Re: Reasonable Minimal Size for a 2-Man Capsule?
- From: henry@xxxxxxxxxxxxx (Henry Spencer)
- Date: Sun, 3 Dec 2006 23:54:25 GMT
In article <1165023151.679335.158150@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Jonathan Goff <jongoff@xxxxxxxxx> wrote:
...would need to support the two-man crew for up to 8 days (ie a
1-2 day lunar stay), and include a hatch, and room for two spacesuits.
The idea would be to have it just roomy enough to be workable for 8
days, but otherwise as small as reasonably possible.
With one major exception (see below), volume and mass are pretty much two
separate issues. The mass penalty of making the thing roomier and hence
easier to live in is quite small. Even the heatshield mass scales much
more strongly with capsule mass than with capsule volume, because a
fluffier capsule sees a gentler reentry environment.
...The 3-man Apollo CM was about 5800kg, and the 2-man Gemini
was about 3800kg. t/Space was talking about 4000kg for a 4-man reusable
orbital capsule, and SpaceX is looking at 7 people for less than 9000kg
including the escape tower. I know that George Herbert has suggested
that a 1-person capsule could be doable in the 300-400kg range...
I once did a back-of-the-envelope estimate suggesting that you could do a
3-man capsule for around 1000kg (including occupants). But that was an
aggressive and severely Spartan design, basically a 3-man Mercury -- no
maneuverability except retrofire, short in-space life, no amenities --
meant as a crew ferry to a space station which had its own tug to bring
you in (and with the launch-window issues of first-day rendezvous sort of
glossed over...). And I never carried the details far enough to decide
whether it was really workable.
Hempsell's "Excalibur" is a reasonable example of a fairly aggressive
modern design, 4 men for 10 days in 10100kg... and about half that mass is
fuel, giving it 2km/s of delta-V, so it's more like 5000kg if you don't
require much maneuverability.
One rule of thumb I've long held is that since giving each guy his own
Mercury capsule takes about 1500kg/man, any vehicle heavier than that is
inexcusably overweight unless it's doing something rather more ambitious.
(Gemini and Apollo are not far above that, and probably get a passing mark
due to greater capabilities. NASA's new barge is another story.)
I guess I'm having a hard time coming up with a realistic number
because most of the previous manned capsules were done with 60s era
manufacturing, electronics, and power systems...
You can safely assume that the electronics weigh essentially nothing, and
power systems will be a lot lighter because they have to do much less.
One snag for a *lunar* capsule is radiation shielding: of some
significance because of deep-space background radiation, somewhat more
important due to the possibility of giant flares, and utterly vital for
passage through the Van Allen belts. This is where having a lot of bulky
heavy equipment and an overbuilt heatshield win -- Apollo had about
8g/cm^2 (aluminum equivalent) of protection even with no mass explicitly
budgeted for shielding. And this is one place where volume matters, or
more precisely, surface area matters.
Modeling a two-man capsule crudely as a 2m-dia hemisphere, it has about
10m^2 of wall area including the base, or around 800kg at 8g/cm^2. Now,
that's a conservative number because aluminum is not actually the best
shielding; something with a lot of hydrogen in it will be better. If we
assume that the heatshield and related structure are enough shielding for
the base (ca. 3.5m^2), and that elsewhere the walls and other structure
contribute 2g/cm^2, and that added shielding is high-density polyethylene
(perhaps twice as good as aluminum), that's maybe 200kg of shielding,
which isn't too bad. Significant, but not a showstopper.
Anyway, given that and the longer in-space life and a high-energy reentry,
you definitely can't be as aggressive as that lightweight design sketch of
mine. But you really shouldn't need as much mass as a Gemini. Take the
geometric mean of the two and call it 2000kg, as a first guess, given firm
design discipline and no severe externally-imposed requirements (e.g., a
demand for much more radiation shielding than Apollo had).
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
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