Talks about fun and interesting things which certainly ought to be possible, and even feasible.

Sustainable Spaceflight Beyond LEO

Henry Spencer

Once we achieve the access to low earth orbit that we want, where should we go next, and how should we get there?

The Moon would make a good target for exploration; having a base there is seocndary. Mars should wait, because any programme that gets us there soon is likely to be a re-run of Apollo: "crash" progams are so called because of what they do after they've achieved their single objective. Better targets would be near-Earth asteroids or even Phobos and Deimos (which might make good stepping stones to Mars).

For sustainable flight you need reusable hardware, for three reasons:

. it's cheaper if you do many flights . it avoids "breakpoints": when the Satrun V production line was shut down, the end of Project Apollo was clearly visible . by re-flying the same vehicle, you can test it enough to make it truly reliable [or develop it into one that is reliable]

Building a big launcher is a bad mistake, not least because the biggest payload you can imagine launching has a habit if getting bigger. Apollo would have been in serious trouble in Wernher von Braun had believed the initial numbers that were given for the capsule. Fortunately, he designed the Saturn V to lift much more than they said it would have to, and even so, it was only just big enough. A far better approach is to accept that you will do some assembly in space (even Apollo can be said to have done this, if you remember how they docked the CSM to the LM) and plan to launch your spacecraft in pieces from the get-go. That way, when your mission gets bigger, you don't need a new launcher, just more pieces. When your existing launcher is flying weekly or even daily, it's time to think about a bigger ones.

After all, much of the mass of a deep-space mission is the propellant, which can easily be subdivided. In fact, an orbital depot is a really good idea. Equipment for transferring fuel and handling tanks can sit there instead of every vehicle having to bring its own set.

The downside of a single base is that it will not often be in the right place to start an efficient transfer orbit to the moon. [Long discussion of orbits skipped, partly because I don't have his shetches.] For Apollo, the angle at which the Sun shone on the landing site at the time of landing was also important, so that the astronauts could land visually, but this need not be an obstacle with modern equipment. For example, a robot mission could release a swarm of penetrator probes with a camera in the nose and a radio transmitter in the tail. Each probe sends back pictures that it took just before impact, and you home in on the one whose pictures look like safe terrain.

Exploratory missions should be designed like naval voyages: redundancy (Magellan took five ships, Columbus three), safety margins, and no attempt to provide for instant rescue or bail-out in emergencies.

Use of lunar resources is not a good long-term solution. The schemes that have been floated for extracting oxygen from lunar soil would make a chemical engieer laugh himself silly. They require chemicals that would have to be brought from Earth, and therefore recycled with nearly 100% effectiveness. Sadly, many chemical processses have some side reactions that consume these chemicals. For example, lunar soil often contains sulphur. If you use hydrogen to extract the oxygen, you will get some sulphuric acid, which will (besides not being the desired product) corrode your equipment. Also, lunar dust is savagely abrasive [the dust particles have not been rounded by erosion like Earthly ones].

Concerning the hydrogen deposits at the lunar poles, it is not yet established that they consist of water; they might be ammonia from comet impacts. If there is wate rthere, it will be harder than most rocks, and mixed with lunar dust, so mining will be a real headache, especially since the conditions in those craters will be dark and bitterly cold. The hardware will need to be debugged on site and redesigned.

Leaving half your lunar lander on the surface (think "descent module") is not sustainable. At least, the pieces directly needed for landing should be designed to be packed up and lifted off again. The habitable part of the lander can be left on the surface, because we *want* habitat modules down there.

Laser Launch: It's Steamship Time

Jordin Kare

Rockets powered by energy beamed from ground-based lasers are on the brink of becoming affordable (or rather, the lasers are).

First, a very brief history of laser launch ideas.

Technology

Think of a rocket with one side flattened out, and that side covered by a flat-plate heat exchanger, heated (by any radiant source) to about 1300K. You force liquid hydrogen through the heat exchanger, and it comes out of the nozzle at 6,000 m/s [that's at least 30% better than any real-world chemical engine]. Such a heat exchanger can be made fairly cheaply by electroforming nickel; silicon carbide could be used (at higher cost) for slightly higher performance, but there's no need for tungsten, hafnium, etc. When you need higher thrust for a short time, such as at liftoff and shortly afterwards, you can inject nitrogen downstream of the heat exchanger (though this will cut the exhaust velocity).

There's no need for one big laser; it's better to build many modules, each containing lasers and a beam director (that's a telescope in reverse). Failure of a few modules doesn't affect the vehicle much. More can be added when more performance (or a bigger vehicle) is desired. The heat exchanger can absorb about 10MW per square metre. A vehicle weighing about 5 tonnes at liftoff, using a 25 square metre heat exchanger and powered by 100MW of lasers, could deliver a payload of about 120kg to orbit.

