Interview with Benjamin Clinton Diven, March 10, 2004

Nevada Test Site Oral History Project University of Nevada, Las Vegas Interview with Benjamin C. Diven March 10, 2004 Los Alamos, New Mexico Interview Conducted By Mary Palevsky © 2007 by UNLV Libraries Oral history is a method of collecting historical information through recorded interviews conducted by an interviewer/ researcher with an interviewee/ narrator who possesses firsthand knowledge of historically significant events. The goal is to create an archive which adds relevant material to the existing historical record. Oral history recordings and transcripts are primary source material and do not represent the final, verified, or complete narrative of the events under discussion. Rather, oral history is a spoken remembrance or dialogue, reflecting the interviewee’s memories, points of view and personal opinions about events in response to the interviewer’s specific questions. Oral history interviews document each interviewee’s personal engagement with the history in question. They are unique records, reflecting the particular meaning the interviewee draws from her/ his individual life experience. Produced by: The Nevada Test Site Oral History Project Departments of History and Sociology University of Nevada, Las Vegas, 89154- 5020 Director and Editor Mary Palevsky Principal Investigators Robert Futrell, Dept. of Sociology Andrew Kirk, Dept. of History The material in the Nevada Test Site Oral History Project archive is based upon work supported by the U. S. Dept. of Energy under award number DEFG52- 03NV99203 and the U. S. Dept. of Education under award number P116Z040093. Any opinions, findings, and conclusions or recommendations expressed in these recordings and transcripts are those of project participants— oral history interviewees and/ or oral history interviewers— and do not necessarily reflect the views of the U. S. Department of Energy or the U. S. Department of Education. UNLV Nevada Test Site Oral History Project 1 Interview with Benjamin C. Diven March 10, 2004 Conducted by Mary Palevsky Table of Contents Introduction: While attending graduate school at the University of California at Berkeley, Dr. Diven was recruited to the Manhattan Project by J. Robert Oppenheimer. 1 Dr. Diven describes living conditions and scientific experiments conducted at Los Alamos, New Mexico during the early stages of the Manhattan Project. 3 Dr. Diven shares his reactions to the Trinity test and the bombing of Hiroshima and Nagasaki. 11 Manhattan Project scientists strove to improve upon the design of the Trinity device. 15 Dr. Diven completed graduate school at the University of Illinois and studied under fellow Manhattan Project scientist Bruno Rossi. 19 Later, Dr. Diven returned to Los Alamos National Laboratory to begin work on thermonuclear weapons that were tested in the Pacific. 22 Brief discussion of the Bravo thermonuclear test 30 Los Alamos scientific experiments at Nevada Test Site measuring bombs as neutron sources in place of accelerators. 31 George Cowan does first line- of- sight experiments from underground tests using time of flight 33 Dr. Diven explains equation- of- state experiments 34 Conclusion: Dr. Diven narrates a series of photographs of underground tests conducted at the Nevada Test Site. 37 UNLV Nevada Test Site Oral History Project 1 Interview with Benjamin C. Diven March 10, 2004 in Los Alamos, New Mexico Conducted by Mary Palevsky [ 00: 03: 02] Begin Track 2, Disk 1. Mary Palevsky: Maybe you could start by just talking a little bit about your background, where you were born, when you were born. Ben Diven: Yes. I was born in northern California on a farm near Chico in 1919, and so I grew up there, went to school in the Chico schools. And then for two- and- a- half years I went to Chico State College, which was close enough to home that I could live at home while going there. I took all of the physics and mathematics that they taught there in that two- and- a- half years and then wanted to transfer to Berkeley to finish up. But lacking money for that I made a deal with an organization that was starting a lumber business, and so for a year or two I worked there without pay with the agreement they would send me through Berkeley for the other year- and- a- half I would have left. That worked very well and so I went to Berkeley. I forget the exact year I graduated, but at any rate then I started graduate school there after I got my bachelor’s degree. And as a teaching assistant I had enough money to live on from then on. I had it made once I got that far. Well, during that period, well, the war had already started in Europe. We got into the war [ 00: 05: 00] then when I had just finished up my first semester of graduate work. That was in December of 1941 when Pearl Harbor came along, and I continued there working for the physics department. Well, I finished up that school year, and then in the fall of 1942, I had decided that I would have to get into the military service. Since all of my friends were in that was the thing to UNLV Nevada Test Site Oral History Project 2 do. But the chairman of the physics department immediately told me to go talk to [ J. Robert] Oppenheimer. And who was the chairman at that time? Raymond T. Birge. Yes. So anyway I went up to Oppenheimer’s office and he told me that he was going to have a