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