Semiconductor laser diodes are compact, efficient, and long-lived, but they are not coherent, so there is a big problem focusing the beam. However, three different solutions to this problem have recently emerged; two (fibre lasers and diode-pumped alkali lasers) involve using diode lasers to pump other kinds of laser that have good coherence, and one (spectral beam combining) involves putting many diode lasers in a cavity such that ... [I'm not a laser scientist]. All these result in coherent beams that can be directed to the vehicle by relatively cheap telescopes.

You can do test flights with no chance of seriously damaging the expensive part of the system, because it stays on the ground!

Market

Given the hoped-for reductions in prices of lasers, the 100MW launch facility could be built for about a gigabuck and could thoeretically launch 3,000 tonnes of payload a year. But it doesn't scale well to smaller vehicle sizes because of the need to "light up" a heat exchanger at distances of hundreds of km. A technology demonstrator [probably incapable of reaching orbit] could be built for a few tens of millions.

Links

http://www.niac.usra.edu/files/studies/final_report/897Kare.pdf

http://www.niac.usra.edu/files/library/meetings/fellows/mar04/897Kare.pdf

Regolith Return Tether

Vincent Cate

A propulsion system whose fuel is rock

Technology

If you put a rotating tether in orbit around the Moon, with the speed of the tip equal to the orbital speed of the anchor satellite, then the tip can touch down on the Moon at zero speed [modulo any mountains that may get in the way] and pick up a load of lunar surface rock or regolith as we call it.

[Paraphrasing, because I didn't find his explanation easy to follow.]

As the tether spins its way around the Moon, the speed of the anchor will vary: decreasing when the load of regolith is behind it, at minimum when the load is above the anchor, and increasing when the load is in front, to a maximum when the load is again below the anchor; this maximum equals the anchor's original speed. If the anchor includes a winch (which can be powered by solar cells), we can shorten the tether; applying conservation of angular momentum about the centre of mass of the (anchor + tether + load) system, this makes the anchor and the load move faster relative to each other, so the variation in the speed of the anchor increases, and when the load is below the anchor, it is somewhat above the lunar surface, and moving backwards, while the anchor is moving faster than its original speed (if you're worried about conservation of energy, the extra energy was supplied by the winch). If we now release part of the load, then the anchor has gained speed. If we released the right fraction of the load, then when the rest of the load is above the anchor, the anchor is once again moving at its original speed, so we now release the rest of the load, which is going faster than lunar escape speed. Wow!

Market

There have got to be some people who would pay plenty to own a little chunk of moon rock.

Mitchell Burnside-Clapp

Pioneer Rocketplane

Mitch tells us what he's been thinking about lately.

Pioneer Rocketplane and Rocketplane Ltd. are now separate entities, due to what are known as "creative differences".

Mitch has come up with a new cycle for a pump-fed rocket engine. It uses the temperature difference between (say) kerosene at room temperature and (say) LOX at cryogenic temperature to run a heat engine; he leans towards the Brayton cycle, though Stirling and Rankin are also possible; he expect to be able to achieve chamber pressures of a few MPa. This, he explained, lets you decouple the pump startup from the ignition of the engine, thus making it safer to start and restart a liquid-fuelled engine, which may make vertical landing safer. [The coupling of pump startup and ignition is blamed for some instance of Rapid Unscheduled Disassembly of rocket motors.]

He also mentioned a technological advance that has surprising benefits for space vehicles: lithium-ion batteries such as are now used in cellphones etc. These are lightweight, compact, and powerful enough that an auxiliary generator can perhaps be dispensed with, saving some complexity and risk.

The Supply-Side Approach

David Hoerr, co-author of The Rocket Company
David argues that you should sell vehicles, not rides,

David began by saying that a consensus on what the industry should be doing would be useful because it would focus efforts. In particular, a suitable market should be found: one that is large enough, is growing or can be grown, and offers some profit. There are many potential markets and it is hard to see which one will be best.

The consat market is mature [and not easy for small companies]. The advent of RLVs is likely to mean, at least in the short term, a decline in overall launch revenue, not growth. The market for launch vehicles may well grow faster and provide better cash flow. [He showed some charts, which admittedly were a bit hypothetical.] If we sell too many vehciles, there will be overcapacity and the price of a ride will go down ... which is exactly what we want.

What would motivate customers to buy? National prestige, competition, desire to do space research, desire to participate [I may have forgotten some].

David also mentioned network effects. A piece of equipment can become more valuable if other similar pieces exist.

He showed charts of the numbers of civil aircraft built in the US in the 1920's and 30's, and the amount airlines were spending on them, versus the total revenue airlines were taking in.

He discussed what a "sensible space transport" should be like to attract customers. Affordable, reuable, not very large, flexible, and able to carry people. He pointed out that the DC-3 was not very big. This points to an emphasis on low cost rather than high performance, on use of existing technologies, on operational simplicity, and on careful choice of propellants. To cross the gap from "space programme" to "space transport" requires high flight rate and utility.

He presented his preferred concept for a transport, with a fairly short first stage on a vertical trajectory [presumably landing vertically -- I forget] and an orbital second stage that would land under a parafoil. [This was different from any other concept I saw that weekend. Consensus is great.]