The 100/10/1 Rule.

Henry Spencer wrote:
In article ,
Pat Flannery wrote:
...keeping the ET's changing center of mass within the
limited gimbal range of the SSMEs absolutely dictated putting the LOX tank
as far away from the SSMEs as humanly possible.
I thought the gimbaling would probably be the main problem... the thing
is already fairly unstable without getting it completely unstable by
sticking the LH2 on top and having it constantly trying to go out of
control.

As I understand it, aerodynamic stability did not end up being an
overwhelming concern, because the SRBs are such a large part of the
stack's mass for most of the atmospheric flight. (Their nozzle gimbals
also handle most of the control then.) By the time they depart, the air
is thinning out fast and the importance of the aerodynamics is dwindling.

The main issue was reducing movement of the center of mass and keeping it
within SSME gimbaling range, so that the off-center SSMEs can always have
their thrust vector pointed toward it. The SRBs and the ET LOX tank
totally dominate the stack's center of mass for most of the flight, so
their centers *had* to be in roughly the same direction, as seen from the
SSMEs.

Was there any particular reason the LOX ended up on top in the Atlas?
Gimbaling limits again?

Gimbal range wasn't an issue with the engines directly under the tanks.
Reducing aerodynamic instability, to go easier on the control system,
might have been. I don't think I've ever seen a discussion of exactly why
Atlas has the LOX on top.

I note that both Jupiter and Thor had it on the bottom.

Designers with different priorities, probably. The Atlas's balloon tanks
minimize the structural-mass penalty of putting the heavy LOX high up, so
the more conventional structures might be expected to encourage putting it
on the bottom. Max Hunter, who was chief engineer for Thor, in later
years was big on saving structural mass by putting the LOX right above the
engines, so that may have been the issue for Thor.

My consideration is the structural efficiency gained from having GEM-60s
span the engine thrust structure to the oxygen intertank segment,
especially with GEM-60 thrust vector control. The idea of a large
lightweight pressurized hydrogen tank at the top of the stack, with no
heavy payload attached to it, other than a nose cone reentry vehicle, is
intuitively appealing for high g levels at the top of the flight path.

How much trouble that presents atmospheric flight is another matter.
Certainly a hydrogen powered vehicle isn't moving too fast down there.
The center of pressure for a simple symmetric SSTO rocket is pretty much
dead center, so I'll write a program to more accurately simulate it all.

On the other hand, simply upgrading to large shuttle style SRBs makes
the attachment points come out ok again, with the oxygen up on top.

--
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"Pat Flannery" wrote in message
...
Without staging, one of the main arguments for a seacoast launch site
vanishes,

No, you still want it near a coast, so it doesn't accidentally fall on
someone important.

That's why the spaceport in NM is fine- the worst that would happen is it
falls on Texas.

Pat Flannery wrote:

:
:
:Fred J. McCall wrote:
:
: Actually, no. You still have all the usual downrange range safety
: issues. This is why launches over water are preferred. If something
: goes wrong, you're less likely to hit something you don't own.
:
:
:I say Polar launches take off from North Dakota and fly over Canada,
:then if something goes wrong it won't fall on something _we_ own. :-(

Not only that, but we could claim that they shot it down and invade.
It's all about oil, you know...

BLAME CANADA!!!


[:-), for the humour impaired.]


--
"Adrenaline is like exercise, but without the excessive gym fees."
-- Professor Walsh, "Buffy the Vampire Slayer"

In article ,
Pat Flannery wrote:
We've read up on your "Brown Bess" booster concept; if you were going to
make an unmanned expendable SSTO, how would you go about it, and what
propellant combo would you use?

First, as Richard observed, I'd drop the "unmanned". If it's reliable
enough for expensive cargo, it's reliable enough for people. Like, for
example, me. :-)

(There are people who suggest building relatively unreliable rockets to be
used for bulk cargo -- water, fuel, etc. -- only. I don't think this
actually works out well. You still need moderately good reliability, say,
80-90%, if only to avoid being fined for littering :-). I don't see a
significant cost or complexity advantage to be had from the difference
between that and the 98-99% of conventional expendables. If you can
dependably get 80-90%, it should cost very little extra to hit 98-99%.)

