In article ,
Mr Jim wrote:
... Very light tanks, probably pressure-stiffened like the old Atlas.

Does this choice place too many burdens & costs on ground handling and
checkout facilities/personnel?

I think not, overall, but it would need looking at. That's why I said
"probably". :-) With a relatively high launch rate, it pays to invest in
mechanization and automation of production and checkout, which does reduce
the amount of handling needed.

...Von Braun's folks reportedly hated the
pressure-stabilized approach, apparently judging it just couldn't fit with
their culture of protracted stacked checkout...

Yes, they distrusted the idea from the start; both the Saturns and the
shuttle had a "no balloon tanks" ground rule. (The shuttle guys were
deeply displeased to discover that the shuttle ET LOX tank had to be
pressurized slightly during filling or there was risk of wall buckling at
one point -- it was fine empty or full, but marginal at one liquid level
in between.)

Protracted stacked checkout is something I definitely wouldn't do. Roll
out to the pad only when *ready*, and then you fill the tanks and *go*.

Would you chose steel, like Atlas, or Aluminum alloy?

Atlas went with steel mostly because of aerodynamic heating in worst-case
(depressed) trajectories. I'd favor aluminum or composites, with some
ablative thermal protection if necessary (my guess: might need a bit on
the nose).

... Boost pumps at the bottom of the tanks, or possibly the
bottom of the feed lines...

How are such pumps driven? Do they have their own turbines or can the engine
pumps drive them via shafts/gearboxes?

Generally they haven't been shaft driven, especially if they're
tank-mounted. Sometimes they have their own turbines, sometimes they're
jet pumps. The SSME has sort-of boost pumps, although they're located in
the engine compartment rather than in the tanks: if memory serves, the
fuel pump is driven by a hydrogen expander cycle, while the LOX one is
driven by a LOX hydraulic turbine (using LOX from the main LOX pump).

... One interesting option is to make the
boost pumps jet pumps, recirculating a bit of the output from the main
pumps to the jets in the boost pumps. (That too has been done.) ...

By "jet pump" do you mean something like a water-driven eductor pump?

Terminology varies, but yes, you've got the right idea.

...Is it bad to have yet more
high-velocity/high-pressure fluid piping running between pairs of pumps?

It's a nuisance, but it may be the least of assorted evils. I wouldn't
want to run at the kind of pressures the SSMEs run at anyway, so it's not
as bad as you might think. It's especially not a big deal if the boost
pumps are at the bottoms of the feed lines instead of at the tops.

...The aerospike provides altitude
compensation and also permits a light, compact nozzle with a very high
expansion ratio in vacuum.

Henry, do aerospike engines have to be carefully integrated with the
particular airframe? If so, is that a problem?

There's no big integration issue, with the caveat that the aerospike does
comprise the whole base of the vehicle. Load paths have to be thought
about, but they shouldn't be a big problem.

Would such an engine use
differential throttling to control pitch and yaw?

I'd like to. Needs some analysis, and perhaps some test flights, to
establish whether it's sufficient. I'd want throttling at least for trim,
but it might not be enough for worst-case control. My fallback would be
fluid-injection vectoring on the aerospike's central plug. It's possible
to gimbal an aerospike, but the large diameter makes it unappealing. Even
RCS thrusters might be worth considering, given that the worst-case
control requirements don't last long (just after takeoff, in turbulent air
with solid objects nearby; and windshear in the stratosphere).

How about roll control during boost?

You don't need very much, if you make a point of keeping roll torques
down. Atlas II used a tiny thruster pack for post-booster-engine-drop
roll control. And the Athena series just let the rocket roll, although
they did have thrusters available to limit roll rate if it became a
problem. That example notwithstanding, I'd go for full roll control.
Warm-gas thrusters using either pump-drive gas or tank-pressurization gas
(see earlier posting) are the obvious choice; there are possibilities for
being clever but it's probably not worth it.

Wouldn't your engine layout lend itself to a tripropellant arrangement?

Maybe, but I'm not convinced that tripropellant schemes show enough gain
to be worth the extra complexity.
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