Lightship

by Stephan Besik

  “I’d like position announcements for a startup team from you in two weeks. Include suggestions for candidates if you have them. The Foundation will post the announcements and provide you with space for interviews, etc. If we know of potential candidates we will refer them to you for consideration. Oh, by the way- you might want to apply a bit of your obfuscation skills to the job descriptions. We don’t want our objectives to become public until you’ve made quite a bit of progress. That will be tricky, but I think you can pull it off.

  “We will provide you with office space here at the Foundation until we can find you a suitable facility. You should start thinking about what kind of space you will need to begin fabrication, assemble a prototype and then design the production kit. We’ll do business like some of the space agencies- the prototype will serve as a test bed, and eventually will be an exact copy of your first ship.

  “You should give a lot of thought to make or buy options; much of the hardware fabrication should be contracted. We aren’t much of a heavy industry or aerospace organization and you should try to buy more than make. What can you buy from someone versus what will your team have to build? We think that will be a big concern.

  “One other thing. We are going to leave your requested salary as Principal Investigator at the rate in your current proposal. In essence you’re not getting a bump because of the doubling of the budget. If things look good at the end of your first year, however, that is likely to change. Is that understood?”

  “Yes sir, that’s no problem. I just have to break it to my advisor. He’s not going to be happy that I’m dropping my dissertation work completely.”

  “I think I can help with that. We want you working full time on this project. That means you’re not likely to be able to get back to any kind of dissertation work for the next three years, maybe more. This will be a big step. Will you be okay with that?”

  Kevin smiled. “Yes, sir, not a problem. Uh, my topic wasn’t great, anyhow.” He hesitated for a minute, and then smiled again. “This sounds like a lot more fun.”

  Chaz smiled back and nodded. “Yes. Probably for us all.”

  As Kevin left the Smith Foundation he was a mixture of happiness and trepidation. He had just agreed to a really big deal. For starters, he had just gotten a terrific bump in salary to do something that might turn out to be hugely important. No more living on a grad assistant’s pay. He wondered if the University would let him stay in campus housing. He didn’t want to move and he didn’t want to waste time looking for a new place.

  There was a lot he didn’t know about what he was getting in to; a lot he had to learn. Contracting for space launches, for example. Things were pretty regular nowadays; there were satellites being sent up all the time and costs had really dropped. But what kind of lead-time was needed? How much would transport cost? He had no idea how many trips they would have to schedule. One thing was sure; the ship itself had to be like a Lego set. Anything that could fit in a cargo pod should be pre-assembled on Earth. Parts had to fit together easily once in orbit. The ship was going to be big compared to most of the junk in space. He didn’t think it would be possible to get even close to building the entire ship on Earth.

  He’d have to figure out how to do final assembly while the parts were in orbit. Could robots handle the assembly? If they had to send people up to do final assembly it would probably cost a bundle. It was just one more unknown to toss on the pile.

  A snapshot of the blond was taking up space in his head, too. It wasn’t often he ran into anything female that looked like her. Out of his league, of course, and probably engaged or married to some super-rich guy. His thoughts turned even more prurient; she might be the big man’s mistress. But she might be single, and available. A guy could dream.

  Now wasn’t the time for dreams, though. At least not sex dreams. Walking back to his car he looked like a man possessed. Lots to do; lots to plan. He started to walk faster. Time to get moving.

  The Design

  Kevin was working twelve-hour days on the project, having put his doctoral program on hold. That had been one of many things the Smith Foundation had helped with. His advisor had been none too happy to find Kevin working on another project entirely, but he was mollified somewhat by Mr. Smith’s interest in and commitment to the new project. It helped that Dr. Delsun used some of the funds from the “grant” to buy out Kevin’s position on the advisor’s project.

  His budget wasn’t really a grant any longer, either. Charity had arranged another meeting in which he was processed through the foundation’s paperwork as an employee. He’d ended up with a truly massive budget line, with authority and everything. It was pretty scary. Even seeing the tall blond again had been intimidating.

  When he went back to the proposal, he was once again struck by how much work there was to be done. It wasn’t just building a ship; it was a project in space. Space might not even be the hardest part.

  The ship started out in Kevin’s mind as a cylinder with a large sheet of solar fabric at the front, similar to the tiny deep space explorer designs that relied on the pressure of the solar wind to push them along via a lightweight sail. It was an interesting concept, but the solar wind was not very dense and very low energy. Nothing of any size could be moved with it unless the sails were very low weight and very large relative to the payload.

  In Kevin’s original concept, the sail was to be a large circle, to provide strength and save weight. It was to be placed some distance from the hull of the ship, on long wires or perhaps on thin struts, so sunlight wouldn’t be blocked from the sail by the main part of the ship. With light energy at Earth orbit of nearly 1.4 kilowatts per square meter, the sail didn’t have to be huge- just fairly large. That assumed that the ship could convert a good chunk of that free energy into motion somehow. That’s where solar panels generating electricity and an efficient motor came in.

