An improved electrothermal device is disclosed. An electrothermal thruster utilizes a generally cylindrical heat exchanger chamber to convert electricity to heat which raises the propellant temperature. A textured, high emissivity heat element radiatively transfers heat to the inner wall of this chamber that is ion beam morphologically controlled for high absorptivity. This, in turn, raises the temperature of a porous heat exchanger material in an annular chamber surrounding the cylindrical chamber. Propellant gas flows through the annular chamber and is heated by the heat exchanger material

Marinos, Charalampus

Mirtich, Michael J.

Penko, Paul F.

Sovey, James S.

Zavesky, Ralph J.

English

NASA Technical Reports Server

United States Patent [I91 
Zavesky et al. 
[54] HEAT EXCHANGER FOR 
[75] Inventors: Ralph J. Zavesky, Parma; James S. 
ELECTROTHERMAL DEVICES 
Sovey, Strongsville; Michael J. 
Mirtich, North Olmsted; 
Charalampus Marinos, Middleburg 
Hts.; Paul F. Penko, Berea, all of 
Ohio 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United States of America as 
[21] Appl. No.: 636,463 
[22] Filed: Jul. 31,1984 
[52] 
[58] 
1561 References Cited 
[51] Int. Cl.4 ............................................... M05B 9/00 
U.S. Cl. .................................... 60/203.1; 219/275 
Field of Search ............... 60/203.1; 219/271, 274, 
219/275, 374, 381, 382 
U.S. PATENT DOCUMENTS 
1,091,637 3/1914 Davis .................................. 219/271 
1,803,334 5/1931 Lehmann ............................ 219/271 
3,210,927 10/1965 Brinich et al. ....................... 60/35.5 
3,339,260 9/1967 Burne et al. ......................... 219/381 
3,350,884 11/1967 Colombani et al. .................. 60/203 
3,425,223 2/1969 Browning ............................. 60/203 
3,439,501 4/1969 Moore ................................... 60/203 
32- 
34 
[ I I ]  Patent Number: 4,608,821 
[45] Date of Patent: Sep. 2, 1986 
3,635,824 1/1972 Brandes et al. ..................... 219/381 
3,718,805 2/1973 Posey .................................. 219/275 
3,956,885 5/1976 Davis et al. ................... 60/39.46 M 
4,038,557 7/1977 Gildersleeve, Jr. et al. ....... 290/1 R 
4,106,891 8/1978 Schladitz ............................. 219/274 
4,164,642 8/1979 Ebert ................................... 219/347 
4,169,351 10/1979 Barber ................................... 60/203 
4,211,072 7/1980 Twardy et al. ....................... 60/203 
4,232,093 11/1980 Miller .................................. 244/123 
4,297,563 10/1981 Berry ................................... 219/275 
4,322,946 4/1982 Murch et al. ...................... 60/203.1 
4,362,917 12/1982 Freedman et al. ........... 219/10.55 F 
4,523,429 6/1985 Bingley .............................. 60/203.1 
Primaly Examiner-Louis J. Casaregola 
Assistant Examiner-Donald E. Stout 
Attorney, Agent, or Firm-Gene E. Shook; John R. 
Manning 
ABSTRACT [571 
An electrothermal thruster utilizes a generally cylindri- 
cal heat exchanger chamber. A textured, high emissiv- 
ity heater element radiatively transfers heat to the inner 
wall of this chamber that is ion beam morphologically 
controlled for high absorptivity. This, in turn, raises the 
temperature of a porous heat exchanger material in an 
annular chamber surrounding the oylindrical chamber. 
Propellant gas flows through the annular chamber and 
is heated by the heat exchanger material. 
6 Claims, 2 Drawing Figures 
,IO 
18 
14 
https://ntrs.nasa.gov/search.jsp?R=19870007442 2020-03-23T15:09:40+00:00Z
U.S. Patent Sep. 2,1986 4,608,82 1 
32- 
34 
FIG. I 
+ 44 
FIG. 2 
4,608,821 
1 
HEAT EXCHANGER FOR ELECTROTHERMAL 
DEVICES 
ORIGIN O F  THE INVENTION 
The invention described herein was made by employ- 
ees of the U.S. Government and may be manufactured 
and used by or for the Government for governmental 
purposes without the payment of any royalties thereon 
or therefor. 
TECHNICAL FIELD 
This invention is concerned with improving electro- 
thermal devices. The invention is particularly con- 
cerned with providing rugged long life heaterheat 
exchangers for electrothermal thrusters. 
An electrothermal thruster uses resistive heaters and 
heat exchangers to convert electricity to heat which 
raises the propellant temperature. The heated propel- 
lant is accelerated in a nozzle to yield high propellant 
exhaust velocity or specific impulse. Very low thermal 
losses can be achieved through thermal insulation and 
effective heat transfer mechanisms making an electro- 
thermal thruster a highly efficient device. 
Many different types of heat exchangers for electro- 
thermal propulsion devices have been suggested. These 
include fast heatup, thermal storage, concentric cylin- 