military project. It was going to be run by the Army and the people who were going to be there would all be in the Army, and that I would be a second lieutenant in the Army and would I agree to go. And well, I said yes. Now, did you know Oppenheimer? Had he been one of your teachers? Oppenheimer had not been one of my teachers. I would see him, of course, and he would give lectures of a general nature, but no, I didn’t know him before that. I got to know him quite well. I bet you did, yes. Then sometime later, probably around the beginning of 1943, he stopped me on the stairs of the physics building, said that things had changed, that the place was not going to be completely militarized, and that I would not be in the Army if I went. Would I still go? Well, by then I was of course intrigued, had some idea about what was surely going on, and so I said I would. Let me stop you for a second. When you say “ some idea,” this was from your knowledge of what had been happening in physics, or were people sort of talking about it? The rumors about what was going on with the 180- inch cyclotron that had been started before the war, but suddenly everything was closed up there, and it didn’t take very much guessing to guess what sort of thing was happening. Well, it kept being postponed. Everything was late, but in early March Oppenheimer called me up to his office and there were John Williams and Hugh Bradner. And he said he was sending them out to Los Alamos to find out what was going on, that everything kept being UNLV Nevada Test Site Oral History Project 3 delayed and he only got information from General [ Leslie R.] Groves who got it, you know, down through the chain of command. It was always obsolete by the time he [ Oppenheimer] heard what was happening, so he wanted someone on the site, and so he said I would go out with them. And I got here [ Los Alamos] then on March 13 [ 1943] in Santa Fe, New Mexico, and Hugh and John had arrived the day before. This was on a Saturday; I got in Saturday night. That was my first airplane flight; that was from Oakland out here. You flew? Yes, we flew out. Wow! So many people tell the story of coming in on the train. Yes, it took a special kind of priority to get on an airplane then. Yes. [ 00: 10: 00] Yes. But anyway I got here. And one other person who was a resident of Santa Fe was Joe Stephenson, and he was called the project manager to start with. He was a local person, had local contacts, and could help with whatever local arrangements had to be made. I don’t know what happened between him and Oppenheimer. They had a falling out and Joe kind of disappeared after a few months. But anyway there was no housing and yet people from various projects— for instance, the project at Princeton to separate U- 235 was cancelled and all those people were to pick up the Harvard cyclotron and bring it to Los Alamos. Well, the project was over, they had packed up the cyclotron and they were on their way, but there was no housing, and that was true for everybody else. They had the whole group of people who were going to come here and figure out how to build this thing and there was no place to put them. So Hugh Bradner’s job was to go UNLV Nevada Test Site Oral History Project 4 around, along with Joe Stephenson, and take over whatever housing they could find. They took over dude ranches and some of the houses down at Frijoles Canyon, and, well, that was his job. John Williams had the job of finding out exactly when to expect housing to be finished, laboratory buildings finished. There was not a single laboratory building finished and not a single apartment house ready to be occupied, nor any dormitories. There was a few houses of the schoolmasters and there was the big house that housed the students and Fuller Lodge [ of Los Alamos Ranch School], of course. Well, what they did then was to essentially confiscate dude ranches and any housing they could find that was in the general direction of Los Alamos, a little closer than Santa Fe. But John Williams and I then spent every day talking to the construction people. You would find the carpenter foreman and ask him when that building was going to be finished, you talked to the plumber foreman, and so on, and the electricians. The idea then was that these people really knew what was a reasonable schedule. Right. And these people were working then for the Army Corps of Engineers? They were working for the Corps of Engineers, private contractors. OK. Well, there were no telephones in Los Alamos and so we stayed in a hotel in Santa Fe. Do you remember which one—? Yes, it was El Fidel. And every evening we would come back to Santa Fe and, you know, John would call Oppenheimer. I would’ve been taking notes all day, like a little boy following John Williams, who was an important professor, but making notes about when one building or another might be ready for occupancy. And then John would call Oppenheimer on the telephone and report. Well, it wasn’t only when a building might be finished but there were deficiencies. These UNLV Nevada Test Site Oral History Project 5 plans had been drawn up in a great hurry, of course. People who were bringing their equipment here would have given very crude specifications of what they needed in the way of a building, but then by the time that was put on paper, made into drawings, and it was getting built, why