Fermat's Library | The Earth as a Man-Controlled Space Ship annotated/explained version.

**Darol K. Froman** (1906–1997) was a Canadian-born American nuclea...

**MeV** stands for mega-electronvolt (10⁶ eV), the unit commonly us...

### Specific Impulse ($I_{sp}$) as a Measure of Rocket Efficiency ...

In 1962, controlled thermonuclear fusion research was still in its ...

In a 1960 Brookhaven lecture, physicist Edward Purcell demonstrated...

### Theoretical Engines vs. Practical Fusion Purcell analyzed tw...

The reaction that fuses four protons into helium (the main energy s...

### Deuterium Abundance Deuterium ($\text{D}$, or $^2\text{H}$) ...

The estimate works like this: hauling an asteroid home means speedi...

### Composition of Asteroids in the Solar System When Froman wro...

### Relativistic Time Dilation in Interstellar Travel Froman is ...

### Calculating the Proton Beam: 5 mA and 6 BeV The specific val...

Froman cites British astrophysicist Fred Hoyle. In the mid-20th cen...

#### Calculating the Net Energy Barrier The minimum energy requi...

THE EARTH

as a MAN-CONTROLLED

The folloiving is based on an after-dinner talk at

the Colorado Springs meeting of the Division of

Plasma Physics of the American Physical So-

ciety in November 1961. The author retired in

January of this year from his post as Technical

Associate Director of the Los Alamos Scientific

Laboratory after having served as a member of

the LASL staff for more than eighteen years.

SPACE SHIP

By Darol Froman

AM very happy to have the opportunity tonight to

express the pleasure of the Los Alamos Scientific

Laboratory in cosponsoring this meeting on Plasma

Physics. The fine facilities and cooperation of the Air

Force and the excellent accommodations in this hotel

certainty constitute a fitting environment for the schol-

arly papers and discussion I heard today.

Now, I don't know much about plasma physics, so

I'll talk about something else, but something which

touches on plasma physics and fusion. I would like to

make a few remarks about possible long-range applica-

tions and economics of fusion without much attention

to some of the practical aspects. However, what I shall

say is based upon such fundamental concepts as the

conservation of energy and momentum.

We all know the interest of the Air Force in space

and one can hardly study so potent a physical phe-

nomenon as fusion without looking for its possibilities

in space applications. To begin with I wish to remind

you that it is essentially impossible to make a return

space-ship trip to a near star and return in a human

lifetime. Edward Purcell

pointed this out very clearly

about a year ago. He considered two of the best imagin-

able engines: one which derived its power by conver-

sion of protons to alpha particles and one in which

matter and antimatter were annihilated. We can't ap-

proach such high performance with D-D and D-T re-

actions, but on the other hand, if I understand some of

the plasma physicists correctly, there is some chance

we may learn how to get such reactions in a controlled

way very soon—perhaps even in less than a million

years.

We can, of course, get it explosively now. We

are a long way from learning how to get four protons

to combine and even further from making and contain-

ing half a million pounds of antimatter.

Let's get a few basic numbers in hand. The oceans

contain about one third of a billion cubic miles of wa-

ter. The total deuterium content is about 5 X 10

tons.

The total energy available in complete D-D and D-T

Edward Purcell, Brookhaven Lecture, November 1960.

combustion is about 5 MeV = 8 microergs per deuteron

or 2.4 X 10

ergs/ton. Thus, the total energy available

from all the deuterium in the oceans is about 10

ergs,

an awesome number.

Now, let us invent a rocket engine which either

squirts the products of the D-D and D-T reactions out

the back end at their velocity of formation or allows

the products to thermalize. If the reaction goes fast

enough to burn essentially all the deuterium, there will

not be much difference in the specific impulses in these

cases.

What kind of specific impulse would we have?

A simple calculation shows it to be 2.2 X 10° sec. Per-

haps I should remind you that specific impulse means

the number of pounds of thrust exerted per second per

pound of propellant ejected. In proper units it is equal

to the exhaust velocity of the propellant gases meas-

ured relative to the vehicle, divided by g, the accelera-

tion of gravity, and has the dimension of time. Table 1

gives other specific impulses for comparison. Our engine

is not so bad when one considers that only the first

three or four of those listed are in current practice or

close to it.

Now with our rocket and D-D engine we can go out

to capture asteroids and bring them home. This is a

Table 1. Specific Impulses

Propellant

Solid Propellant

LOX-kerosene

LOX-H

H2 at 3000°K (nuclear heat)

235

fission products

D-D, T products

4H->He»

Matter-antimatter

in sec

2.25 X10

2.75 X10

4X10

1x10*

1.3X10

2.2X10

3.7X10

3X10

772o/rM{,*

200

3.2

1.001

1.0005

1.0003

1.00003

* Mass ratio of single-stage vehicle for escape from earth's gravita-

tional field.

JULY 1962

well-known and ancient idea. The order of magnitude

of the effort in accelerating and decelerating an asteroid

might be like two earth-escape missions with it—i.e.,

— (1.0005)

= 1.001. So at the best, not count-

ing getting our locomotive to the load and with a 100%

efficient engine, we will need to burn about 1 ton of D

for each 1000 tons of asteroid we bring back. Now I

don't know what asteroids are made of, but maybe

they are half nickel. Nickel is worth about 50# a pound

and Do about $100 a pound. So for $1 million we can

buy 5 tons of D

and with it bring back 2500 tons of

nickel worth $2.5 million. IVe been thinking of organiz-

ing the American Asteroidal Mining and Transportation

Co.

whose equipment is illustrated in Fig. 1. With a

A.A.M.& T CO.,;*

substantial subsidy from Uncle Sam to pay part of the

development and operations costs, it might be a good

thing. Anyone in the audience with a large bank ac-

count who wants in on the ground floor should see me

after the meeting.

Now let's look at more distant horizons. I do not

understand at all why would-be astronauts want to go

tootling off into interstellar space. The quarters and

food are likely to be miserable. To get anywhere and

back in a lifetime, the speed will have to be very high

so as to take advantage of the relativistic change in

clock rates. Let's say the speed is to be 99% of the

velocity of light. To attain this is not too hard on the

pilot and crew. It takes only about a year at an ac-

celeration of g to reach such a speed. There is, how-

ever, as Purcell pointed out, a little shielding problem

because of the interstellar hydrogen. It is estimated

that there is one atom of H per cubic centimeter in

space. A fair-sized rocket traveling at 0.99c through

this stuff would then receive about a 5-mA current of

6-BeV protons. This 30-MW beam would produce quite

a radiation field. Some of the problems of such space

travel are illustrated in Fig. 2. There should be some

way to get around these troubles. I got to feeling sorry

Fig. 1. An American Asteroi-

dal Mining and Transporta-

tion Company locomotive

bringing home a cargo

Fig. 2. The mode of space

travel in vogue in this cen-

tury presents some difficulties

20^century

space TRAVEL

PHYSICS TODAY

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