Re: Messenger - long way to Mercury?

From: Henry Spencer (henry_at_spsystems.net)
Date: 08/04/04

  • Next message: Peter Stickney: "Re: Mountains Rush and Moore" 
    Date: Wed, 4 Aug 2004 00:07:28 GMT

    I wrote:
    >>...would it be a better tradeoff to spend more money
    >>on having a larger mass buget, than to spend money on squeezing
    >>everything into a (much) smaller amount of mass?
    >
    >Yes, sometimes -- perhaps even fairly often -- it would be worth at least
    >considering such tradeoffs; the cost cap should really be on total mission
    >cost, rather than having separate caps on development and launch. Such
    >moves to bigger launchers to simplify spacecraft development have been
    >done a few times in the past. But the Discovery rules, alas, pretty much
    >forbid it.

    An afterthought: such tradeoffs could be done *within* Discovery rules if
    Discovery proposal evaluation gave more weight to margins and development
    flexibility. There's no law of nature that says a Mercury orbiter, for
    example, *has* to be right at the maximum weight a top-end Delta II can
    lift. But after a promising early start, I think Discovery has at least
    partially fallen back into the old trap of loading up the proposals to
    barely fit within the cost and mass caps, so everything is pushed to the
    limit from the beginning.

    Indeed, I see a common thread going back all the way to Vanguard (!) of
    proposal evaluation over-valuing promises of massive science return and
    under-valuing easy, quick engineering with large margins. It's easy to
    slip into this trap when:

    (a) decision processes are often dominated by scientist input, with
    feasible engineering seen as a constraint rather than easy engineering
    seen as a major objective;

    (b) institutional memories tend to be dominated by spectacular successes
    at difficult engineering challenges, with failures pushed into the dim
    background;

    (c) the program is a random grab-bag of missions, with no continuity in
    targets or engineering, so the incentive is to get maximum return out of a
    unique flight opportunity, rather than to ensure that the first one works
    so the experience can be used to improve the second and third. 

    -- 
"Think outside the box -- the box isn't our friend."    |   Henry Spencer
                                -- George Herbert       | henry@spsystems.net
  • Next message: Peter Stickney: "Re: Mountains Rush and Moore"

    Relevant Pages

    • Re: Am I a racist?
      ... It involves finding the center of mass of a semicircular solid. ... And you can't just go to the department store and buy kite string. ... precision string and it will cost much more than the computer. ... place in the engineering and design field. ...
      (rec.arts.poems)
    • Re: Aircraft Scaling Problem / Question
      ... There's more to it than mass and volume, ... And that makes it an engineering problem, ... If we tried to lift the same proportionate ... Now I assumed the density was constant throughout the plane. ...
      (sci.math)
    • Re: Torsional Vibration and PSRU Design
      ... with complete engineering information. ... misalignments, nominal and max twist in degrees, weight and mass moment ... You really want to say "inertia", ... torsional stiffness. ...
      (rec.aviation.homebuilt)
    • Re: Spaceship One stepping-stone or dead-end?
      ... >>Speaking as someone with an engineering degree, ... >>Cost usually scales strongly with complexity and less strongly with size. ... This does not disprove my assertion that complexity, ... aerospace measures like propellant mass to payload mass or dry mass to ...
      (sci.space.policy)