there would be glaring discrepancies. We found there was no control room for the cyclotron. You couldn’t have the control room in the building with the cyclotron; it had to be some distance [ 00: 15: 00] away. We found such things as chemistry laboratories which were to have an acid drain in the back of the work benches, that got interpreted as a little groove in the hardwood bench top, and that would hold pencils nicely but had no relation to an acid drain. So these were things that we would refer to Oppenheimer and he could get a hold of the appropriate people who were going to occupy these sites and have them take care of getting corrections in. Well, that went on for a couple of weeks, I guess. I don’t remember when Oppenheimer arrived but it had to be a couple of weeks after the fifteenth. I know the laboratory official history by [ David] Hawkins says he arrived the fifteenth of March, but he certainly didn’t. [ See Hawkins, Project Y: The Los Alamos Story. 1983, Tomash]. Great! That’s a great little fact. Yes. But I don’t know exactly when he got here. No, but that’s interesting, just that you can say that Hawkins is off by probably a week or two, I would say. Yes. Yes. Right. Well, first we lived in a hotel, then as the other people began to arrive I moved to Rancho del Monte, a dude ranch outside of Santa Fé in the Tesuque area, for a little while. And Oppenheimer must have been here by then too. And I was no longer doing this job, running around behind John Williams, but Oppy asked me to do something about all of this equipment that was arriving. The man who was to be in charge of supply and property, Dana Mitchell, was UNLV Nevada Test Site Oral History Project 6 a physics professor but he had become an expert in supply and property and how to get things in a hurry. I think he was on the radar project at MIT [ Massachusetts Institute of Technology]. And he came here, actually ended up being an associate director. But there was no one here to do anything about all the equipment, the truckload after truckload of equipment pouring in. There wasn’t even a warehouse to put it in. The first building that was finished was the building that was to be a cryogenics building to liquefy hydrogen to make a hydrogen bomb, and of course there was no conceivable use for such a building so that became temporarily a warehouse. It was just a great big empty building, was just a shell essentially. So these truckloads of things came in and Oppy said, Well, do something about it. And so we just had this huge pile of boxes and such things and so we would try and sort them out to go to the various groups, but then these people just had to come and get their own. We did have an army dump truck that we could use for hauling these things around but if Bob Wilson wanted the equipment that was coming in for the cyclotron, he could get his people to come there with him and load up that truck and take it down themselves. So anyway that’s what we did. I just tried to get the things that were pouring in to go to the appropriate people. And I did some other jobs, little gofer- type jobs Oppenheimer, if he wanted something and didn’t want to disturb an important person, I could go do it for him. And he promised if I would do this for a few months, that then he would put me in the best possible group so I would learn the most. And he did that. After Dana Mitchell had been here awhile and I had still then been [ 00: 20: 00] working for Mitchell to try and get this supply and property confusion sorted out, he gradually got people who knew how to do these things to come in. And finally Oppenheimer said now I could go into a scientific job. He put me in Bruno Rossi’s group. Rossi and Hans Staub UNLV Nevada Test Site Oral History Project 7 were co- group leaders. Originally I think it had been intended to be two separate groups. But it was a group whose job at that time was to develop instrumentation for other people to make the kind of measurements that were desperately needed. And so I was assigned to learn how proportional counters worked and then work up a system such that if somebody wanted to build a proportional counter, that we had charts and various sorts of data that would, given the kind of conditions the counter had to work under, that they would know how to build one, put it together. And other people were studying ionization chambers and whatever instrumentation that was needed. So I learned a lot about instrumentation until about the end of that year, I suppose, is when [ Emilio] Segrè discovered that plutonium- 240 was spontaneously fissioned and put out lots and lots of neutrons all by itself and that the plutonium that was going to be available was going to have a lot of plutonium- 240 in it. And that made it impossible to use a gun weapon because assembly was too slow. Already Seth Neddermeyer was working on implosion systems. But anyway obviously, well, the uranium- 235 device would work clearly with a gun- type weapon and we had Captain [ William S.] Parsons— a Navy captain, who was an expert on guns— had that well under way, and clearly we were going to be ready to have the gun- type weapon when they found out they couldn’t use plutonium. And of course it turned out that there was hardly any U- 235 and there was going to be buckets of plutonium, so— You said “ buckets of plutonium”? Well, it was going