der, vortex flowheater, contact resistance, and radia- 
tive type heat exchangers. 
A major problem has been encountered in the heat 
exchange devices of the prior art because a heating 
element is immersed in the propellant. This positioning 
causes material/propellant interactions that may lead to 
heat exchanger failure. 
In radiative heat exchanger devices the electrical 
heater is not immersed in the propellant. This type of 
device is a less efficient heat exchanger because of the 
optical properties of the heating element which exhibits 
low emissivity as well as the receiving surface which 
does not have high absorptivity. Thus, such a means for 
heat transfer to the propellant is not desirable where 
high efficiency is important. 
Current radiatively heated thrusters use wire heaters, 
in a coiled-coil form, which readily sag and do not have 
great potential for operating times of a few thousand 
hours at temperatures in excess of 1800" C. Heater life- 
time may be limited because of crack flaws in the heater 
wire as well as grain growth and creep effects. If the 
heater element fails the thruster remains operational, 
but because of the lower specific impulse operation, 
larger quantities of propellant may be consumed. 
It is, therefore, an object of the present invention to 
provide an electrothermal device with a long life, high 
efficiency heat exchanger having a coiled tube heater 
which is not exposed to flowing gas for radiatively 
heating a heat exchanger material in contact with the 
gas. 
BACKGROUND ART 
Davis et a1 U.S. Pat. No. 3,956,885 discloses an elec- 
trothermal reactor which controls the decomposition of 
chemical compounds and generates gas flow. In Gilder- 
sleeve U.S. Pat. No. 4,038,557 radiant energy is con- 
verted into thermal energy, utilizing forced convection 
through a fluidized bed. 
Barber U.S. Pat. No. 4,169,351 shows an electric 
heater surrounded by a coaxial tubular copper jacket 
and a resistanceheated chamber in an electrothermal 
thruster. An electrothermal thruster is also disclosed in 
Browning U.S. Pat. No. 3,425,223. Murch et a1 U.S. Pat. 
No. 4,322,946 discloses an electrothermal thruster for 
satellite control. 
U.S. Pat. No. 3,350,884 to Colombani et a1 is con- 
cerned with an electro-thermic ejector. Moore U.S. Pat. 
No. 3,439,501 discloses an electrothermal gas feeding 
system. U.S. Pat. No. 3,210,927 to Brinich et a1 shows 
an electrothermal rocket while Twardy et a1 U.S. Pat. 
10 No. 4,211,072 discloses the thermal decomposition of 
liquid fuel by an electric heating coil. 
5 
DISCLOSURE O F  THE INVENTION 
This invention relates to an electrothermal thruster 
l5 which utilizes a generally cylindrical heat exchanger 
chamber with a coiled tube electrical heating element. 
An annular jacket filled with radiantly heated refrac- 
tory metal, or ceramic particles surrounds the coiled 
tube heater. Propellant gas is admitted into the intake 
2o end of and flows through the annular jacket. This gas 
discharges into a thrust nozzle at the outlet end of the 
chamber. 
An important feature of the invention is that the 
coiled tube heater element which is not in contact with 
25 the propellant gas radiatively heats a heat exchanger 
material which is in contact with the gas. Also, the 
refractory, ceramic, or cermet particles used in the heat 
exchanger are sized to allow the proper gas through 
BRIEF DESCRIPTION O F  THE DRAWINGS 
The details of the invention will be more fully de- 
scribed when taken with the accompanying drawings in 
FIG. 1 is a vertical section view of an electrothermal 
thruster having a heat exchanger constructed in accor- 
dance with the present invention, and 
FIG. 2 is a vertical section view of an alternate em- 
3o puts while preventing clogging. 
35 which 
40 bodiment of the invention. 
BEST MODE FOR CARRYING OUT THE 
INVENTION 
Referring now to the drawing there is shown in FIG. 
45 1 an electrothermal thruster 10 having improved heat 
exchanger means constructed in accordance with the 
present invention for heating a gaseous propellant. This 
propellant is supplied to the thruster 10 through a sup- 
ply line 12 from a suitable source (not shown). The 
50 heated propellant is discharged through a nozzle 14 in a 
manner well known in the art. 
The heat exchanger of the present invention includes 
an inner hollow cylindrical housing 16. The inner sur- 
face of this housing 16 is ion beam textured to morpho- 
55 logically provide for high absorptivity. The outer sur- 
face of the housing 16 is similarly textured to provide 
for high emissivity. 
An outer hollow cylindrical housing 18 surrounds the 
inner housing 16 in spaced relationship. The space'be- 
