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‘“ A; Makes“’:any warrantyor representat’i’on, e~ress

f .,*-, ‘,

or implied,withrespectto the accuracy,completeness,

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or thattheuse of any information,apparatus,method,or

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B. Assurn%sany liabilities with respectto theuse

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As used h– the above,“personactingon behalfof the

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to,any informationpursuantto his employment or contractwiththeCommission.

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8

LAMS-2186

NUCLEAR ROCKET ENGINEs

This document consistsof 21 pages

-QQ@s, ‘=QLOIS ALAMOS SCIENTIFIC LABORATORY

OF THE UNIVERSITYOF CALIFORNIA LOS ALAMOS ~!EwMEXICO

REPORT WRITTEN: March 1958

REPORT DISTRIBUTED: March 14,1958—

VERIFIED UNCLASSIFIED

VEFUFXEII UNCLASSFED

Per /l&% ,4-249-79

By m “,&&. /..~ +-~

F

SOME SCHEMES FOR NUCLEAR PROPUU310N

By

C. Longmire

‘. F. Re~nes

S. Ulam

UN~.A*frFIED

C@ltractW-’(4O5-ENG.36 withthe U. S. Atomic Energy Commission

1

Thisisan informalreportwhichhasbeenprepared

fora specialpurpose.It isintendedprimarilyforI

..

-.. -L internalLosAlamosLaboratorycirculation andisnot

enerzddistribution.

%== ●

● b :0

a$:i! ‘:’”$’”” ● 0 ● boeoa ● e

ODD

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S* ● **

● av

●

● ● .* .,:*.*

9* ● ●:00 ● *

● 9** ● ●:O :-O ● -

NUCLEAR ROCKET ENGINES

LAMS-2186

AEC Classified

AEC Classified

Technical Library,

Technical Library,

DMA

DRD

UCRL, Livermore

Manager, SFO (PaulAger)

Los Alamos Report Library

.

● mo ● ● ** 9** 9* ●°0 : ●

●

● ●-

●

● .9:0B-H*H 9** ..9 ● ** ● .

00 ***0*0 ● *

1-5

6-10

11-13

14

15-50

!,

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It is intended to present here a qualitative

tai.nschemes for nuclearly propelled rockets. The

the sequel stem from the schemata proposed by some

INTRODUCTION

description of cerideas sketched in

of us in the past.

Various details and technical points were discussed in a Rocket Group

which meets weekly in our Laboratory.

The scheme discussed in Part I might be considered as intermediate

between the one outlined in H-19551 and the ones where the idea is to

propel a nuclear rocket by having a gaseous fission reactor operating

2

inside the vehicle. New ideas on isothermal gaseous reactors are discussed in Part II.

‘On a Method of Propulsion of Projectiles by Means of External Nuclear

Explosions, Part I, Everett and Ulam, September, 1955.

%-821, Nuclem? Chinese Rockets, C. Longmire, May, 1956.

3

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PART I

C. Longmire and S. Ulam - Internal Explosions. Briefly speaking, we

imagine a great

cession,, These

surrouncied“bya

ous material or

number N of very mild explosions taking place in sucexplosions involve bomb-like assemblies of either r.etal

small amount of high explosive and essentially hydrogen-

‘k cores. Each of these explosions is supposed to

heat the total mass involved only to very moderate temperatures. To

fix the ideas we consider temperatures of the order of ~ ev, i.e.,

9,000°C, although temperatures up to a few ev may be useful. Each of

these explosions will involve

and several tens of kilograms

total yield of the order of a

equivalent. These explosions

only several kilograms of active material

of hydrogenous material, and therefore the

few hundreds of kilograms (sic!) of TNT

are, properly speaking, “fizzles” resembling burning rather than a true nucleer detonation. One imagines a

large chamber with steel walls of roughly paraboloidal shape with the

“explosions” taking place at its focus. The chamber my be considered,

for the purpose of this discussion, as being evacuated except for the

material.to be exploded. The linear dimensions of the chamber are large

coriparecLto the assembly which is exploding. For orientation, we may

4

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UNCL4551

assume the diameter of the chamber to be of the order cf 4 meters,

whereas the cliameterof the bomb, together

is say of the order of 40 cm. Each of our

as being in a liquid or solid state before

with the enclosing hydrogen,

bombs should be thought of

the explosion. This

sion will convert its whole mass into gas which will expand and

the chamber with high velocity psxtlcles impinging on the walls

ultimately escaping from the chamber.

explofill

and

A

The “bombs” are brought in in rapid succession from a storage

chamber and brought to the “nozzle” chamber where they are exploded.

