: broad spectrum of neutron energy up to 15eV. No moderator.

NTR Concept

Neutron Reflector: typically Beryllium shield

Propellant: H2 (high Ue), CH4 (better storage density) etc.

Types of Reactors

- NERVA (Nuclear Engine for Rocket Vehicle Applications )

Graphite fuel rods with uranium-carbide fuel particles – coated to protect

from hydrogen.

Coolant passes through channels in the rods.

Most fully developed, but low T/W

Particle Bed Reactor (PBR)

Particles of uranium-carbide fuel (coated) are packed between two porous cylinders. Hydrogen (or helium) is directly used to cool them.

PBRs have a higher fuel density and thus higher T/W. The configuration also allows a higher temperature in the working fluid before the fuel melts – Thus higher specific impulse.

inisjp.tokai.jaeri.go.jp/ ACT95E/11/1104.htm

“The technical risks of the PBR include:

+ Challenges in fabricating the high temperature fuel particles that are the key to this technology -- efforts to date have failed to conclusively demonstrate that fuel particles can withstand the rigors of the reactor operating environment;

+ The low thermal capacity of the reactor core increases the risk of thermal damage to the core in off-normal conditions, or during reactor cool-down;”

lifesci3.arc.nasa.gov/.../thomas/ Adv.prop/advprop.html

CERMET-core

- Source: www.ms.ornl.gov/.../ SCG/programs/OTT_HVPSM.html
- “Cermets: composite materials made by combining a ceramic and a metallic alloy.”
- “Cermets are made by 1) milling together powders of the desired materials, and 2) forming a shape, and then 3) sintering (heating) those materials together to form dense parts. The sintering process causes the component materials to bond together to form a new material with structural characteristics that are better suited for the intended use than either of the starting materials.”

- CERMET fuel: Hexagonal fuel elements. Uranium oxide particles imbedded in tungsten / tungsten-rhenium matrix; uranium oxide in molybdenum.
- Advantage: very long life ( > 40 hours).
- Uses fast fissioning reactor that does not depend as much on a “moderator” material to slow down the neutrons in the fission reaction.
- A newer system that still requires development
- Has potential for multiple restarts (very robust fuel elements)

“ 6 - Cermet Reactor ( http://lifesci3.arc.nasa.gov/.../thomas/ Adv.prop/advprop.html )

General Electric (GE) is the leading proponent for the Cermet Reactor, which has been evaluated for SDI(16) and SEI applications.(17) .. advantages :

+ .. improved thermal conductivity compared to metal oxide fuel elements.

+ .. extensive fuel test engineering heritage exists from the ANP and 710 programs.

+ … high retention of fission products in the fuel matrix. .. experimentally demonstrated in the 710 Program, in which most test fuel elements demonstrated fission gas fraction release of less than 10-9, while some fuel elements released fission fragment fractions in the range of 10-5 to 10-4. This should produce less stringent containment and confinement restrictions on Ground Test Facilities.

+ .. cermet fuel may offer improved swelling behavior, but this remains uncertain.

- .. lower fuel density relative to metal fuel elements, a potential disadvantage of cermet fuels is large core size, and thus greater core and shielding mass.

- In order to improve weldability, Rhenium is a potential Cermet cladding material. “

Candidate Working Fluids

Working Fluids: coolant, exhaust gases

As the propellant, we want something that can be easily heated and results in a high Isp.

Low molecular weight is preferred, but propellants with higher molecular weight may be used depending on storage volume constraints.

Hydrogen: MW = 2.016

Methane: 16.043

Carbon dioxide 44.01

Water: 18.015

Note: the above offer the possibility of replenishing propellant from extra-terrestrial sites.

For a given temperature limit, we can determine the specific heats from equations or tables and find the other quantities

such as :

NTR Performance Example heats from equations or tables and find the other quantities

Assume a PBR NTR. Hydrogen is heated to 3000K at a total pressure of 60 atm The engine operates in vacuum with e = 200, At = 100 cm2, hCF = 0.97, hC* = 1.0. Find vacuum specific impulse, thrust and mass flow rate of hydrogen.

At 3000K,

This gives heats from equations or tables and find the other quantities

At heats from equations or tables and find the other quantities = 200, =1.289,

Not counting any turbine/pump losses (I.e., closed cycle) heats from equations or tables and find the other quantities

Reactor Power heats from equations or tables and find the other quantities

The reactor provides the power necessary to heat the propellant to the required temperature.

Heat of vaporization

Specific power required. Depends on propellant initial / final temperatures

60

H2

is 48 MW/(kg/s)

For H2 at 3000K,

CH4

CO2

0

2000

T

3500

Reactor Mass heats from equations or tables and find the other quantities

From Fig. 8.21 in Humble, (Reactor Mass vs. Reactor Power for different

NTR technologies ) the mass of the reactor is estimated as 500 kg.

Reactor Shielding heats from equations or tables and find the other quantities

We also typically provide a “shadow shield” between the reactor and the

payload for in-space applications. A typical shield consists of layers of

Lithium hydride (LiH2) which is a neutron absorber, Tungsten to shield

against gamma rays, Beryllium as a neutron reflector. A typical shield may

be 25cm thick and have a mass-density of 3500 kg/m2 of surface area

Total mass = Reactor + shield + nozzle + turbopumps + core containment vessel

Typical thrust-to-weight ratio of an NTR is from 3 to 10.

Preliminary Design Decisions heats from equations or tables and find the other quantities

NERVA type core limited to ~ 2360 K temperature

CERMET: 2500K

PBR: 3200K

Determine gas properties

Determine Nozzle expansion ratio and Isp.

Sizing the System

Inert-Mass Fraction (when using hydrogen propellant): 0.5 to 0.7

Lack of database of nuclear engines prevents good estimate of inert mass

fraction: iterate.

From Isp and required thrust, find propellant mass flow rate.

Determine required reactor power to heat propellant to required temperature

at the required flow rate.

Determine system pressure levels: empirical correlations heats from equations or tables and find the other quantities

Prometheus Nuclear Engine: JIMO mission heats from equations or tables and find the other quantities

Jupiter Icy Moons Orbiter: Nuclear electric primary propulsion

- Two basic types of technology under consideration:
- radioisotope-based systems
- (2) nuclear fission-based systems.

RadioIsotope Thermoelectric Generators heats from equations or tables and find the other quantities

Heat from plutonium dioxide and solid-state thermocouples to

convert directly to electricity. Cold outer space is the cold junction.

General Purpose Heat Generator: 250 watts

Multimission Radioisotope Thermoelectric Generator: 100watts, 14+ years

http://spacescience.nasa.gov/missions/MMRTG.pdf

Stirling Radioisotope Generator heats from equations or tables and find the other quantitieshttp://spacescience.nasa.gov/missions/Stirling.pdf

Closed cycle Stirling cycle free-piston machine. Heat from GPHG with 600g

Plutonium dioxide at 650C. Heat rejected from other end at 80C. Closed-cycle

Engine converts heat to reciprocating motion – linear alternator produces

62-65 watts AC – converts to 55w DC.

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