Orbital Reentry shield/landing system?

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?

Rüdiger Klaehn wrote:
Why not have a reservoir of water at low pressure and below boiling
temperature that flows into the heat shield, evaporates and leaves the heat
shield at the center?

As a general rule, it very much pays you to vent any steam at the
highest temperature possible because then it carries away more heat from
the vehicle per kg of coolant. This is easier if you are keeping the
water under pressure so that it stays liquid as long as possible. (The
alternative is to make labyrinths that the steam goes through, but this
leads to costly fabrication and design issues.)

The steam would flow from the center of the heat
shield to the sides and provide an additional protection for the shield.

Because then you have doubled up the landing equipment and doubled the
water needed.

For example, wings come in at about 7-10% of the dry mass, and you'd
need another ~10% water. That's up to 20% of your dry mass. That's your
payload gone right there. Or, you could go with parachutes and landing
rockets and recover some payload, but there are worse reliability issues
with that than with steam landing rockets.

This is a really good idea for a pure reentry vehicle. An OSP capsule that
would use this for landing would be much nicer than an OSP capsule using
parachutes and solid retro rockets like Soyuz. This might be exactly what
NASA needs to sell an osp capsule design as "high tech" :-)

But for a VTVL space transport you really want to use the main propulsion
system for braking and landing.

That's a design choice, although, it might well be possible to use some
of the takeoff nozzles for landing.

Certainly, using the full main propulsion system gives issues with
reliability, turbopump spool-up, chill down, and doesn't help you with
reentry survival in any way.

A steam powered landing would complicate incremental testing so much that it
would not be worth it even if it should save some mass.

I couldn't disagree with this point more. Not 'saving some mass' really
means slashing the payload you carry. The payload size is one of the
main factors that determines the profitability or otherwise of the
vehicle. The reentry and landing equipment mass comes straight out of
your payload; and are likely to be of comparable masses. With the steam
rocket scheme- the masses overlap.

Besides, testing of a steam landing system is trivially performed-
there's nothing that says you can't take off with full steam pressure on
test flights.

It is so
low-tech that it could even be built by an enthusiast with some free time,
an amateur group or a small commercial startup.

The core idea yes, but the 'heat exchanger' for the hypersonic reentry
thermodynamics may require some exotic testing facilities ;-)

The exhaust velocity of steam rockets is only about 1000m/s, so you would
need a lot of steam for deceleration from 100m/s terminal velocity. You can
not afford more than a few seconds of hover time, so the landings would
look really sporty...

Yes. It's likely to be a seriously sporty landing at say, 4g. Compared
to non steam approaches however, it seems very much safer- no turbopumps
or complicated start-up sequences, and you can have a pinpoint landing
with no long runway.

regards,

Rüdiger

In article ,
Ian Woollard wrote:
Back in the 50s or 60s there was a test where they reentered a solid
sphere of copper... Now for ICBMs they
could live with it; and I believe that atleast some ICBMs use copper for
their heat shield.

The early ICBM warheads (and the suborbital Mercury flights) used copper
or beryllium "heat sink" heatshields, which work exactly that way, soaking
up the heat rather than getting rid of it. However, that approach was
quickly abandoned when ablative heatshields proved practical, because
heat-sink heatshields are *heavy*.

If you look at photos of early Atlas and Thor missiles, you'll see very
blunt noses -- "Chinese hat" shapes, cones so wide they are almost flat.
Those are heat-sink heatshields. But later Atlases have more-or-less
pointed noses, and those are ablative.

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.

Sometimes, and sometimes not. Titanium and stainless-steel tanks, in
particular, do sometimes make it down. The metal is thin, so the tank is
quite light and decelerates very high up, in very thin air where heating
rates are not huge. Aluminum tanks generally will break up even so, but
titanium and stainless hold their strength to higher temperatures and will
often survive. (Note also that such tanks often start out *inside* other
structures, which protect the tanks for a while.)

...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.

Unfortunately for this theory, usually the gas will heat up quickly enough
that pressure integrity is lost early. After that, it's up to the metal
of the tank to survive as best it can. Compressed gases really don't give
you much useful cooling. Liquids, however, can be another story.

