Unconventional Space Access

I’m doing a piece for Popular Mechanics on alternatives to rockets, and I was going to cover rail guns, gas guns, space elevators, sky hooks, and perhaps the launch loop. Does anyone have any other suggestions?

[Update a while later]

Folks, when I say alternatives to rockets, I am including all vehicles that employ chemical rocket engines, including airbreathers. As I said, unconventional.

[Update a few minutes later]

OK, I’m thinking of three categories: cannons (whether EM, chemical, whatever), external energy (laser, Orion), and momentum exchange (tethers, space elevators, compression towers). I know the latter isn’t really momentum exchange, but it fits sort of. The former don’t work well for passengers, but are well suited to bulk delivery of low-cost stuff (e.g., propellants), and the latter require very high up-front capital costs, in general. With a lot of tech risk.

67 thoughts on “Unconventional Space Access”

  1. John Hare, I agree completely. We should design with what we have or know we can do in the very near term. Other ideas should be explored, but not as the basis for any current mission (which only serves to make them show stoppers for some length of time.)

  2. Don’t forget the short-circuiting sewing machine that became a space drive in Frederick Brown’s WHAT MAD UNIVERSE.

  3. Under momentum exchange – see aerovator:

    encycl.opentopia dot com/term/Aerovator

    It is more workable than other designs which can provide full orbital velocity.

  4. John Hare said:

    > All this fiddling around looking for different way to do the job is a waste of time. Chemical rockets are more than good enough to provide economical orbital access without looking for some new magic technology.

    He is correct. If a reusable rocket were developed which could reliably fly to orbit once a day, and last for a thousand flights, cost per-kg to orbit could be in the $50 – $100 range. High flight rates are the key.

    Unconventional orbital access methods are unlikely to beat that figure.

  5. cost per-kg to orbit could be in the $50 – $100 range

    Ermm, if you can get it to passenger jet level efficiencies, then propellant costs are roughly a third of the overall cost per kg. That would be perhaps $100 per kg for kerosene/LOX, and $300 per kg for LH2/LOX.

    As to beating that figure, keep in mind that the actual cost of energy to get to orbit is something like $10 per kg. All of the scheme categories, even if they don’t eliminate the need for chemical propellant engines, do at least make a partial trade of chemical propulsion delta v for delta v that has a marginal cost proportional to the energy used. Some of them also lower the performance margin needed for chemical rockets.

  6. Cost of a kilogram of hydrocarbon/LOX propellant (lets assume methane/LOX for now) at an ideal mixture ratio should be well under one dollar (lets assume $0.60). If you can achieve payload fractions of 3%, you’d be looking at a propellant cost of about $20 per kilogram of payload delivered to LEO. Assuming airline-like total costs of three times propellant costs, and you are looking at around $60 per kilo to LEO.

    Note: LOX is very cheap, and makes up between 65% and 85% of propellant mass, depending on the fuel (and the engine).

    LOX/LH2 doesn’t make much sense for sea-level operation, but for a second stage, the 30% higher ISP can generally make a pretty convincing argument.

  7. Ermm, if you can get it to passenger jet level efficiencies, then propellant costs are roughly a third of the overall cost per kg. That would be perhaps $100 per kg for kerosene/LOX, and $300 per kg for LH2/LOX.

    If we assume say 25% RP-1, 2.5% payload fraction and negligible LOX cost, then one is talking about a 10:1 fuel to payload ratio. At say $0.5/kg for kerosene that works out at ~$5/kg. With a three fold multiplier for total cost (I think a reasonable assumption – a third each for fuel, capital and O&M seems typical across many transport systems) that would be $15/kg to LEO. The costs for LH2/LOX should actually be fairly similar (say 4x the energy/cost for LH2/kg, twice the fuel ratio and twice the payload fraction). Interestingly, improvements in light weight engine and tank design benefit LH2 far more than RP-1 (which makes me think LH2 might actually be the future).

    Topping out with a rotovator would I think then be the next logical step for reducing cost below this pure rocket vehicle. And should ultimately reduce costs down to only a few dollars a kilogram to LEO.

    It should work out at roughly 10 time the price of a long distance flight, although one thing that should be noted is that rockets are capable of much higher flight rates than long distance aircraft – perhaps reducing capital and operational costs significantly.

  8. Uh huh. Disposable rockets don’t cost a lot either. The price of launch has more to do with capital investment and fixed costs than it does with the margins.

    Yes they do, even at $500/kg for dry mass (passenger jets typically come in at around $1000/kg), one would not be able to do better than around a $1000/kg to LEO. Expendables are fundamentally ~2 orders of magnitude more expensive than what reusable vehicles could be.

  9. Pete, and no-one is doing better than $5000/kg.. clearly the price of rocket launch has little to do with the technology. It’s the markets, or the lack thereof.

    At launch costs generally significantly above ~$1000/kg, sure. Expendables can work just fine in a dinosaur market, as the last fifty years has well demonstrated.

    But to paraphrase many people, if one is not on a development path to ~$100/kg, then one is not going anywhere.

    Expendables are not compatible with CATS because vehicle capital cost has to be amortized over one not 1,000-10,000 flights.

  10. Expendables are not compatible with CATS because vehicle capital cost has to be amortized over one not 1,000-10,000 flights.

    Are not as compatible. My take is that there’s still room to go with expendables. Among other things I think there will be a means to transition gradually from expendable to fully reusable.

    Pete, and no-one is doing better than $5000/kg.. clearly the price of rocket launch has little to do with the technology. It’s the markets, or the lack thereof.

    Currently. Keep in mind that there used to be only one supplier at that price point, Russia (and to a very limited extent the Chinese). With increased competition from US launchers, that’s going to encourage everyone to lower their prices.

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