to be measured in lots of pounds [ laughing]. There would be plenty of it. So anyway, the lab was reorganized and everyone pushed towards finding out how to make an implosion device. And so Rossi’s group was assigned to help out with the RaLa [ Radio Lanthanum] experiment. This was an experiment in which they would use an extremely intensely radioactive source of radiolanthenum to study an implosion: to put this very intense radioactive UNLV Nevada Test Site Oral History Project 8 source in the center of an implosion system, set off the high explosive. And well, let’s say, this is a shell of something to mock up plutonium that was going to collapse and by putting ionization chambers to measure the current from the radioactivity, as the shell collapsed it would thicken and the current would decrease and that way you could study the progress of the implosion. All right, so Rossi first was asked to supply the detectors to do this, and Luis Alvarez was in charge of the experiment. I’m afraid he made a horrible mess out of his first trial, and [ 00: 25: 00] Rossi said he wouldn’t work with that man. Either Rossi would take over or Alvarez did it himself. In layperson’s terms, what was the mess that he made, can you—? Well, he just was unprepared. He didn’t have any proper way to get the radioactive source from the radiochemists and put it inside of the bomb. He had some ridiculous scheme, using a fishing pole or something. Anyway, it was all right for a trivial little practice source but when the real ones came, why, you couldn’t do it that way. And there were other things about the electronics Rossi didn’t approve of. It just had been too casually done. It wasn’t carefully prepared and Rossi was a person who was meticulous with everything, and he expected things to work the first time. So anyway he was given the job of taking over the RaLa experiment, the whole thing. So at that point then, he called all the members of the group together and said this is what they were going to do and he hoped everybody would join him but if anyone didn’t want to, why then they could find themselves a place in the physics division. But he was going into the new division that would study implosions. UNLV Nevada Test Site Oral History Project 9 OK, so I started learning things about explosives. I had the job of getting the source from the chemists, removing it from the big lead pig it would arrive in— “ Big lead pig”? Pig. Just a cylinder of lead several feet in diameter with a hole down in the center where this tiny little extremely hot source would be located, with another plug of lead on top of it. And so I’d take it out of that. Well, I guess I should back up and say first, of course we had to supply the chemists with the piece of equipment that the source was to go in because it had to fit inside of a bomb. So this would be on a very thin stainless steel metal cone that they place at the bottom for this small volume of this radioactive material, probably a fraction of a centimeter in diameter. Now, did that plutonium come down from Hanford or was that—? Oh no, this— This plutonium was made here? We’re not talking plutonium. This is radiolanthenum that came from Oak Ridge. From Oak Ridge. Yes. It had a half- life of a matter of hours, I forget, maybe twelve hours, something like that. OK. So it would come by train or something from Oak Ridge or—? No, I don’t know how it came. A car or something like this. It probably came by either truck or train. OK. Don’t let me get you off- track. I’m just curious about that. OK. So anyway then we first had to know what the implosion system was going to be. I had the job of handling the explosives too. And so the explosives people would have a new system that they thought would be better than the last one. And these were to be roughly, I think, half- scale UNLV Nevada Test Site Oral History Project 10 bombs, half as big as the real one was going to be, because you had to have something small enough that you could get— the gamma rays from the source had to penetrate and still have enough intensity to get a good current in your detectors. So we mocked up plutonium using cadmium. It was very much less dense than plutonium. They hoped that it would bear a little resemblance, as far as its hydrodynamics was concerned, to plutonium. But anyway [ 00: 30: 00] uranium would have been a good mock- up for plutonium, for instance, as far as density goes, but it’s so dense that you couldn’t get enough current, couldn’t get enough radiation through it. So we had to study mock- ups. Not a very satisfactory way of doing it, but that’s all we could do. So we would then, knowing what the implosion system was going to be, then we would have to arrange that a conical section would be cut out— of course it would come in pieces, it wouldn’t be a complete cast sphere, and these pieces would all fit together to make a sphere. So one of them, the one right at the top, had to have a conical hole cut in it that would accept a very thin metal cone that at the tip would have the radioactive source that would end up then at the center of this sphere of explosive. So that was my job, then, to handle the explosive, get the source located in the center. And then other people in the group did the electronics and so on, and built the detectors. Well, they would have a large number of detectors all around the