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

It's been suggested, typically using either water or liquid hydrogen.
Phil Bono's base-first-reentry SSTO designs, in particular, ran LH2
through their bases for reentry cooling.

...Ideally you keep it under it's
critical pressure, and then the water doesn't boil to over 350C.

I think you mean *over* its critical pressure. Which means it doesn't
boil at all, ever -- there is no sharp liquid-gas transition, just
gradual expansion as the temperature rises.

Unfortunately, the critical pressure of water is rather high, about
3200psi. Moreover, supercritical water is extremely corrosive, which
considerably increases the structural problems. Supercritical operation
is a whole lot easier with LH2, alas.

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.

Actually, it does, because you can roughly quadruple the effectiveness of
the coolant by venting it through the heatshield, so it fends the hot air
off, keeping it away from the surface. (There are some wee engineering
problems, mind you...) And that way, you don't need high-temperature
high-pressure tankage.

...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

Hmm. An interesting idea. My gut feeling, *without* having done the
numbers, is that it's heavier than expendable coolant plus ordinary
braking engines. But it does have its attractions.

- 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.

As Jeff Greason is fond of pointing out, for reusable vehicles you need to
think really hard about abort modes, and this can affect your design a
lot. Unfortunately, if you need main-engine restart for some abort modes,
that reduces the attraction of not needing it for normal landings.

- 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.)

That percentage depends greatly on issues like shape. It can be far
under one percent.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
In article ,
Ian Woollard wrote:
...Ideally you keep it under it's
critical pressure, and then the water doesn't boil to over 350C.

I think you mean *over* its critical pressure.

English is so ambiguous(!); actually either at or somewhat below; it's
fine if it boils off somewhat, as the coolant absorbs the latent heat of
vapourisation at that time.

Which means it doesn't boil at all, ever

Surely all liquids vapourise above their critical temperature?

Moreover, supercritical water is extremely corrosive, which
considerably increases the structural problems.

I was planning on using slightly subcritical coolant, but corrosion is
very much a problem with this scheme I would think.

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.

Actually, it does, because you can roughly quadruple the effectiveness of
the coolant by venting it through the heatshield, so it fends the hot air
off, keeping it away from the surface. (There are some wee engineering
problems, mind you...)

Actually, venting some does make it much better than none; you're right.

...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

Hmm. An interesting idea. My gut feeling, *without* having done the
numbers, is that it's heavier than expendable coolant plus ordinary
braking engines.

At the moment I think it's potentially lighter than using wings.

Unfortunately, if you need main-engine restart for some abort modes,
that reduces the attraction of not needing it for normal landings.

It does possibly depend on details of the main engines. Restarting the
main engines at exactly the right time may not be reliable enough to do
routinely for landing; it might be say, only 99% successful -enough to
land in an emergency, but not enough for routine use.

(Henry Spencer) wrote:

If you look at photos of early Atlas and Thor missiles, you'll see very
blunt noses -- "Chinese hat" shapes, cones so wide they are almost flat.
Those are heat-sink heatshields. But later Atlases have more-or-less
pointed noses, and those are ablative.

Nit: Any Atlas you see with a pointed nose is a test bird. After the
heat sink equipped RV, Atlas used the sphere-cone-cylinder-flare
warhead which was the intermediate form that proceeded the pure
conical version.


http://www.fas.org/nuke/guide/usa/icbm/atm-10.jpg shows the heat sink
style RV, while
http://www.fas.org/nuke/guide/usa/ic...e_atlas_01.jpg shows the
final form.

D.
--
The STS-107 Columbia Loss FAQ can be found
at the following URLs:

Text-Only Version:
http://www.io.com/~o_m/columbia_loss_faq.html

Enhanced HTML Version:
http://www.io.com/~o_m/columbia_loss_faq_x.html

Corrections, comments, and additions should be
e-mailed to , as well as posted to
sci.space.history and sci.space.shuttle for
discussion.

