May 19 1944 i jor Genen l L R Groves P O Box 2610 Washington D C Dear General Groves Consideration of the delivery problem in its broad aspects has highlighted many questions which one might assume could be taken up later or are being handled by other mechanisms unknown to Ramsey and myself However as I emphasized to you Dr Oppenheimer Dr Bush and Dr Conant the isolation of this project while justified for security reasons also imposes a duty on those connected with it which would not exist with projects channeled normally through ilie High Command For this reason I believe that it is necessary to start action along certain lines fairly soon because unless plans have been made which will pennit certain highly desirable delivery· conditions to be met we may find perhaps this Christmas that these conditions cannot be met either by us or by the High Command in which case a much less positive gadget or method of delivery will have to be accepted You are better qualified than we to make an exact prediction from a production standpoint as to the first date at which the delivery problem should be solved An estimate that it could not be before this Thanksgiving and might well be before Easter 1945 is about as close as I could guess This time consideration is linked with other considerations when it comes to setting a date at which the military delivering group should be ready in a material sense One such consideration concerns the necessity for an overall test of the gadget before its use against the enemy There is also the dee isi on as to whether a single gadget not to be followed by a second and third model for a period of several months would be used as soon as ready If a military requirement of a minimwn of three gadgets were set up then the date of first use against the enemy would naturally be months later A further consideration which has a bearing on the time scale is the directive now in effect that a minimum of 1 000 tons TNT equivalent is required If there should be a possibility of producing three or more 500 ton equivalent gadgets by the same date as one probable 10 000 ton equivalent gadget the directive would require modification It is desirable to state the assumptio ns under which the design and delivery problem is now being attacked lhese assumptions should be critically considered by the Military Use Committee a The implosion method is definitely possible of accomplishment and may with good breaks permit construction of a 1 000 or more ton equivalent gadget by the end of 1944 c J • ' - lJ I £11 ll -- J · 7 9 ft L _tf 7 - Li Oll ±-1 1 t - - Major General L R Groves - 2 - May 19 1944 b The most probable size and shape of the implosion gadget is a large sphere probably about 60 O D stabilized with a box tail The total weight of this assembly will be about 10 000 lbs and its overall length will be less than 130 c The only known airplanes which can carry a gadget of the above overall dimensions without major modification are the B-29 and the British Lancaster It is believed that the present production B-29s with the bomb rails removed from the forward bomb bay can accept a sphere up to 65 diamete·t· -out the box tan -'· could not be more than 60 on a side d Detailed planning and tests looking toward military use have been conducted only with a B-29 plane Undoubtedly a long period in terms of months would elapse if any other airplane Lancaster were ordered made ready for this delivery e The primary and so far only contemplated method of delivery toward which the testing program is oriented is high altitude about 30 000 feet above sea level horizontal bombing with provision for detonating the bomb well above ground relying prima ri ly on blast effect to do material damage In th is connection the present thought is to use a height of deton tion such that with the minimum probable efficieµoy there will be the maximum number of structures dwellings and factories damaged beyond repair f Delivery should be able to be accomplished either by day or by night g The fact that the power of the contemplated gadget could only be equaled by nonnal bombs carried by 2 500 bombers requires a degree of reliability in functioning of the airplane selected for this task which is well beyond that now acceptable in military aircraft To accomplish this extraordinary precautions are being taken in the design and acceptance tests of suspension and release equipment and the fuzing including safety control of the gadget It is now planned to protect the spherical high explosive charge with armor probably in the outer shell surrounding the charge Electric circuits will be required in the release system and in the gadget and these will be duplicated and power supplied by batteries which can operate down to very low temperatures minimum not yet determined ' h In line with paragraph g above it is planned to install additional armor to protect vital personnel radar and engine positions Tliti c oiume-nr N 1ti0n il ·r c i i 1 - w · i' 1- -r i 0tltaim ' _ Major General L R Grovea - 3 - May 19 1944 h continued The weight devoted to this will naturally reduce the effective range of the airplane Therefore the amount of annor which can be so used will be determined by the tactical range which must be ave ila le from take-off to target 1 i For positive delivery it is planned that the airplane be equipped with APQ-13 navigational and bombing radar This is on the assumption that delivery may be required in 1944 Better equipment might be available in 1945 j Availability of S S Loran equipment in the airplane with corresponding transmitters located to permit navigation to the point of delivery by this method is highly desirable in order to give assurance of successful completion of such a crucial mission In a later paragraph the implications of this statement will be discussed k Since the assembled gadget is heavy and awkward to handle and also includes two tons of ordinary high explosive which would completely destroy the active material even under conditions which insure a nuclear dud it is planned to develop a gadget which will be assembled near enough to the point of take-off so that transportation and unloading operations can be minimized The above paragraphs outline assumptions under which the Ordnance Division of this project is now opera tine the above developments being to some extent under the control of this project There will be outlined below certain actions which may control the date of use or theatre of use or the reliability and positiveness of delivery1 1 The minimum number of B-29 planes which should be fully modified and prepared to carry out the delivery mission This includes trained and specially selected crews The method of organization of the delivering unit and the time required to activate such an organization 2 Fighter protection throughout the critical stage of delivery 3 Availability of B-29 planes perhaps specially armored and armed to act as escorts for the B-29 plane carrying the gadget 4 Sub-paragraphs 1 and 3 above are critically affeoted by the assignment of B-29 operating theatres 5 Maximum operating range which will be required of the delivering B-29 Major General L R Groves - 4 - Ms y 19 1944 6 Recognition of the fact that proper anticipation of earliest date of readineaa of the gadget will probably result in a period of waiting by highly trained and keyed-up military personnel with attendant loss of morale lt is• not known here what the present plans are for malg ng S S Loran available for navigation in the vicinity of primary targets in The Pacific If present plans do not now include making S S Loran available near all targets where this gadget may be used by the end of 1944 these plans should be reconsidered in view of the marked increase in reliability of navigation which this development affords The above para raphs have presented the situation as seen here in May It is believed that considerations given above require certain 1944 speoiPic -actions namely 1 a Full consideration by the Military Use Committee of the assumptions under which the gadget iii being developed by this project b Consideration by the Military Use Cormnittee and by the High Comma d of strategic and tactical preparations which should be made in the light of probable de_velopments c A relatively minor point - by July 1944 inakin_g available one of the battle -delivery B-29s in addition to the experimenwl B-29 now in use Availability of this additional B-29 will permit modification of its bomb bay and other equipment while the present B-29 i retained in its present condition · Sincerely W S 6 Parsons WSP hg Captain USN ·1r 1 1 •• _r law L - n -- cf 1 · i C l ltt r t ' in J i ersvn i ptohibitod i - 1 1 1