Re: New design for spacecraft with Orion propulsion

The original poster has been delayed. He will be posting
on his own behalf in a couple of days. In the meanwhile
he asked me to post his comments:

Thank you to everyone who has taken the time to plod through the
inital post and respond. I put together a few comments on the open
points I have seen so far. I'm going to try and respond to all the
input on this forum and also the thread on rec.arts.sf.science. Since
there's content overlap and my response time will contiue to be
pitiful, I didn't respond to everyone individually. Please keep the
feedback coming, it's extremely helpful.

Jarrod

==========

Pusher Plate Form: I looked at concave, flat, convex, and really
exotic pusher plate forms before deciding to present this one. I do
not have a tremendous amount of confidence in the choice but there
was some rationale for it. After some very crude finite element
analysis, the concave pusher plate looked bad. The hoop stress must
be counteracted by adding mass near the rim. This is the worst place
to add mass for two reasons. You need a lot of mass to go all the way
around the plate and that mass has the longest possible lever arm to
the reaction at the center of the plate. Statically the flat plate
looks good, no hoop stress and no compressive stress. However,
dynamically there may be a problem. With the pressure load resisted
only at the center, the plate tends to bow back and forth when force
is applied. (I can try to draw this if it’s not clear.) This will
cause fatigue problems within the plate as it vibrates. To deal with
this problem I wanted a solution that would increase the rigidity of
the plate without increasing the mass any more than necessary. I
chose the recurved convex profile. Although the compressive stress
may be an Achilles heel, there are a couple of things going for this
proposal. First it’s more efficient to add mass at the center to
counteract the compressive stress, than it is to add mass at the
perimeter against hoop stress. Second, the pressure from the plasma
cannot be uniform. I read figures of 100,000 psi at the center and 0
psi at the edge (obviously). The highest pressure would be applied at
the flattest part of the curve, reducing the compressive stress
substantially compared to the uniform pressure assumption.

If I ever go further with this design replacing the homogeneous plate
with trusses and an ablative skin will be high on the list of things
to do. I will also be altering the plate as I learn more about the
plasma effects. Right now the design is driven almost solely by
structural concerns.

Exit Holes: The exit holes in the pusher plate are a weak spot. No
argument there. I dislike the rotating door solution more the more I
think about it. First it probably requires a fragile motor to operate
it. Second it requires sealing on two surfaces in relative motion.
This cannot be done reliably. The comment about no protection vs.
x-rays and neutrons is certainly valid and brings up a few thoughts.
The propellant mass must have some shielding value? Is it a big deal
if some radiation gets past the plate? It will hit the pulse units
first and there is no line of sight to the crew cabin. Will the
neutrons scatter secondary effects into the crew cabin? Can we add a
flap inside the tube that will stop the radiation but hinge out of
the way when a pulse unit pushes past it on the way out? Why not just
have a hinged door over the hole(s)? As an orbit-to-orbit vehicle the
detonation frequency can be very low so the door will have plenty of
time to close. I still like the idea of a magnetic coil because there
are no moving parts in the pusher but there are other solutions. Need
more input.

Suspension: I love the energy recovery idea. Wish I had connected the
dots myself.

Navigation and Star Sightings: I don’t think the navigation problem
is worth worrying about for a couple of reasons. 26 meters of
suspension travel is enormous! I went through an applied mechanical
vibration short course earlier this year. The focus was on automotive
suspension systems where several Gs are damped into a comfortable
ride in the space of a few cm. This thing will ride smooth! Also the
acceleration phase is a miniscule fraction of this vehicle’s total
mission time and there is no almost penalty to starting and stopping
the drive. It would be trivial to coast between pulses long enough
for even sextant and slide rule (S&S) technology to navigate.

Vehicle Rotation: I should make clear that I proposed to rotate the
whole vehicle. No relative rotation of parts and absolutely no
bearings. Sorry about the Gatling gun. I also figured the rotation
would be slow, maybe about the speed of the space needle restaurant.
As an orbit-to-orbit only proposal everything about this vehicle will
probably be best when it is lethargic. I do not propose rotation as a
replacement for active navigation, only as a supplement.

Even if pulse unit placement error and erosion asymmetry cancel out
as the Orion study concluded (I probably blew their assumptions in
this design.), there are still inherent manufacturing errors to
contend with. Just as physicists won’t let us go FTL, engineers won’t
let us build anything perfect. Consider the pusher dome (I really
like that. Now I need a football team to play in it.) viewed from the
rear. The time averaged input (plasma impact) will be symmetric about
the centroid. The response (vehicle acceleration) will be symmetric
about the center of mass. Any irregularity in the vehicle’s mass
distribution will move these two points apart and induce acceleration
and tumble in unwanted directions. Without getting into any
manufacturing details I would assume the centroid and the center of
mass will diverge by about 10 cm. If the vehicle does not rotate,
this error will always be the same and will accumulate over time
unless counteracted with maneuvering thrusters. If the vehicle
rotates, at least some of the error will sum to zero over time.
Hopefully this will reduce the consumption of maneuvering propellant,
improving the rocket equation’s bottom line. Of course I could be
completely cracked.

The proposal to handle this by selecting the appropriate pulse unit
stack certainly has merit but I would be concerned that one group of
stacks would be prematurely depleted because the inherent
manufacturing error does not move. Uneven use of pulse units will
also skew the mass distribution and parallelism errors in the outlets
will cause additional problems or opportunities.

Metal Storm: The only things I borrowed from the Metal Storm concept
were the series-parallel arrangement and electronic initiation. Past
that the pulse unit magazine is completely different. Although
modeled as a simple cylinder for convenience, I envision a system of
open frame racks that hold the pulse units. Their overall weight is
minimal and most of that is required to prevent buckling when under
thrust. For now I would prefer to leave this a single structure
rather than add ejectable pieces.

I don’t understand the comment about the system being fantastically
complex. There are 7501 nodes communicating in this system. That
doesn’t strike me as overwhelming in any way. 7500 of the nodes have
identical hardware and their communication needs are going to be very
low. I would like to think the pulse units could triangulate their
position by chatting amongst themselves to so that each could avoid
anything dangerous like prematurely firing its separation change.
Additional sensors on the pulse units could be very basic and
confined to communication within the pulse unit. Smart fuel.

I’m also unclear on my failure to identify the true strengths and
weaknesses of this system. I proposed this concept because I think it
is very robust and compatible with the suspension system. There are
some features, like the potential detonation rate, which I am not
taking maximum advantage of, but the goal is to get the best possible
performance out of the whole system within stringent bounds of
reliability, not to optimize any one subsystem.
.

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