In article ,
Derek Lyons wrote:
If you look at photos of early Atlas and Thor missiles, you'll see very
blunt noses -- "Chinese hat" shapes, cones so wide they are almost flat.
Those are heat-sink heatshields. But later Atlases have more-or-less
pointed noses, and those are ablative.

Nit: Any Atlas you see with a pointed nose is a test bird. After the
heat sink equipped RV, Atlas used the sphere-cone-cylinder-flare
warhead which was the intermediate form that proceeded the pure
conical version.

That's why I said "more-or-less". The cylinder-flare designs weren't
exactly conical, but they were a lot pointier than the "Chinese hats".
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Here, Henry Spencer wrote:

(One of the best processes for waste recycling is SCWO, SuperCritical
Water Oxidation. Oxygen in supercritical water will attack *anything*.
Plastic, human waste, dead bodies, broken machinery, you name it -- it
comes out as water, CO2, nitrogen [plus some nitrogen oxides], and sterile
metal-oxide ash. A minor problem is that it's terribly energy-intensive;
a major problem is, what do you build the reaction vessel out of?)

Fused quartz? I hope?

--Z

"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*
* Make your vote count. Get your vote counted.

(Derek Lyons) wrote

Nit: Any Atlas you see with a pointed nose is a test bird. After the
heat sink equipped RV, Atlas used the sphere-cone-cylinder-flare
warhead which was the intermediate form that proceeded the pure
conical version.

Second-order nit: the RVs that look like pure cones aren't: they have
a spherical nosecap with a very small radius of curvature. "Tangent-
sphere-cone" is the term used to describe them. This is actually
significant, as the radar cross section of such an RV oriented toward
the radar depends on the nose radius. (There's also a base rcs effect.)

(Allen Thomson) wrote:

(Derek Lyons) wrote

Nit: Any Atlas you see with a pointed nose is a test bird. After the
heat sink equipped RV, Atlas used the sphere-cone-cylinder-flare
warhead which was the intermediate form that proceeded the pure
conical version.

Second-order nit: the RVs that look like pure cones aren't: they have
a spherical nosecap with a very small radius of curvature. "Tangent-
sphere-cone" is the term used to describe them.

Third-order nit 01: That appears to differ somewhat between USN and
USAF birds. Declassified pictures of USAF RV's show the radius to be
a fraction of an inch, while the declassified diagrams of USN RV's
show the radius to be somewhat greater. It occurs to me that
slightly blunter tips may allow the nose fairing to be fractionally
shorter, a small but significant win when dealing with the cramped
confines of a SLBM tube.

Oddly enough the cylinder flare warhead of the A-1 is the reverse of
this, it has an ogival tip as compared to the flatter tips of the
Atlas/Titan I. This may have to do with the underwater portion of the
A-1/A-2's flight., or the mechanization of the tube closure diaphragm.

This is actually significant, as the radar cross section of such an RV
oriented toward the radar depends on the nose radius. (There's also
a base rcs effect.)

Third-order nit 02: The primary reason for the radiused tip is not to
control RCS, but to avoid large changes in tip profile during
re-entry. Such changes increase dispersion.

D.
--
The STS-107 Columbia Loss FAQ can be found
at the following URLs:

Text-Only Version:
http://www.io.com/~o_m/columbia_loss_faq.html

Enhanced HTML Version:
http://www.io.com/~o_m/columbia_loss_faq_x.html

Corrections, comments, and additions should be
e-mailed to , as well as posted to
sci.space.history and sci.space.shuttle for
discussion.

The real purpose of the TPS is to convert orbital velocity to heat
energy and transport it to somwhere that isn't the crew compartment.
From an operational standpoint, wouldn't it make more sense to haul up
enough delta-v to perform a larger retro burn? Not enough to bring the
orbiter to a full stop relative to the surface, but enough so that a
titanium hull like the X-15 or the SR-71 could withstand the rest.

Also, is there any way on a winged orbiter to do a phased reentry to
control temperatures (stall and fall till heat rises, then straight an
level till heat drops, repeat till on or underneat ground)

Lou