60 tween the inner housing 16 and the outer housing 18 
forms an annular chamber 20 which serves as a conduit 
to conduct the propellant to the nozzle 14. 
The propellant is preheated in a long path provided 
by a coiled wire 22 which is positioned between the 
65 inner housing 16 and the outer housing 18. This length- 
ened path improves the efficiency by taking heat from 
the housings 16 and 18 that would normally be con- 
ducted to the back of the thruster and lost. 
4,608,82 1 
3 
A suitable heat exchanger material 24 is retained 
within the chamber 20. This material is preferably in the 
form of discrete refractory, ceramic, or cermet particles 
having diameters between about 0.001 inch and 0.100 
inch. It is contemplated that the heat exchanger mate- 
rial 24 also may be fabricated from hydroformed refrac- 
tory felt metal composed of fine wire filaments. 
Annular end caps 26 and 28 are provided at opposite 
ends of the chamber 20 to retain the heat exchanger 
material 24 therein. Each of these end caps has a plural- 
ity of small drilled flow passages to enable propellant to 
Wow through the chamber 20. The small holes in these 
end caps contain the particles of the heat exchanger 
material 24 but enable the propellant gas to throughput. 
Suitable radiation shielding (not shown) may be pro- 
vided around the outermost surface of the outer housing 
18. The entire device is preferably enclosed in suitable 
insulation (not shown). 
A coiled tube heater element 30 is mounted along the 
axis of the hollow housings 16 and 18. The surface of 
the heater element 30 is textured to provide for high 
emissivity so that heat is radiatively transferred to the 
inner wall of the hollow cylindrical housing 16. 
The heater element 30 is connected to a suitable sup- 
ply of electrical power (not shown) by leads 32 and 34. 
A thermal choke 36 forms a portion of each electrical 
lead 32 and 34 to reduce conducted thermal energy. 
Suitable radiation shielding 38 is provided to reduce 
radiated heat loss. 
In operation, a gas propellant in the line 12 is supplied 
to the thruster 10. This propellant may be hydrogen, 
ammonia, nitrogen, hydrazine, steam, carbon dioxide, 
methane, or other suitable gases. The propellant flows 
through the chamber 20 where it is progressively 
heated by the heat exchanger material 24 whose tem- 
perature has been raised by the radiatively heated hous- 
ing 16. The propellant is heated to a temperature be- 
tween 1200" C. and 2200" C., preferably 2000" C. A 
platinum alloy is used to make the various components 
of the heat exchanger when the operating temperature 
is to be at the lower end of this range. Rhenium alloys 
are used for high temperture operations. 
More particularly, the coiled tube heater element 30 
radiatively transfers heat to the inner wall of the hous- 
ing 16 which is morphologically controlled for high 
absorptivity. This, in turn, raises the temperature of the 
heat exchanger media 24 that are sized to enable effi- 
cient gas particle heat transfer and not be clogged by 
impurities in the propellant. The propellant passes over 
the porous media 24 and attains a maximum temperature 
prior to entering the nozzle 14 of the electrothermal 
thruster 10. 
It is apparent that a long-life, higher efficiency heat 
exchanger is provided. The lifetime is increased because 
the coiled tube heater 30 is not exposed to the propellant 
flows. This eliminates propellant/material capability 
interactions. The tubular heater 30 can easily be oper- 
ated with a 0.25 mm wall thickness which provides 
structured integrity and negligible sag at high tempera- 
ture. Because the propellant is not in contact with the 
heater element 30, many different propellants may be 
used in the system that are compatible with the porous 
media only. By passing the gas over the porous media 
there is a more uniform heating of the gas and a shorter 
gas path length. 
Another advantage of using this configuration is that 
it enables a proper value of resistance to be chosen for 
the heater element 30, thus allowing the use of porous 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
50 
55 
4 
media. The use of porous materials makes this heat 
exchanger very efficient and extremely rugged. Also, 
the fabrication does not depend on close tolerances 
thereby reducing the cost of fabrication. 
Propulsion is enhanced by thermodynamic processes. 
Low thermal losses are attained by using radiation 
shielding and insulation. Effective heat transfer mecha- 
nisms from the heat exchanger material 24 and the pro- 
pellant make the thruster 10 a highly efficient device. 
High reliability is achieved by using a rugged tube- 
heater which is isolated from the propellant. 
DESCRIPTION OF AN ALTERNATE 