outside of this sphere and they from the very beginning could tell that the implosion was not simultaneous enough. Some pieces of explosives would go off before other pieces and the result was that you would just get a useless mess in the center. You couldn’t make a bomb that way. It had to have much better simultaneity and a better shape to the shock front that was going to accelerate the metal. And they were able to tell just at first that things weren’t good enough, and later to make real measurements about how much compression you got in the metal. UNLV Nevada Test Site Oral History Project 11 Now, I shouldn’t pretend that we were the only people studying implosions. The whole laboratory had been turned into a study of implosion systems using all different kinds of schemes for doing it. This was just one. It turned out to be a very important one because it could make a quite respectable measurement of the compression of the metal. Not real plutonium, just cadmium, but still they could tell that it had a good chance at working, at least. All right, so we did that until it was fairly clear, I suppose around early in 1945, that there was a system that would work. And of course it was going to be tested at Trinity. And one of the important measurements was the alpha measurement. That was studying how fast the reaction rate would take place. It would be an exponential reaction and you needed to measure that with the time constant of that exponential. It would be in the neighborhood of a small fraction of a microsecond, and at that time we didn’t have electronics that was fast enough to quite do that. And if the bomb failed, then that would be an important measure to know why it failed, and so they wanted to have a good measure of this let’s call it the time constant. And Bob Wilson had the job of making that measurement and he had something that actually worked but [ 00: 35: 00] but didn’t give a very accurate answer, but Rossi thought he had a way of doing it and getting a more precise answer. Wilson nicely agreed that Rossi could share his recording station and gave him some help too. And so I quit the RaLa experiment then and the soldiers who had been working with us— by then we had a whole lot of these SEDs ( special engineer detachment) and they were doing very well on the RaLa experiment. And so we moved to Trinity to make this measurement. So were you then with Wilson’s group or you—? No, with Rossi. You were with Rossi at Trinity. UNLV Nevada Test Site Oral History Project 12 Strictly with Rossi, but Rossi couldn’t have squeezed in at this late date without Wilson’s help. Wilson was a good guy and he gave Rossi all the help that you could imagine. Anyway, Rossi’s experiment worked very nicely. Just barely, but nicely. So that was sort of the end. So just for a chronology, you move out to Trinity how soon before the test, would you say? Oh, a month I suppose. A month, and you’re living out there? Oh yes. Yes. That must have been an interesting experience. I haven’t been there in the middle of the summer. They don’t let you go if you’re a tourist. I was there in October and in March and it was hot enough then. Yes, it was very warm. Yes. And you lived in—? Barracks. Barracks? Yes. And I saw the McDonald House there. Were you working—? We had a room in the McDonald House where they could assemble the detector that was going to follow the nuclear explosion. And where were you when Trinity actually went off? Were you in this recording place or—? No. No, I was about twenty miles away where all the onlookers were, people who weren’t really needed in the— well, nobody was— where the data were all recorded in underground bunkers and UNLV Nevada Test Site Oral History Project 13 nobody was there. The timing and firing system was in another bunker quite far away, I don’t remember how far. Anyway, all the people who were essential for that were there, and then— And was Rossi there or—? Rossi was up on the mountain where I was. There would be people, for instance, I suppose it was Dave Nicodemus who was in charge of the detector and— well no, it wouldn’t have been Dave. Whoever it was who was to go in and recover the photographic film and take it to Los Alamos and develop it and so on. That person would have been at the base camp, which was still farther away than where the bomb was set off. So there were a few, of course Oppenheimer and Groves and various important people were right there where the timing and firing was done. But otherwise the people who were essential for recovering equipment would be at the base camp, and then those who had no real use there anymore were— I think I was about twenty miles away. But anyway, there were lots of people there. Yes. And what was that like when you saw it? What did you think? Well, we thought, Oh boy, it worked. Well, of course, there was a flash of light and simultaneously heat, suddenly like the sun shining on you, and then nothing for quite a while [ 00: 40: 00] until the shock arrived, and then you had the loud boom! and then the reverberations as the sound reflected off the surrounding mountains, and just a whole long series of booms! and this really impressive cloud. Well, so we knew it worked. And well, I was with Rossi and he had an Army car and we drove back to Los Alamos and the car was full. I don’t remember who all was in it. By then the job was