Power supply design for unmanned electric propulsion missions to outer planets utilizes a store of curium-244 in compact array of capsules as energy source. Supply subassemblies are: heat source, converter equipment which supplies power, and safety equipment. System is designed for a 72,000 hour mission

Gietzen, A. J.

Heath, C. A.

Homeyer, W. G.

English

NASA Technical Reports Server

May 1972	 B7240212 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Radioisotope Thermionic Power Supply for Spacecraft 
The preliminary design of a radioisotope thermionic 
power supply for unmanned electric propulsion mis-
sions to the outer planets utilizes a store of curium-
244 in a compact array of capsules as an energy 
source. Heat evolved by curium disintegration flows 
to thermionic converters, where part of it is recovered 
as electrical energy, and the balance is rejected into 
space. The converters are connected in series-parallel 
to yield 5 kW of electric power at 15 volts DC. 
Protection from radioactivity is provided for per-
sonnel and adjacent hardware while the vehicle is on 
the launch pad or while it is in launch by an envelope 
shield that contains special safety devices and a heat 
rejection system. For example, while the power sup-
ply is on the ground, the envelope provides a lithium 
hydride neutron shield and cooling by a flow of 
water. During launch, heat pipes cool the reactor by 
providing thermal conduction paths to the outer skin 
of the envelope. Should a launch abort be necessary, 
the shield will prevent dispersal of the isotope by 
returning the power system to earth intact and by 
providing secondary containment. A launch escape 
system may be added to separate the power supply 
from the launch vehicle in the event of an explosion 
on the pad or of a low level abort. The entire shield 
and safety equipment subassembly is to be jettisoned 
once a heliocentric trajectory is established. 
The power supply system, including its safety 
equipment, is installed on the launch vehicle in front 
of the payload package. The three major subassem-
blies are: the heat source, the converter equipment 
which supplies power to the mission, and the safety 
equipment subassembly which is jettisoned when no 
longer needed. The mass is 426 kg before and 142 kg
after jettison of the envelope and safety devices. 
The radioisotope heat source is a drum-shaped en-
closure containing 136 cylindrical capsules of 244Cm302 
in a geometrical array with their axes in parallel 
alignment. Interspersed with them are the ends of 69 
emitter heat pipes that conduct energy to the therm-
ionic cells in the converter subassembly; heat is trans-
ferred from the capsules to the pipes by radiation. 
Another array of 408 auxiliary heat pipes reject heat 
while the system is on the ground or in the launch 
phase; this array is part of the safety equipment that 
is withdrawn from the heat source when the outer 
envelope is separated. 
The thermionic converter subassembly consists of 
69 converter cells with their heat pipes, and auxiliary 
equipment and supporting structures. The two elec-
trical elements of a cell are in the form of concentric 
cylinders, the inner being the emitter and the outer 
the collector. The pressure of cesium vapor in the 
sealed space between the emitter and collector is 
maintained by a graphite sorption reservoir in the 
recess in the insulator below the emitter. The emit-
ting surface is formed by vapor deposition of a 
0.25-mm (10-mu) layer of tungsten on the end of the 
emitter heat pipe. Waste heat is removed from the 
converter by a long heat pipe terminating in a space 
radiator array. 
A beryllium neutron shield protects the payload 
and power conditioning equipment from radiation 
while in space, but for added protection, the power 
supply is pushed away from the payload by extension 
of a 6-meter boom after the outer envelope has been 
jettisoned. Stranded aluminum electrical transmission 
lines conduct the power across the gap. 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000212 2020-03-17T03:33:36+00:00Z
The safety equipment includes: 
1. The 408 auxiliary heat pipes (potassium in stainless 
steel) which remove heat from the radioisotope 
capsules prior to and during launch. 
2. The helium-filled containment shell which provides 
secondary containment of the radioisotope and 
protects refractory metal components from oxida-
tion in the earth's atmosphere. 
3. The water container and water circulator to re-
move isotopic heat prior to launch. 
4. The lithium hydride neutron shield to protect per-
sons from neutron radiation prior to launch or 
following launch aborts. 
5. The graphite reentry shield to protect against aero-
dynamic heating following a high level launch 
abort. 
6. The aerodynamic flare which reduces the ballistic 
coefficient of the power source and assures a stable 
reentry attitude. 
7. The parachutes which reduce the velocity of im-
pact of the power source on the earth. 
8. The impact energy absorber which protects the 
secondary containment shell and other safety 
equipment from damage during impact on the 
earth's surface. 
9. A launch escape system which removes the radio-
isotope heat source from the vicinity of the launch 
vehicle in the event of a fire or explosion on the 
launch pad, or a low level launch abort. 
The power system has been designed for a mission 
of 72,000 hours. During the first 18,000 hours and 
the final 18,000 hours of operation in space, it must
develop full power, but the power requirement is 
reduced during the intervening 36,000 hours. A mass 
of 19.5 kg of the isotope, or 21.7 kg of 244Cm203 must 
be supplied as a source of 52 kW of thermal power to 
yield 5 kW of electrical, power in this design. Operat-
ing temperatures are estimated as 2030 °K for the fuel 
capsules, 1900°K for the emitters, and 1000°K for 
the space radiators. 
Notes: 
1. The planned launch vehicle is Titan III-D/Centaur. 
2. NASA mission analyses based on characteristics of' 
this design have indicated encouragingly high per-
formance levels for missions to Saturn, Uranus and 
Neptune. 
3. Requests for additional information may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94085 
Reference: TSP 72-10212 
Patent status: 
No patent action is contemplated by NASA. 
Source: William C. Homeyer, Albert J
.
 Gietzen,
and Cohn A. Heath of
Gulf General Atomic Company 
under contract to

Ames Research Center 
(ARC-10438) 
B72-10212'
	 Category 03


Radioisotope thermionic power supply for spacecraft

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server