EMBODIMENT 
Referring now to FIG. 2 there is shown an electro- 
thermal thruster 40 which comprises an alternate em- 
bodiment of this invention. Propellant from a supply 
line 42 is heated and discharged through a nozzle 44 as 
in the embodiment of FIG. 1. The thruster 40 has an 
inner cylindrical housing 46 and an outer housing 48. 
This embodiment is characterized by approximately 
fifty tubes 50 or circular channels milled in the annular 
heat exchanger. The tubes 50 replace the annular cham- 
ber 20 in FIG. 1 and contain the heat exchanger mate- 
rial. 
Not shown in FIG. 2 are radiation shields or thermal 
insulation which preferably surround the heat ex- 
changer. It is further contemplated that a swaged heater 
may be provided in conductive thermal contact with 
the inner housing 16 shown in FIG. 1. Such a heater 
would replace the coiled tube heater 30 for situations 
where lower temperature is required. 
Still another alternate embodiment is contemplated in 
the use of a cermet rod heater in lieu of the coil tube 30. 
It is further contemplated that the heaterheat ex- 
changer functions may be combined using porous cer- 
met material. 
While several embodiments of the electrothermal 
thruster have been described, it will be appreciated that 
various structural modifications may be made without 
departing from the spirit of the invention or the scope of 
the subjoined claims. 
We claim: 
I. A heat exchanger for heating a gaseous propellant 
to a temperature between about 200" C. and about 2200" 
C. in an electrothermal thruster having a nozzle com- 
prising 
a hollow housing forming a cylindrical chamber adja- 
cent to said,nozzle, said hollow housing having a 
textured inner surface to provide high absorptivity 
and an oppositely disposed textured outer surface 
to providerhigh emissivity, 
an outer housing surrounding said cylindrical cham- 
ber in spaced relationship thereto thereby forming 
an annular chamber for conducting said gaseous 
propellant to said nozzle, 
a porous heat exchanger material selected from the 
group consisting of refractories, ceramics, and cer- 
mets contained within said annular chamber, hous- 
ing 
a wire coiled about the outer surface of said hollow 
housing in engagement with said outer housing for 
providing a lengthened spiral flow path for said 
propellant to said annular chamber, 
an electrical heating element comprising a coiled tube 
having a wall thickness of about 0.25 mm and a 
textured surface for providing high emissivity 
mounted within said cylindrical chamber in spaced 
5 
relationship with said textured 
radiatively heating said hollow 
4,608,82 1 
6 
inner surface for 
housing and heat 
selected from the group consisting of platinum alloys 
and rhenium alloys. 
exchange; material without contactingsaid gase- 
ous propellant, 
means for supplying a gaseous propellant to said 
lengthened spiral flow path and seal annual cham- 
ber whereby said propellant is uniformly heated by 
said hollow housing and said porous heat ex- 
changer material as it flows therethrough in a mini- 
mum gas path length, and 
a thermal choke formed in said electrical heating 
element adjacent to said coiled wire for reducing 
conducted thermal energy. 
2. A heat exchanger as claimed in claim 1 wherein the 
hollow housing and the outer housing are of a material 
3. A heat exchanger as claimed in claim 1 wherein the 
porous heat exchanger material is in the form of discrete 
4. A heat exchanger as claimed in claim 9 wherein the 
discrete particles of heat exchanger material have diam- 
eters between about 0.001 inch and about 0.100 inch. 
5. A heat exchanger as claimed in claim 1 wherein the 
10 porous heat exchanger material is in the form of hy- 
droformed refractory felt metal. 
6. A heat exchanger as claimed in claim 1 including a 
plurality of tubes for forming said annular chamber 
surrounding the inner cylindrical chamber. 
5 particles. 
* * * * *  15 
20 
25 
30 
35 
40 
45 
50 
55 
BO 
45 
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
P A T E N T  NO. : 4,608,821 
D A T E D  : September 2 ,  1986 
INVENTOR(S) : Zavesky e t  a1 
It is certified that error appears in the above-identified patent and that said Letters Patent is hereby 
corrected as shown below: 
Column 1, l i n e  68, cance l  "resis tanceheated" and i n s e r t  
- - res i s tance-hea ted- -  
Column 4, l i n e s  59 and 60, cancel  "housing" 
Column 6, l i n e  6 ,  cancel  "9" and in se r t  - -3 - -  
Signed and Sealed this 
Twenty-third Day of December, 1986 
Airesr: 
Attesting Oficer 
DONALD J. QUIGG 
Commissioner of Patents and Tmdemarks 


Heat exchanger for electrothermal devices

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