done and soon after Trinity we were allowed to go home. So for the first time I went home, visited relatives. I had a pretty good idea of when the bomb was going to be dropped because, well, of course the uranium bomb was already over there or on the way. UNLV Nevada Test Site Oral History Project 14 And we knew that the plutonium parts were coming out at a sensible rate and that it was going to take just a reasonable amount of time to get the core over to Tinian and then for the Army Air Force to do their things. So we didn’t know when the day was going to be but knew approximately about the earliest it could be, so from then on I started wanting to see the newspaper every day as soon as it came off the press. Yes. And this is when you’re home visiting. Yes, when I was home. Yes. Yes. And you were single at the time, is that right? Yes. So you were visiting your parents? Yes, and friends. Yes. So were you still at home when Hiroshima happened or were you back here? I was there when Hiroshima happened, I was home, and so I immediately came back because I knew there were going to be all kinds of celebrations. So I hopped on a bus and came back. Yes. So then you were here for the second bomb. No, I don’t’ remember. Yes, I’m almost sure I was. Yes, I was. That’s right because I knew that they hadn’t hit the target squarely, you know. I had to have been here. Yes. Oh, they hadn’t hit the target squarely? Well, you know, it was a cloud cover and they couldn’t do the first target. They took a second target and it was partly obscured. It was a difficult job but it still worked well. Yes. Just sort of a detail question that it never occurred to ask but you sort of raised it in my mind. You’re working on the implosion. Are you simultaneously making duplicate sets in case it works so that you have that whole setup to ship out to the Pacific? UNLV Nevada Test Site Oral History Project 15 Well now, remember we were working on a scale model that didn’t apply to a real bomb, and not only a scale model but we of course weren’t using plutonium. But we were studying various schemes of implosion. What was finally used of course was the explosive lenses that created a pretty good spherical shock wave. For them to work, maybe the most important thing was to have electric detonators that were extremely precise and that all go off at exactly the same time. Until those were developed— and incidentally it was Luis Alvarez mostly developed those, and I maybe made some nasty comments about him— [ 00: 45: 00] They weren’t nasty. Anyway, the most important thing of all I think was getting simultaneous detonators, a whole lot of detonators, all to go off at the same time. Right. But when you’re doing the Trinity test and it works, then you have this other— That’s right, the conditions of making these explosive systems were somewhat primitive, I suppose, and there were elaborate schemes for studying whether there might be bubbles or any imperfections in the explosives. So there were lots of methods of nondestructive testing, so every one of these pieces would have been just studied to death and the very most perfect ones picked out, the ones they had the most confidence in. And yes, of course, there would have been duplicates and duplicates. OK. So once you have the success of Trinity you can duplicate it within a reasonable period of time. Right. Right. Of course, once Trinity worked, why we knew we could’ve done a lot better. Just the system I had been working on, the RaLa experiment, by the time of Trinity we were already testing much more sophisticated implosion systems. Once you knew Trinity worked, it was a shame in a sense to use such a super conservative design that was not as efficient, not nearly as UNLV Nevada Test Site Oral History Project 16 efficient, as it could have been. But the Trinity shot, everything was aimed at something that for sure would work, and it was very, very conservative, and it could have been much more efficient. Actually as soon as Trinity went, a matter of it could only have been days, maybe even before it went, people knew that if it worked you could make it better and you could use the fissionable material that was available to better advantage. So when you say “ conservative” let me see if I’m understanding you correctly. You’re saying things are going to behave in a certain way and we give ourselves a wider range which makes it maybe less efficient and then you see that you can narrow it down in some sense? Yes, we started out— there were design parameters, for instance, for the gun gadget. It was going to be a long skinny thing and the length of the gun would be the longest that a B- 29 could carry— even though you modify the bomb bay— and so the B- 29 was modified to take the longest, skinniest thing that it could carry. All right, then the fission bomb was going to look like a sphere and there the diameter of the sphere that we could use was the biggest diameter that a B- 29 could be modified to drop through a bomb bay. All right, so that fixed that, and that nobody would’ve modified. You might as well use all the explosive you can. But you could configure the solid material much more efficiently. A real bugaboo was that you knew you would get the maximum compression— and therefore the most efficiency— if you would just accelerate a thin shell, because as a shell collapses it goes fast