Compared to t,heproposals made in JJM-1955, the present scheme differs

in the following respects: The explosions am of smaller yield. Their

number is greater by a factor of 10 or 20. They will be of longer

duration and lesser violence, and therefore, by order of magnitude,

the individual accelerations given to the body of

5

the rocket in each

UNCLAJJIFIID

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UNCLASSIFIE

push will be smaller. Secondly, they are made internally, which allows

a greater f’ractionof mass to be used in imparting the momentum. This,

of course, is more than counter-balanced by the greater n~ber of suPercritical assemblies that one has to employ. Let us say from the beginning that the total amount of fissionable material expended will be of

the order of a few tons, at least for a first design. This mkes it

appear, offhand, that the primary use of such rocket motors would be to

have large satellites and vehicles for interplanetiy travel, rather

than for stockpiling in large numbers.

We shall employ the following notations:

N= total number of exploding assemblies.

Ei i=l...N = the energy released in the i-th explosion.

Mi = the total amount of mterial exploded.

m. = the mass of fissionable mlx?rial in the i-th “bomb.” 1

R= the diameter of the nozzle chamber.

Pi = the pressure

d= the thickness

v = the velocity

e

on

of

of

the wall of the nozzle.

the wall.

propellant mass escaping the chamber.

P = mean molecular weight of the bomb material.

T = the temperature to which the mass Mi is brought as a

i

result of the nuclear reaction.

Ww = weight of the walls of the psxaboloid.

w = weight of the propellant.

P

Wa = weight of the structure of housing of the bombs, and

injecting mechanisms> instruments and “payload”.

.

6 UNCLASSIFIED

APPROVED FOR PUBLIC RELEASE

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w = Wv + Wp + Ww total weight

We ncw give a tentative set of values in e.g.s. units for our

quantities.

N = 103

m~=y=5.lo4gms

The

would be

R

T

d

P

effective volume

V_nR2~ = 3.14 X

= 2.102 cm

‘:ev

=3cm

=

3

V of our p=aboloid with a length of 300 cm

(2.102)2 ● 3.102- 4.107 cc.

Ww- 2TRd~p ~ 6.3x 2.1c2 ● 3.102X3 x8~10 tons

w - 103 X 5.1C4- 50 tons

P

Wa - 10 tons

w- 70 tons

The exit ve~locity Ve of tie propellant will be sensibly higher than

the thermal velocity of our material at the temperature obtained in the

nuclear explosion. This is so because of the effects of the recombination of the molecules and ions. If T=3ev,

u

K = 3, the thermal velocity v is about 6 Km/sec., and the final v

e

about 10 Km/sec. The

energy Ei of each explosion is then given by E -~ mv~ = 4 12 *X5X1O.1O

= 2.5 X1O 16ergs, about 500 kgs of TNT equivalent. The pressure on the

walls will be of the order of P - (?’- 1); = “4X 2.5 x 1016 -2.5 x

4 x 107

7 U}ICIAS$FIED

108

———

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-250 atrnc,spheres-4000 lbs/sq.in.

“Inthe first discussion we shall assume that the quantities are

independent of i, that is to say, each assembly and explosion have

constant characteristics.

!l?henumerical data above represent merely an order of magnitude

orientation about the scheme and are, of course, in no way opti~l.

There are many degrees of freedom in this scheme.

the fissionable material is “wasted” and we could

Ei within a very wide range of values - also the

hydrogenous material surrounding the bomb and its

Obviously, most of

choose our yields

composition of the

mass in proportion

to the nass of L?35isat ourdisposal, inalarge measure. Thegeome-

“ tries of the chamber, etc.,seem not to be limited from above by the

numbers adopted here.

