Grant Bonin is having a debate on the appropriate launch vehicle size for exploration. My attitude is either use what you have, or if you're going to spend billions of dollars developing new vehicles, focus it on something that actually reduces cost and improves reliability.

“Grant Bonin is having a debate on the appropriate launch vehicle size for exploration. My attitude is either use what you have, or if you're going to spend billions of dollars developing new vehicles, focus it on something that actually reduces cost and improves reliability.”

Spending “billions of dollars” developing new vehicles seems stupid as such vehicles would require flight rates far beyond what a reasonable exponential growth in new and existing markets could cover with regard to their development costs. If one assumes significant ongoing cost reductions then space transport designs will become obsolete in only a few years. Billion dollar space transports will not have long enough economic lives to pay back their development costs. By the time the market is big enough to sustain such large vehicles, their design, if started now, would long be obsolete. Hence near term billion dollar space transport development programs will necessarily lead to yet another Shuttle. It would be far better to do nothing.

A $100 million could probably pay for the development of a new low cost space transport, assuming that was the focus, and so even paying for existing launch vehicles is questionable. Three 250 kg payload space transports each flying once a day would launch near as much mass as the world currently does annually, so even this would require new markets, (especially considering the small payload size). With tether augmentation, more reasonable fleet size and further flight rate increases, order of magnitude increases in market size would be required again. Hence any space transport larger than the practical minimum is overkill and not matched to a reasonable expectation of initial exponential market growth.

The current optimal size for a space transport is as small as is practically possible. This minimizes amortised development costs, which currently dominate, and maximises effective flight rate, (and thereby reliability).

A payload of 200-300 kg is probably near optimal. This allows for the initial carrying of one to two people and the boot strapping of a rotovator which could quickly increase space transport payload, eventually perhaps up into the 2000-3000 kg range.

Of course this requires significant orbital assembly and perhaps air launch to mitigate aerodynamic drag and range costs. Not exactly prohibitive considering the order of magnitude plus reduction in development and launch costs.

Pete,

Say you want to build something big, like a space station or a transport to take men to mars. How are you proposing to assemble all these 250 kg payloads into something useful. Construction in space or on the moon isn't like building a house in Houston.

Big vs. Small is the wrong debate. Did you know that about 10 years ago the FAA was for a time the fourth largest telephone company in the U.S? I worked as a contractor in the TM&O of the ANM regional headquarters. However, before the breakup of MaBell, each regional office only had one or two people that ordered phone services from THE phone company. That was it. After the breakup, they hired a new army of field engineers because phones are used to relay voice and data; radar, ground to air communication, vortacs, Etc. They don't use 'unreliable' services like the internet. It took years of negotiation resulting in a huge contract to move back toward that smaller direction, where a single clerk could order the service instead of a standing army providing it. You woundn't believe the mismanaged millions per month that was wasted on unused but paid for services even with their own phone network.

That's the NASA problem. They need to replace their standing army with a clerk with a checkbook and a phone.

...and the FAA?, they used microwave relays to create this huge network, with all it's inherent reliability problems, rather than fiber optics because there was no way they could manage the right of way issues.

I like Griffin, but I think he makes a mistake in presuming only the government can produce critical path items.

Capitalism naturally solves the big vs. small debate in the way it should be resolved.

More like 15 years ago... time is a flying.

Designing lower cost space transports is primarily about finding ways of making them smaller. A design metric which most have decided to ignore, and so there has been minimal design effort in that direction.

Plug and play kitset approaches are possible at this scale that require little more than docking, hence the term orbital assembly. For example, house sized inflated habitats, (say 15m by 7.5m diameter), can be so docked in shells, each shell weighing less than 250 kg. Once pressurised they could be internally out fitted, in a fashion that does not require tools or extensive labor. Similarly one would have plug in solar power modules, heat radiators, tanks, farming shells, energy storage flywheels, etcetera, that massed less than 250 kg.

Early on in the piece I would expect to see small manned/unmanned tugs with robotic arms. These would be used for precise docking/assembly tasks. Also the assembly of much larger inflatable hangers, there are simple ways of doing this from 250 kg payloads, including the hanger doors. These hangers would enable hands on work. While not required for general settlement assembly they do enable detailed onsite maintenance and development work. This is a capacity currently lacking at the ISS and needed for serious space settlement. Personally I see no need for people in space suits. If a module is in need of fixing or upgrading, unplug it and take it back to a hanger.

Such large hangers could be used for the construction of one piece Mars ships, but I doubt one would want to depart from the modular approach here. One should be sending a settlement ship to Mars that has the capacity to take off and repair its own modules.

Of course once the tether starts being boot strapped the space transport payload increases dramatically. The first payload of tether is almost sufficient to double the effective space transport payload, (excepting power systems, etcetera). But the point remains, person size is the initial practical minimum payload size determinant, not orbital assembly limitations. One should initially be designing the space transport to the minimum size that enables tether boot strapping. This means around 100-150 kg if capable of unmanned flight and 200-300 kg if not.

Frankly, I'm not convinced that assembly in orbit is that big a deal compared to the costs of a launch vehicle that is used infrequently or perhaps not at all.

Frankly, I'm not convinced that assembly in orbit is that big a deal compared to the costs of a launch vehicle that is used infrequently or perhaps not at all.

Well, there's assembly and then there's manufacture. Putting together 10,000 kg modules so that we don't have to design a huge underusable heavy lifter, that's assembly. If the interconnections between modules add a few hundred kg to the weight of each, that's fine - you just launch one extra module.

Putting together 250kg modules? That's not assembly, that's manufacture. Maybe you can send up fuel that way, but you'd better have a bigger launcher for most of your parts... in which case why can't you save money by using the bigger launcher for everything?

Tethers might help, but they don't make momentum come from nowhere. Space elevators still require unobtanium, and large amounts of it to boot. Even rotovators need to get their momentum back from somewhere, and they need to get it back fast before drag ruins their own orbits - and I don't think a vehicle that can only lift a few hundred kg to LEO is going to be able to carry a new engine (or even just new fuel or reaction mass) to the orbital altitude of a useful rotovator.

Roy: “Putting together 250kg modules? That's not assembly, that's manufacture.”

Why? Because that is not the way it has been done in the past? Do you think that say, putting Lego together is manufacture? Where do you draw the line?

I draw the manufacturing line at the likes of material cutting/welding/forming processes. What I advocate is a long way from that, likely not even requiring basic assembly tools.

Habitat modules larger than those currently used for the ISS can be assembled from 250 kg modules. What part of a space settlement do you think can not be sub divided into 250 kg modules? Tanks? Power systems? Life support? Storage? Propulsion? Tether systems? From what I can tell the least sub dividable items are habitat shells, as stated previously this is manageable well within 250 kg.

Even at a payload of 250 kg the space transports would be seriously under utilized such that major subsidization would be required for a number of years until the market grows to the necessary levels. This is up front expensive, high risk and it delays payback many years. Though a wide range of new low cost payload modules will have to be developed anyway, it is unfortunate that this will probably have to be included in the development costs of new space transports.