(Getting to 99.9% is harder, as witness the fact that no existing
expendable has definitely achieved it -- there are a few uncertain cases
where moderate production runs simply had no failures -- with the
*possible* exception of the Soviet-era Soyuz launcher. It should be
feasible, given careful design, a high flight rate, and automated
production. Even 99.99% is probably not out of reach for expendables,
if you sweat hard on things like systematic process improvement. Beyond
that is strictly reusable territory.)

The real major dividing line is reusable vs. expendable. Here, by
definition, we're talking expendable.

After that is the big question of whether whoever's paying for it has
constraints to impose: use existing engines, no Russian subsystems, a
minimum payload size, etc. They also might have opportunities to offer,
e.g. use of shuttle-ET production facilities. Many of these things can
severely constrain the design. Assume none of this.

The major subsystem question is engines: buy or build? Buying means you
don't have to get into the engine-development business, which saves a lot
of trouble and may look less risky to potential investors. There are some
downsides: (a) it's a lost dimension of competitive advantage, (b) the
choice of existing engines is somewhat limited and can severely constrain
design choices (in particular, ruling out many unconventional approaches),
and (c) buying engines tends to be expensive and to involve a lot of
hassles. I'd favor build, if only to relax design constraints.

The major specs issue is, how much payload to what orbit? Orbital
inclination affects delta-V requirement by determining how much help you
get from Earth's spin. The big question for orbital altitude is whether
the orbit is low enough for a direct-ascent trajectory -- continuous burn
all the way up, like Gemini or Apollo -- or requires a Hohmann ascent like
the shuttle, injecting into an elliptical orbit and then doing a final
insertion burn at apogee. Hohmann ascent would always be more efficient
if the atmosphere didn't get in the way. In real life, direct ascent
usually incurs little penalty up to 300-400km, but gets rapidly worse
thereafter. The nice thing about direct ascent is no engine restart.
And even with Hohmann ascent, you pay a price for higher orbits. Absent
outside constraints (e.g. cargo delivery to ISS), I'd favor direct ascent
to 250-300km, high enough to last a little while and give the payload time
to maneuver higher or be picked up by a tug.

As for how much payload... depends on whether there's a specific mission
constraint. If not, I would favor relatively small payloads, giving a
small launcher and frequent flights, and relying on orbital infrastructure
(assembly base, tug, fuel depot) to assemble larger systems. Smallness
actually is not that important -- launcher cost scales much more strongly
with complexity, thinness of margins, and closeness to the leading edge of
technology than with sheer size -- but frequent flights are beneficial in
many ways.

*How* small depends on how much inconvenience you're willing to accept.
There are cutoff points where inconvenience rises sharply because you can
no longer launch particular objects in one piece, plus a general slow rise
in inconvenience as orbital assembly operations multiply.

If you want at least the option of launching people, that obviously sets a
minimum size. For serious orbital operations, I see a high payoff for
being able to launch a two-man ferry spacecraft, sort of a stripped-down
Gemini, in one piece: it lets you have one pilot and one passenger, so
the passenger doesn't need exhaustive training in emergency procedures for
the ferry. Gemini weighed a bit under 4t, with early-1960s technology and
greater capabilities than the ferry really needs. An aggressive modern
design could come in quite a bit lighter.

For serious orbital operations, the other thing that it would be nice to
launch in one piece is a minimal habitation module. Perhaps inflatable...
but with an expendable SSTO, a "wet workshop" approach using the spent
stages is also very attractive. Say: Launch #1 carries a life-support
module with consumables, integrated with the spent stage and with a
docking hatch at the top. Launch #2 carries a multi-hatch docking node
integrated with the spent stage, and a tug; the tug maneuvers it to mate
with #1, and sticks around to supply attitude control and reboost. A
ferry docks with one of the ports on the node, and you're in the
space-station business. (Actually berthing would be better than docking,
but that's a detail.) How much does each load have to weigh? That would
need more study, but it's interesting to note that the Apollo-Soyuz
Docking Module was about 2t.

