guest blogger john hare

It is a requirement of living that you make assumptions about things. If you have to have all conceivable information on any subject, and have all of it verified to a high degree, before you do anything, you will never do anything. You have to assume you will get paid for the work you do, or not do it. You have to assume your vehicle won’t suddenly explode on the way to work, or don’t go.

In almost anything to do with spaceflight, we have to make assumptions based on less information than in most of the rest of our daily lives. This is because the accurate information is just not there. Is there a market for space tourism? Probably. Is it big enough to support a few companies? Probably. Will the first fatality kill the industry? Probably not. Is helium three worth going after on the Lunar surface? Depends on a dozen factors, I’d say not now.

All these assumptions are best guesses based on the best information available. Of course the best guesses themselves are based on your personal viewpoint. It is safe to assume that Marks’ assumptions are almost opposite Rands’ for instance.

What got me thinking about it this time was a comment by ‘the other Bob’ at Rands place. He was concerned that the existence of propellant depots might somehow prevent the development of better in space propulsion systems. I can barely follow the logic of this. If gas stations (propellant depots) exist, there will be no market for electric and hybrid (ion, tether, and laser) vehicles, or incentive to develop them. The way I see it, even if launch prices drop to $500lb, every ton of propellant you don’t have to buy  is a megabuck of savings.

The assumption seems to be that if you have a boat load  of bucks invested in depots, you don’t want to do anything to make that investment worthless.  My assumption is that if Hareball Aerospace has a nifty propellantless drive, Masten Orbital Refuelers is on their own. In a free market with multiple aggressive players, somebody is going to use anything useful for competitive advantage. Bobs’ assumption seems to be that one player is going to dominate space for the foreseeable future. Mine is there will be many. Differing assumptions based on the apparently the same information give very different answers.

The recent airbreathing vs rocket discussions have some underlying assumptions that may not be obvious at first glance. One assumption I’ve not seen stated exactly is that you must develop both systems yourself. On that assumption, rockets will win almost every time. It is easier to set up a shop to develop two rocket engines for the two stages than a turborocket plus a rocket for the upper stage. If you have to develop both simultaneously, I’d say build a large rocket for the first stage and a smaller one based on the same tech for the second.

If however, a mach 4 turborocket system is available COTS, then the assumptions change fast. If first stage propulsion development consists of  a check to the order of Bossards Turbospeedy Engines, years of development time and a lot of uncertainty just left the building. I would bet that XCOR would even have looked at this for the Lynx if it had been available substantially before airframe work started. 

Another annoying assumption by some is that Isp is everything. Laser/hydrogen is the wave of the future because Isp is 50% higher than the best that chemical rockets can do. Scamjets are the wave of the future because they will reach mach 20 without onboard oxygen. And so on. My assumption is that choosing a launch system based on Isp alone is like choosing a wife based on bra size alone, the superficial attraction occasionally hides a really nasty disposition.

One of my personal assumptions is that turbopumps are not that tough, if done right. Most knowledgeable people will disagree  with this. People in the business have limited budgets and developing a pump system adds another  whole set of problems as if they didn’t have enough already. I assume that the increased engine performance will reduce problems in other areas to the point that the pumps pay for themselves during development even before the vehicle flies. I make the assumption that 30psi fittings will be easier than 300 psi fittings.

Perhaps the most important assumption most of us  make is that space is worth doing and will be profitable. There are many with the opposite assumptions though, and they can’t be dismissed out of hand. One of the most important things we can all do is find ways of altering the base assumptions of those that are both interested in space, and have a negative attitude toward the profit potential. Not with words alone though, we need proofs that will be accepted by the ones we need to help us. Why we want to go must be answered, and it must be answered with integrity.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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8 Responses to Assumptions

  1. Andrew Swallow says:

    Good points.
    First stage LV exist and so do second/upper stages like the Centaur. So make sure that your new air breathing first stage can lift a Centaur (or rival) to the required height and velocity.

    Air breathing jet engines have a higher Isp than rockets. Second stages normally start in the atmosphere.

    Propellant depots can sell reusable space tugs the propellant Argon.

  2. John,
    Great article. Hopefully I’ll be able to rejoin the bloggy goodness soon.

    ~Jon “needing a vacation from his vacation” Goff

  3. Pete says:

    I would quite like to see an article detailing the short, medium and long term competitive economic advantages and disadvantages of space for humanity (like a SWOT analysis). It might go along ways toward focusing efforts in constructive directions.

  4. MG says:

    Ah, the ol’ “what assumptions are we making, that we don’t recognize?” problem.
    Sensitivity analyses can help determine what the “important” variables are.

    PS: I concur with your assessment regarding the need for pumps for high-performance first stages. BUT… if the mission is something other than maximizing deltaV in the first stage, then the trade space can open up quite a bit.

  5. johnhare john hare says:


    I’m not familiar with SWOT. Could you expand a bit on what you would like to see?

  6. Rhyolite says:

    “Perhaps the most important assumption most of us make is that space is worth doing and will be profitable.”

    Are you making an assumption about what kind of ‘space’ you are talking about here?

    The commercial satellite communications side of the space business as a whole has been profitable for years (decades really). Companies like Intelsat, Telesat, Inmarsat, DirecTV, Thuraya, and many others, make money in space by providing services that would be expensive or impossible by terrestrial means. Their suppliers, Loral, Lockheed, Boeing Space Systems, Astrium, ILS, Ariane make up the bulk of the manufacturing side of the industry. The total revenue stream for commercial satellite communications business is tens of billions per year.

    This is not a bad place to be starting from. The question is how to expand the size and number of profitable markets in space.

  7. Robert Lynn says:

    I am a little surprised that there hasn’t been more made of the implications of VASIMR on LEO scenarios.

    It really does change things because it enables atmospheric scoop mining.

    We can now build a propellant depot/space station with a megawatt or two (@~5000kg/MW) of solar power and a few VASIMR engines on it (~1500kg/MW), stick it at 200km altitude and drop a tethered collector down to maybe 100-120km to scoop up and compress atmosphere.

    1MW VASIMR with 65% efficiency at 3000Isp gives about 45N thrust, enough to overcome about 45N/7200m/s=.006kg/s = 540kg/day, 180 tonnes per year.

    Think GE90 sized turbomolecular pump, feeding secondary compressors, partially powered by 7km/s gas impact on the oblique inlet blades as well as solar, power down tether or perhaps other beamed power from above.

    20 tonnes in LEO would go a long way towards getting this all working.

    Ditch any gases you don’t want. Use about 25% of Nitrogen collected to fuel the VASIMR drive, pull tether up every few weeks to offload collected atmosphere.

    With CO2, N2 and H2O there are quite a few useful materials that can be made, as well as fuels for Moon or Mars Landings etc.

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