SPeaki~ quli,~tively, the possible advantages of our scheme ~e

as follows:

l.. If we admit

the nuclear explosion

that the temperature of the material heated by

is of the order of 1/2+ 1 ev, the expansion of

this material i.ntie vacuum of the nozzle chamber will convert most of

the energy released and initially present in the form of thermal energy

to kinetic energy of the particles with the corresponding cooling of

the gas. The velocity of the escape of the propellant will be therefore

of the order of 10 kilometers per second, that is to say, the velocity

of a satellite. For the velocity of the final “payload” to be of this

order, one needs only a ratio ~ between the mass of the propellant

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U!’wuwlm

.

and the mass of the installation and instruments, etc.

2. We mentioned a ratio of about 10 between the linear dimensions

of the nozzle chamber and those of the explodin~ assembly. The density

of the gas which will fill the chamber before impinging on the walls

will be therefore 1/1000 of the original density. This means that the

pressure on the wall will be moderate. The tensile strength of the wall

of a fixed thickness depends, adversely, linearly on the inner diameter.

If we assume that the pressure on the wall is given by the Bernoulli

formula P =~p(v2) -- since P depends inversely on the cube of the

linear expmsion there is obviously a gain by having the walls of given

tensile strength far apart. This gain obtains as long as the total

weight of the propellant, auxiliary equipment and the “payload” exceeds

sensibly tke weight of the walls of the chamber where the explosions

take place. Heating by neutrons and gamms becomes even less of a problem - when, with the weight of the payload and equipment essentially

constant, the chamber is large.

Considerable computational and experimental work seems necessary

to provide a design of the above sort. First of all one should try to

calculate individual explosions which are to heat the material to be

reproduceable and as precise as possible. This should be done with the

greatest pcssible economy of the fissionable material. Probably experiments have to be made with actually exploding such assemblies in order

to learn about their characteristics. The action of the expanding gas

on steel or tungsten covered structures has to be studied in order to

9

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.

understand the erosion of the material by successive explosions of this

sort. The velocity of the propellant leaving the chamber has to be

calculated - possible benefits from shaping the exit of the nozzle should

be studied. One should discuss the possibilities of cooling of the walls

by “sweathg” if that should be necessary. We have not discussed the

problem of “pumping” individual assemblies at a sufficiently fast rate

and the concomitant engineering difficulties. At any rate, the problem

here involves a shoving in

in intervals of about 1/10

calculated in detail, also

of masses of the order of 50 kilograms each

sec. The problem of neutron heating should

the problem of the residual gas remaining

be

after the i - l-st explosion at the time when the i-th explosion is

to take place, etc.

It seems likely that a shock absorber between the thrust chamber

and the remainder of the missile is desirable, to spread the shsrp

impulses out over time as well as possible. The number of g’s that the

main structure has to stand can thus be reduced to a small number.

It apFears that steel of average 3 cm thickness will, for our

choice of the radius of the wall, contain 4000 lbs/sq.in.

Ths wall can be coated with tungsten to resist the temperature of

the gas accuxlulatingon its surface. The contact of the gas with the

wall is or extremely short duration - in a case like the one illustrated

above abwt 1 millisecond for each explosion, so that heating by conduction is seemingly negligible.

Materials other than steel could be considered for confining

10 UNUNW?’

the

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exploding gas, with greater strength for weights than steel.

The main problem is the construction of “economic” bombs giving

yields of .- 1 ton of TNT equivalent.

We had the benefit of conversation with George Bell on this problem andC. 13.Mills is in the process of calculating critical masses and

“alphas” for such UDk assemblies.

11 UNCI.AWIED

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PART II

F. Reines

*

- A gaseous Fission Reactor Rocket Motor.

1. Introduction —.—

The idea of using a gaseous fission reactor for rocket propulsion,

where the reacting materials are ejected, has the attractive feature

that heat transfer from heat source to working fluid, or propellant, is

extremely rapid and efficient.1,2 Difficulties with such a scheme center on the relatively large amounts of fissile materials which are thrown

out the nozzle. The point of view adopted here is that it is worth many

tons of fissile material to propel a manned ship into interplanetary

space, allowing for a landing on Mars, for example, and returning to

*

This chapter is, with minor modifications, contained in a memo, T-2-50,

dated January 17, 1958.

