Groves' Memo to the
Chief of Staff (George C. Marshall)
30 July 1945
MEMORANDUM TO THE CHIEF OF
1. The following additional
conclusions have been drawn from the test in New Mexico with respect to the
probable effects of the combat bomb which will be exploded about 1800 feet
in the air:
a. Measured from the point on the ground directly below the explo-sion the
blast should be lethal to at least 1000 feet. Between 2500 and 3500 feet,
blast effects should be extremely serious to personnel. Heat and flame
should be fatal to about 1500 to 2000 feet.
b. At 10 miles for a few thousandths of a second the light will be as bright
as a thousand suns; at the end of a second, as bright as one or possibly two
suns. The effect on anyone about a half mile away who looks directly at the
explosion would probably be permanent sight impairment; at one mile, tem-porary
blindness; and up to and even beyond ten miles, temporary sight impairment.
To persons who are completly unshielded, gamma rays may be lethal to 3500
feet and neutrons to about 2000 feet.
c. No damaging effects are anticipated on the ground from radioactive
materials. These effects at New Mexico resulted from the low altitude from
which the bomb was set off.
d. Practically all structures in an area of one or two square miles should
be completely demolished and a total area of six to seven square miles
should be so devastated that the bulk of the buildings would have to have
major repairs to make them habitable.
e. At New Mexico tanks could have gone through the immediate explo-sion area
at normal speeds within thirty minutes after the blast. With the explosion
at the expected 1800 feet, we think we could move troops through the area
immediately preferably by motor but on foot if desired. The units should be
preceded by scouts with simple instruments. The nearest exposed personnel
should not be nearer to the blast than six miles plus the necessary
allowance for bombing inaccuracy and they would require a high order of
discipline and special but simple instructions. As an extra precaution,
extra special dark glasses might be issued to all commanders of units as
large as a platoon. If dropped on the enemy lines, the expected effect on
the enemy would be to wipe out his resistance over an area 2000 feet in
diameter; to paralyze it over an area a mile in diameter; and to impede it
seriously over an area five miles in diameter. Troops which were in deep
cave shelters at distances of over a mile should not be seriously affected.
Men in slit trenches within 800 feet should be killed by the blast.
2. The energy of the test
explosion has been broken down as follows:
Total theoretical energy contained in the bomb at 100% efficiency was
[sensitive information deleted]. Of this amount, 21,000 to 24,000 tons were
converted into actual energy made up of.
Blast - 10,500 tons minimum, 13,500 maximum
Light - 2500 tons
Waste Heat - 8000 tons, about 4000 of which went into the air and 4000 into
the ground. If the explosion had been at the combat al-titude of 1800 feet,
most of the 4000 that went into the ground would have been converted into
blast, making the total blast from 14,000 to 17,000 tons.
3. There is a definite possibility, [sensitive information deleted] as we
increase our rate of production at the Hanford Engineer Works, with the type
of weapon tested that the blast will be smaller due to detonation in
19advance of the optimum time. But in any event, the explosion should be on
the order of thousands of tons. The difficulty arises from an undesirable
isotope which is created in greater quantity as the production rate
4. The final components of the
first gun type bomb have arrived at Tinian, those of the first implosion
type should leave San Francisco by air-plane early on 30 July. I see no
reason to change our previous readiness predictions on the first three
bombs. In September, we should have three or four bombs. One of these will
be made from 235 material and will have a smaller effectiveness, about
two-thirds that of the test type, but by November, we should be able to
bring this up to full power. There should be either four or three bombs in
October, one of the lesser size. In November there should be at least five
bombs and the rate will rise to seven in December and increase decidedly in
early 1946. By some time in November, we should have the effectiveness of
the 235 implosion type bomb equal to that of the tested plutonium implosion
5. By mid-October we could
increase the number of bombs slightly by changing our design now to one
using both materials in the same bomb. I have not made this change because
of the ever present possibilities of difficulties in new designs. We could,
if it were wise, change our plans and develop the combination bomb. But if
this is to be done, it would entail an initial ten-day production setback
which would be caught up in about a month's time; unless the decision to
change were made before I August, in which case it would probably not entail
any delay. From what I know of the world situation, it would seem wiser not
to make this change until the effects of the present bomb are determined.
L. R. GROVES
Major General, U.S.A.
Source: Manhattan Engineer
District -- Top Secret, Manhattan Project File, Folder 4, Trinity Test,
National Archives, Washington, D.C.