  The cylinder and the sail stayed but the sail moved to the center of the ship, closer to where the electricity was needed. There really wasn’t a justification for putting the sail ahead of the ship and there were a number of liabilities to the placement. It didn’t matter where the electricity-generating fabric was, as long as there was enough of it. The idea of a circular sail stayed with him, though, to make the frame holding the solar fabric strong even with lightweight structural materials. The ship’s hull, including motors, controls, fuel and cargo, could sit in a hole at the center of the sail.

  He thought that the only truly permanent parts of the ship should be the sail (or sails, perhaps; he still wasn’t sure what the solar array would look like), the plasma drives, one or two control modules that would house the control systems, and sensors to ensure that the ship knew where it was and where it was going. The rest of the ship would be an empty framework, to which things like fuel tanks and cargo pods could be attached. For a while Kevin’s design was a two-part thing, with all of the permanent parts behind the sail and all of the cargo modules and fuel tanks (which could be rapidly replaced once they were empty) in front of the sail and secured on some type of framework.

  Having all the permanent parts of the ship behind the sail didn’t last very long. Modules shifted around, and new things had to be added. Things like batteries were necessary. Kevin was pretty sure that times when the ship was in shade were unavoidable. There might also be emergencies that required stored power because the sails weren’t working.

  Having control modules that housed sensors behind the sail didn’t make sense. While rear-looking sensors were going to be necessary, knowing only where the ship had been wasn’t exactly smart. Two physically separate control modules were needed, one for normal operations and one for backup, and their best location was some kind of arrangement in the front of the ship.

  He sketched out a number of rough designs, but for quite a while they stayed rough. While it was fun to play with the shape of the ship, the physical layout wasn’t the most important part of his work. (Maybe he shouldn’t have started there, b
ut the ship design had been in his dreams for months.) There were other things coming that he wasn’t sure he could get done.

  Kevin had to focus on the development problems far more than arranging the Lego pieces of his ship design. The heat problem was actually quite a big one- existing plasma engines spent a lot of energy for magnetic fields not only to accelerate the plasma but to keep the plasma away from the walls of the engine so the whole thing wouldn’t melt. The magnets themselves got very hot, requiring that the magnets be cryogenically cooled and the engines stopped regularly to cool down. The result was poor conversion efficiency as electricity from the solar sails was changed into ship motion.

  The engines themselves were highly fuel-efficient, but in the most recent attempts by plasma pioneers they ran so hot their operating time was measured in hours, with a high risk of failure. The magnets took a severe beating, too, which meant downtime to replace them frequently.

  Ideal output for a one-megawatt system was over a thousand horsepower. It was hard to get that output when a lot of energy was being used to cool the system. Even with a lot of energy lost in keeping things cool during operation, the engines might still have to be shut down periodically to cool down. That last bit was truly inefficient- the ship would drift at constant velocity while trying to dump a lot of heat into space. The ship had to come back up to operating efficiency after each shut-down, too. That was hard on the whole drive system- constantly cooling and heating, beating up all the parts.

  The answer Kevin came up with was more complicated than he would have liked, but he thought it had possibilities. The newest magnets were more heat-tolerant, able to operate for longer periods at high temperature. The motors could run hotter and he thought he could get away without cryogenic cooling. That would save a lot of weight.

  The magnets and the magnetic fields had to accelerate the plasma, but he thought the field protecting the interior walls of the motors should be minimized.

  For starters, the walls of the accelerating chamber and the supports around the accelerating magnets would be made of high temperature steel. The exterior engine casings would be ceramic alloy pressure vessels. The linear accelerators and their magnets would be water cooled, with a fairly thick jacket of water inside the ceramic casing surrounding the chamber. The water would still be very hot, actually superheated steam, when the engine was running. As the water heated up it would be sent to regeneration turbines to convert as much waste heat as possible back into electricity, propelling the ship while reducing the overall heat in the system. The remaining hot water might be used to preheat the ship’s fuel. That might add a little more to the performance of the system.

  He thought that the system would be able to operate for longer periods at higher efficiencies, even though there might still be times when the engines had to be stopped to cool them. If there was a way to get a little push out of the engines even as they cooled, that would be helpful. Maybe there was a way to have some kind of low power mode?

  The engines would run at a temperature high enough to produce a decent amount of thrust while low enough that they could run for long periods of time without a meltdown. After all, the idea was to keep the engines running, continuously if possible, to produce continuous acceleration. The longer the ship could accelerate the faster it would go. Even a relatively low-thrust engine would be effective if the ship could run under constant acceleration.

  On Earth, a fraction of a gee of acceleration wouldn’t do much. In outer space, even a fraction of a gee could result in very high speeds if the acceleration could be sustained.