I shcmld like to acknowledge helpful discussions of these ideas

with many people at Los Alamos among whom were: Aamodt, Bell, Cranberg,

Leland, McSnteer, and Taschek. I especially wish to thank S. Ulam for

his interest and stimulation.

.

%teady State Fizzlers, R. W. Bussard, memorandum N-3-134 (1956). In

this memo references are given to considerations of H. T. Gittings (1954)

andD. S. Young andG. A. Jarvis (1955).

2

It should perhaps be added that static tests of such motors would not

be wasteful.of fissile material because the exhaust could be caught and

reused.

12

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earth. Nevertheless it is recognized that some limit on the expenditure of fissile material exists and that any design must keep this

csrdinal :l?actin mind. The limitations characteristic of other schemes

arise from the temperatures and pressures at which materials can be

used. It is conceivable that thermal gaseous reactors are much less

limited because of the possibility of generating heat directly in the

working f:Luidwhile converting the energy into ordered motion. In

principle, this could be accomplished in stages by first passing a

uranium-hycirogengas into a region surrounded by a neutron reflector

so that i-tbesomes supercritical and heats up, then allowing it to

expand through a nozzle ordering its random energy, repeating the process untiILthe desired propellant velocity is attained. The materials

limitation in such a scheme is the local heating in the vicinity of

the walls caused by the friction of the very high velocity propellantreactor gases. In addition, we must consider the heating of the container walls by the neutrons and gamma rays from fission and neutron

capture. These limitations are much less severe than those imposed

in the more conventional schemes in which fission energy generated in

a solid must be transferred out of the reactor and into the propellant.

The hope is

so that the

attractive.

that sufficiently

payload per pound

high propellant velocities can be attained

of fissile material ejected will become

13

—....—

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INCLWIWO .

2. The working fluid, critical masses

——

The working fluid, i.e. , the reactor-propellant, we propose is

a mixture of U235 and deuterium (D2) irrbimat’elymixed.3 Hydrogen (H2)

i.spreferable as a propellant because of its lighter mass but the

neu’tron capture cross section (0.33 barns at 1/40 ev) is so large that

the number of hydrogens per uranium atom must be kept low i.fa chain

reaction is to be

is mostly uranium

hand, we choose a

sustained. Under these conditions the propellant

and hence is extremely inefficient. If, on the other

deuterium-uranium mixture, then because of the low

neutrcm capture cross section in deuterium (0.0057 barns at 1/4.0ev)

reacting mixtures of ~n with n- 10> can be imagined. The critical

mass of L?35 in an infinite D20 reflector at normal temperature and

pressure has been calculated by Be114 to be 900 grams. It seems reasonable cn the basis of calculations given in the AEC Reactor Handbook

RH-1 for 1)20moderated reactors to believe that the gaseous critical

mass, though insensitive to n, should drOp tO 4300 gms for n = 2000.

In a finite reflector, say 50 cm thick, the critical mass might rise to

-600 gins,, The radius of the gaseous sphere would then be -35 cm,

and the pressure 310 atmospheres. Since we wish to operate at a maximum temperature in the range 2000-3000°K the pressure would, for this

size sphere, be 2300-3400 atmospheres. Such a pressure would dictate

3

The U235 might be contained in the gas ~6, the D2 stored as liquid.

4G. I. Bell, J-A-1874 (1955).

14

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an excessively heavy container and consequently an increase in volume

is indicated so as to reduce it to a more reasonable -500 atmospheres.

Because tineneutron moderation occurs primsrily in the D20 walls an

increase in container volume by a factor of 4.6 to 6.7 (or to a radius

of 60-70 cm) should not increase the critical mass of &’35 very much.

