In article ,
says...

On Thursday, June 5, 2014 11:32:53 PM UTC+12, Jeff Findley wrote:

Bigger and bigger launch vehicles aren't the answer to everything.

Agreed. Space travel is a complex business. You have to do many
things well. Even so, we can look at the highlights needed today.

Highly reusable vehicles, ones that have 20 year working lives,
cost less than 0.1% or less their purchase price in maintenance
to reuse, and are usable 2,000x or more without compromising
safety or reliability - is a critical technical achievement.

This is going to take several iterations of designs to get there. The
space shuttle was iteration 1.0, and it sucked. The problem was that
NASA sold it as "operational" after five flights and proceeded to treat
it as "finished". It wasn't. Continuous spiral development could have
improved it quite a bit. Instead, we got a few piecemeal upgrades, some
driven more by replacing obsolete, hard to maintain, hardware rather
than staying "ahead of the curve". In other words, they reacted to
problem rather than being proactive.

It's unclear how much it could have truly evolved though, because the ET
being throw-away with the SSME's on the orbiter was a design decision
that meant that the system could never have been made fully reusable
without a major change to the overall system design. In other words, it
was a dead end in terms of reusability.

Falcon 9R is another approach to resuability, but it's not quite there
yet. The hardest part will be trying to reuse the second stage. I'd
say we have at least 5 to 10 years before we'll know if the second stage
can become reusable. Then, and only then, will we truly find out if
this approach is viable.

Reusable systems with 20 year working times are far, far into the
future. Because of this, I'd rather focus on the present and what can
be done with relatively inexpensive vehicles like Falcon 9R.

However, in concise conversation, bigger and hotter launch vehicles
lower the cost of space access more rapidly than anything else, if
choices must be made about what to talk about in a 4 minute elevator
conversation.

I quite disagree. Going big early (for launch vehicles) locks you into
super high fixed costs and very low flight rates, preventing rapid
spiral development of launch technology. Saturn V had this problem, the
shuttle had this problem, and SLS will have this problem.

Going big early would only work with a massive infusion of cash
amounting to at least 10s of billions of dollars of *additional* NASA
funding per year. THIS ISN'T GOING TO HAPPEN, EVER!!!! Not accepting
this reality is what keeps NASA going down this same FAILED path
repeatedly. This is a lesson that NASA and you should learn.

In
particular, developing technologies like inflatable reentry shields is
one of many enabling technologies that helps to eliminate the "need" for
very large launch vehicles for a manned Mars mission.

Inflatable technologies are near and dear to me. I've been a
proponent of inflatable technologies for low-cost re-entry and entry
into Mars' atmosphere for many years.

Saying we have to ignore one good idea to do another good idea is
the fallacy of 'false choice' - there is nothing in inflatable
thermal protection gear that makes us not choose to build
adequately sized vehicles with high exhaust velocities. (bigger &
hotter)

It would be far better to invest in the following *before* investing in
big launch vehicles:

Inflatable habitat modules.
Inflatable heat shields for Mars atmospheric entry and braking.
Cryogenic storage of propellants.
Transfer of cryogenic propellants (i.e. fuel depots).
In-situ fuel production (lunar as well as Mars).

If all of the above prove successful, the need for large launch vehicles
is reduced or perhaps even eliminated. Most of what you need for a Mars
mission is fuel. That does *not* have to go up on a large launch
vehicle since it's infinitely divisible into smaller pieces.

Putting the launch vehicle first, before you really know how big the
payloads *need* to be is stupid, plain and simple.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer