Space Forum / Space Flight / September 2003

I was thinking about the possibilities of building a water cooled
reentry shield.

Back in the 50s or 60s there was a test where they reentered a solid
sphere of copper. The copper conducted the heat away fast enough that it
didn't melt- it just got hot (a few hundred degrees). Now for ICBMs they
could live with it; and I believe that atleast some ICBMs use copper for
their heat shield.

Now, there's often pictures of fragments of spacecraft that survive
passage through the atmosphere. Quite often spherical helium tanks seem
to make it safely to the ground. Presumably they're often made of an
alloy. What I suspect happens is that any remaining gas in the bottle
gets hot, expands and leaves the sphere through the outlet taking with
it the heat and cooling the sphere.

So I was wondering whether the same thing could be done deliberately,
for example using water as a coolant.

Now the problem is that as the water gets hot, it boils and then you
have problems putting any more heat into it (steam is a much worse
conductor of heat than water, and you need to conduct heat into your
coolant to cool the shield). So it pays you to keep the water under
pressure to keep it from boiling. Ideally you keep it under it's
critical pressure, and then the water doesn't boil to over 350C. Ok, so
now the reentry shield is a big boiler. I did some calculations and the
pressure vessel didn't come out unreasonably heavy even at the critical
pressure. In fact, the water you need is itself much heavier- together
you're looking at maybe very roughly 10% of the reentry mass. On top of
that you have to add the landing system etc.

Ok, so it's fairly heavy, but then inspiration struck. Suppose you keep
most or all of that coolant right down to the ground? It doesn't cost
you much to do that.

I mean, steam is actually a rocket fuel in its own right; the ISP is
pretty bad, but it's not that bad. I did some more calculations and
worked out that there's more than enough steam in the reentry shield
coolant to brake a vehicle from a couple of hundred miles an hour down
to touchdown. i.e. VTVL style landing

You'd need some De Laval nozzles to vent the steam through, and some
valves but that doesn't seem to be too bad at all- and there's no chill
down needed, it's a non cryogenic landing system, there's no spooling up
of turbopumps; so you can be very sure that it would work before you land.

Ok. let's list the advantages and disadvantages:

Advantages:

- lightweight (the landing system fuel is your reentry coolant)
- you can check the coolant pressure well before touchdown (if there
isn't any- bail out!)
- no chilldown
- water is a good coolant
- you land cooler since you've dumped the heat/steam to the atmosphere

Disadvantages:

- if you lose pressure on the boiler during the actual reentry you may
not make it (but you may be able to have redundant cooling systems or
something)
- the nozzles valves may fail to open, or open too early (but you can
have redundant nozzles and valves.)
- steam may be hard on valves and they may leak
- if you fail to make orbit the system doesn't help you land since you
probably won't get enough steam pressure up during a slower reentry (but
you should have fuel left anyway in that scenario, so redundant main
engines may help you there.) But if you are nearly at orbit when you
have a failure, then you'd have no problem.
- the heat shield is no good for suborbital (but you probably don't need
it then anyway, although you could theoretically use water to cool the
engines, but the ISP isn't so great.)
- you are carrying a high pressure steam boiler; if you crash it may
explode (Then again you've crashed, that may be the least of your
problems, or anyway the last ;-) )

Areas of uncertainty:

- depending on the heat shield design you may have to vent some of the
steam during the reentry interface so as to allow the heat shield to survive
- exactly how much water do you need to survive reentry anyway (I was
figuring that less than one percent of the heat actually makes it
through to the heatshield.)

Other similar ideas involve keeping less hot water back for landing, and
passing a better propellent through a heat exchanger; for example
hydrogen. That means you can increase the amount of vented steam during
reentry, or employ less water. Also, if you land more slowly then you'd
need less steam, for example a lifting body could be used and then do a
VTVL landing, or the steam could be used for a powered landing and for
reducing landing speed.

Anyway, it's just a straw man idea at the moment, but it seems to work
on paper; comments?