One final adjustment in these nunbers is required because of the elevated

temperature at which the system operatis. The fission cross section of

U*35 &-ops by about a factor of three when the temperature rises

273°K to 3000°K, thus raising the critical mass by this factor.

izing our numbers we have for an initial gaseous reactor stage:

Sphere: Rc = 60-70 cm

Reactant: U235D2000; 1.8 kgms U235, 31 kgmsD2

Reflector: D20, 50 cm thick

Pressure: 500 atmospheres

Temperature: 3000°K

3. Thrust, fuel consumption from initial stage motor

FUEL

FEED TO

MOTOR

NOT

SHOWN

/

/

Q

WALL OF

PRESSURE

VESSEL

D20 REFI.ECTOR 50CM

(

f

U235 THROAT

D 2000

!’;A’A

//

o

Rc# 65CM

t

\

NOZZLE

Fig. 1 Schematic of Initial Stage

15

from

s ummarAssume D2, disassociated into

2D, but do not count on extra

thrust due to recombination in

the nozzle (conservative).The

velocity of sound in D at

3000% is

Vs=i=’ #

= 1a4 X kl.25 X 10b X 3 X 103

Vs = 4.2 x 105 cm/sec.

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APPROVED FOR PUBLIC RELEASE
.

The ordered meximum velocity Vout of the D leaving the nozzle is

given by

v out = 6.1 x 105 cm/sec.

This corresponds to a

Suppose we ask

neglecting gravity, a

specific thrust of 630 seconds.

for a final rocket velocity Vf = 6 km/see then,

mass ratio

M

initial el .—— =

M

= 2.7

fi~Ll

is required.

M

initial

For Vf = 11 km/see, = e~~~6 = 6.2 is required. The

‘final

acceleration, a, we choose then determines the mean rate of propellant

ejection. Suppose a = 15 g and Vf = 11 km/see, then the time of

.

e~ection = 11 x lo>

’15 = 73 see; for a = 100 g, ‘loo = 11 sec.

15 x 103

M.

These numbers, together with the # determine the nozzle dimensions

*.!.

required. At this point it is instructive to calculate the number of

tons of payload plus vehicle per ton of I?35 ejected which achieves

‘f = 6, LIKxn/sec.

Mi M + Mf

—.

propell. = ~ + “prope11. =

‘:e ‘f ‘f

but M = %+y=~(l+~/~) so that propell.

16

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APPROVED FOR PUBLIC RELEASE
%) —+1

%

Mi

—. 1

‘f

and hence

‘f Mi ‘f

lun/sec q%

6 2.7 10

11 6.2 3.3

Elementsxy structural considerations suggest that a ratio

incompatible with a pressure vessel weight plus tanks for

*

but that 10 is quite reasonable.

of 3.3 is

the propellant

“The mass, M, of a pressure vessel is estimated from the consideration

of hoop stresses which yields the equation

where R. =

P

=

‘t =

M= z ~R3 w ~ 10-6 ~metriC tins ~

0s

t

container radius (cm) + 65 (reactor chamber radius)

+.50 (reflector thickness)

internal pressure (psi) = 7500 psi

permissible tensile stress (psi)

density of container material (gm/cm3).

For steel container (allowing no safety factor)

M

A more reasonable

safety factor, would be

= 2m(115)3 x7500x 7.85 -6 .Xlo

4x lo~

= 1.4 metric tons.

figure on the tank weight, allowing for a small

two tons.

17

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APPROVED FOR PUBLIC RELEASE
U!lcuwl

It is of course possible to imagine schemes which might improve

the performance of this single stage motor such as

cooling” of the container walls so allowing higher

tures. In addition, recovery of the &35 might be

having “sweat

propellant temperaconsidered.5 Though

worth pursving, these suggestions will not be discussed further here.

Instead we will outline the multiple heating proposal mentioned in the

introduction.

4. Multiple .— reheat motor (MRM)

LS>L2>LI

NEUTRON

‘RE’LETP+

eL3a

w+L2z ~

~~d . .. . .,...-..”

&

V///.,,J/... “.....<: .. ”.”,” .“. ”.;

. ..” . .

.. ....... . .. . . . .

.. . . . . . . . . .

. .

“.- . . . . .

‘;:.”I;;;”+-”: :”:’”:”:”i ~.”:”’,+”.” -“ nI-,”; “~ “ “E - . “ -e “+

[P%

Vow .”.....,. . .