  Another possibility for engine coolant was to use the fuel, circulating it in the jacket surrounding the accelerator pipes as it proceeded from the fuel tanks to the motors. The fuels he had in mind weren’t flammable, so they weren’t going to spontaneously ignite under high temperatures. The idea was to ionize, not blow up. Kevin thought that could be a big saving in weight in the sense that there wouldn’t be a completely separate system for water. The other advantage was that the heat being wasted would bring the fuel close to ionization energy. The cooling on one side would heat the incoming fuel, making it easier to ionize and accelerate.

  He was thinking the fuel might be ammonia, rather than some of the exotic fuels that other prototypes had used. He might still have to extract heat from the coolant and turn it back into electricity, requiring some kind of ammonia-based regenerating system, but there might still be merit in the idea. Ammonia wouldn’t be as efficient as water for cooling, but a completely different system for cooling might not be required. Perhaps running the fuel through a regenerator after it cooled the accelerator and then pumping into the acceleration chamber could get two cooling processes out of one set of equipment. Kevin thought that was an idea that should be explored.

  There was still going to be some waste heat in the propulsion system and it had to be prevented from building up over time. His answer for this remaining loss was a set of cooling vanes, probably running along the length of the linear accelerators, surrounded by open space. They might look like tail fins, or rectangular radiators, or perhaps like a last casing around the engines that was open to space on both sides.

  After a lot of thought and a lot of research into the problems of the early plasma rockets Kevin came up with another approach that might help to keep the motor parts from overheating. He thought that he should minimize the weight of the whole system by making the whole thing a barebones frame that did nothing more than hold the magnets in place and cool them. The ceramic cases on the ion accelerators would go. While there had to be a sort of ignition chamber where the fuel was ionized, after it was turned to plasma the job of the motor was to accelerate the particles in a relatively straight line. All that was necessary between magnets was a frame to hang them on, some tubing for coolant and a decent wire to carry electricity. The rest of the motor could be open to vacuum. The only other items needed were a regenerator to turn as much waste heat back into electricity as possible and a large radiator that spent its time radiating the remaining heat in the coolant out into space. Making the plasma motor long and skeletal with a regenerator and a big radiator might solve the problems of heat in the old designs.

  Kevin was also planning on cooling the solar sails. They weren’t likely to break down like the engines, but they were likely to heat up. They would also let a lot of energy simply pass through them. Once again there was a need to grab power from heat as well as light. He had to try to squeeze as much power as possible out of the available sunlight by using some kind of regeneration technique.

  Once again, materials sciences came to his rescue. There were new materials that converted heat directly into electricity. No need for a mechanical generator. He thought he could use the new materials in the construction of cooling vanes for the sails. Perhaps it was possible to make sort of two-layer sails. The first layer would do the direct light-to-electricity conversion. The second layer would soak up the heat from the first step of the conversion, and then convert the leftover heat into additional electricity.

  If the materials were durable, not too expensive and could actually be fabricated on a near-production basis, that might be a winner. Between the regeneration in the solar sails and regeneration that cooled the engines he hoped to bring the overall sunlight-to-thrust efficiency of the ship above sixty percent in a system that could run continuously.

  The solar array would have to be pretty big to get to a megawatt of power. His ship wouldn’t be a racehorse, but a megawatt pushing a fifty-ton ship would have a trip performance better than a short-burn chemical rocket while still being fairly fuel-efficient. That would be especially true when (or if) the ships were going to make long interplanetary runs. The thrust he calculated didn’t sound like much, but with no resistance and a nearly constant push the performance in space would be good. In frictionless space, constant push meant constant acceleration. That’s what he was after.

  Not only was he saving on fuel, but the fact that the ship c
ould be under power a significant portion of trip time meant it could be maneuvered in space. That opened up some interesting possibilities, like being able to do station keeping (staying in one place) and holding to orbits that a rocket really couldn’t manage well. Being as close to the Earth as the Moon was created some odd gravitational situations, such as no place where an orbiting object could remain stationary over the Moon’s surface. If a ship (or a space station) had to hold a position over some part of the Moon, it had to be able to move itself periodically to compensate for gravitational forces that could pull it down to the Moon’s surface or pull it away into the Earth’s more powerful gravitational field. To maintain a lunar orbit at a reasonably fixed height above the Moon, making orbital adjustments would be a frequent necessity.

  The solar sails needed a design. The International Space Station used arrays of old-fashioned solar panels to provide power, but the whole point of his ship was to take advantage of the advances in light-to-power systems to build space transport that was cheap and fast. He had sketched out some ideas, the most interesting of which were retractable sails to help manage power and heat and big gimbals to enable the ship to change direction while the solar sails stayed pointed in the Sun’s direction.

  Once the sail was moved to the center of the ship in his designs, Kevin realized that he had nothing to steer the ship with. For real maneuverability, it would be best if there was a way to turn the engines. Otherwise his lightship wouldn’t even be as good as a conventional chemical fuel rocket. He would lose much of the benefit of having continuous thrust if he had to rely on steering jets or something similar to change course. If he couldn’t avoid that, he’d have to add fairly big maneuvering jets, probably lose maneuverability, and add an additional system to fuel them. More weight, less flexibility.