.“. - . ...... . . . . $/1:’””-.”...y2:”-. “.. ”..VP ,’

\

l~q “....”;.:.“.”;-----. ...

NOZZLE b “ “ (; V2;VI”>VO)

NozzLE ~\” ““ .

FUEL-PROPELLANT mmm-mly

INJECTION

U235

02000

Fig. 2

In

reactant

Crude Schematic of Multiple Reheat Motor. (Not to scale)

this motor the fuel-propellant mixture is injected into the

chamber I where it is heated and expelled into chamber II

through the nozzle which serves to order the thermal energy somewhat.

The mixhn?e is now cooled and at a lower pressure passes into the part

%. w. 13ussardand G. M. Grover, N-3-270 (1957).

18

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● 9 ● ** 90

● ea ●*9 : ●** ●

● omoeo ●

● *O ● b :0: 0

● 00 ● *moo,* .* ..*.*. . . . .

..

.

90 ● m. ● . . .9. . . . . .

● **me

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● ●:0 : 8:0 :e. ● *

of region II which is surrounded by a neutron reflector making it

react and beak up again. It then is nozzled out into the non-reacting

section of’III again cooling down as its velocity increases. The procedure is repeated until the desired exit velocity is achieved.

We have in this brief

the multiple reheat motor.

desirable isothermal nozzle

description indicated the basic idea of

The scheme bears a kinship with the more

which would result if the number of stages

were increased without limit and were made continuous in space. However, for simplicity in explanation, let us consider at this time the

discrete situation outlined above.

The essential features of MRM have to do with the unconventional

fission reactors involved. As the fuel-propellant achieves higher

velocities, fission cross sections drop so that the material becomes

less reactive. This means that more excess reactivity has to be

included initially in the form of more #35 in the fuel-propellant and

the reflector must be progressively enhanced (thickened)for successive

6

stages. In a,ddition,since the time spent in a given region varies

inversely ‘Withthe ordered fuel-propellant velocity the length of each

section must be increased accordingly for a given heat input. In addition we must also consider the effects on criticality of neutrons being

6The fission cross section drops by a factor of -3 as the energy

rises from 1/’3 ev to 2 ev. Other fissile materials should perhaps

%3 considered since their cross sections behave rather differently. U ,

for emmple, has a broad high (a erage - 500 barn) resonance in the

2 ev region. This means that U2J3 is about 10 times better than U235,

i.e., that an order of magnitude less U233 would be required!

● 09 ● ● ** ,aa ● * ●**

● :

●

● : :“ h ;“ ; :

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swept along by the rapidly moving gases.

If this system is to represent an improvement over the single

heating arrangement it should give a greater thrust per unit weight

of fissile material employed. In rough terms we can say that an m

stage MRM will weigh more than m times that of the one stage motor

which is practically completely surrounded with moderator and that further, because of the increased fuel-propellant speeds approximately

‘?35 twice the ~ will be required. Suppose we achieve a V of propell.

2 ev (= 16.5 kilom/see), then for Vf = 11 kiloms/see,

Mi_=e11116.5 =eo.66 =195

●

‘f

or

3=18

~ 0.95=19

This fine ratio is, however,achieved at the cost of a much

heavier rocket motor, more than ten times as heavy, canceling out the

gain over the single stage motor.

!Ihesenunibersare at best crude, but they indicate the need for

an improvement over MRM. The direction in which to go is toward the

continucus or isothermal case, and possibly the use of t?33. The continuous isothermal nozzle reactor

without neutron moderator, and so

fissile mterial. It seems clear

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has only one “open” end, i.e., end

should be much more economical of

that the optimal isothermal system

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from the points of view of the minimal length and niassof fissile

material required

tute the motor.

At pre:sent

the hydrodynamics

energy generation

is one in which many smaller pexallel pipes constia grOUP in tie Theoretical Division is considering

of the isothermal nozzle and is looking into the

characteristics

It should perhaps be added

of such novel flowing reactors.

that static tests of such motors

would not be wasteful of fissile material because the exhaust could be

caught and reused.

.

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