Could this sort of scenario be done with payloads of 2t or less?
Probably, but it might get pretty tight. 5t should be lots. Let's be
mildly aggressive and set the payload at 3t. I'd want to look into
infrastructure issues -- size of manufacturing machinery, size of
facilities, etc. -- and if it didn't look like a slightly bigger launcher
would cross any boundaries that made things significantly harder or more
expensive, make it bigger just on general principles. Materials etc. cost
very little; the infrastructure issues are the main things that make a
launcher cost more just because it's bigger, and they mostly rise in
sudden jumps, not in a steady slope. And far more people have regretted
making a launcher a bit too small than have ever regretted making it
slightly too big.

Anyway, let's cut to the chase -- this has already taken rather longer
than I meant to spend on it :-) -- and look at the launcher. This is
based on some past thought but without rigorous calculation for this
particular design problem.

Shape: a plain cylinder with a cone on top, or possibly a two-slope cone
like the nosecone for Apollo 5 (which has lower drag and more usable
volume) -- simple to make, simple to analyze. More the proportions of
say, a Jupiter than a Delta -- the shorter, fatter shape has a bit more
drag but is a lot stiffer and less prone to bending problems. Very light
tanks, probably pressure-stiffened like the old Atlas. Likewise for the
nose -- that was done on Atlas for SCORE and some other flights. (Here
the nose stays on until reaching orbit, after which it hinges up and over
to expose the payload, staying on the rocket so it goes back down when the
rocket deorbits itself.) Either aluminum alloy or composite -- that would
need more investigation. Composites are stronger and lighter, but more
hassle to make, and there might be minimum-gauge issues with such light
sheets, and composite LOX tanks are still iffy.

Pressurization in the tanks is just enough for structural purposes, i.e.
not very much. Boost pumps at the bottom of the tanks, or possibly the
bottom of the feed lines, add enough pressure to prevent cavitation in
the main pumps. (This approach is out of fashion but it has been done
successfully in the past; it avoids having to make the tanks stronger and
heavier to permit higher pressures.) 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.)

The oxidizer is LOX -- cheap and dense. The fuel is probably propane --
slightly better performance than kerosene, less tendency to leave oily
residues and otherwise misbehave, and it's still liquid and quite dense at
LOX temperatures.

Finally, for engines, I'm partial to the idea of an aerospike with a ring
of small individual chambers. The small chambers help keep the scale of
most engine-development facilities down. The aerospike provides altitude
compensation and also permits a light, compact nozzle with a very high
expansion ratio in vacuum. Expander or gas-generator cycle, preferably
the former if enough heat can be had. (It's been done with propane.)
Post-separation attitude control with propane cold-gas thrusters, and
deorbit by dumping residual propellants through the engines.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article ,
Pat Flannery wrote:
...Without staging, one of the main arguments for a seacoast
launch site vanishes, and you now have all sorts of options open to you
as to where you want your launchpad at.

You'd probably still have to launch over water for development launches,
although once reliability is established, an inland launch site becomes a
possibility.

You could make it a manned launcher, but NASA would try to get their
hands on it, and it would end up being dragged down to KSC.

NASA really doesn't have any say in the matter any more. It's the FAA
that you have to keep happy. In fact, rather than snapping it up, NASA
will do its best to ignore it and/or cast doubt on the whole idea -- after
all, if it succeeded, it would make them look incompetent.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

On 11 Mar, 06:57, (Henry Spencer) wrote:
In article ,
Pat Flannery wrote:

We've read up on your "Brown Bess" booster concept; if you were going to
make an unmanned expendable SSTO, how would you go about it, and what
propellant combo would you use?

First, as Richard observed, I'd drop the "unmanned". If it's reliable
enough for expensive cargo, it's reliable enough for people. Like, for
example, me. :-)

(There are people who suggest building relatively unreliable rockets to be
used for bulk cargo -- water, fuel, etc. -- only. I don't think this
actually works out well. You still need moderately good reliability, say,
80-90%, if only to avoid being fined for littering :-). I don't see a
significant cost or complexity advantage to be had from the difference
between that and the 98-99% of conventional expendables. If you can
dependably get 80-90%, it should cost very little extra to hit 98-99%.)

A slight modification: Allow untried rockets for fuel, water etc.

For example, Atlas and Delta could compete for the valuable cargos (~6
launches). Those two plus Falcon could compete for the fuels (18
cargos). Next time round, all three can compete on both contracts.
Another new entrant could compete on the fuel launches.

I would actually say, rather than fuel, launch "Earth Departure
Stages" with LOx / Kerosene propellant. These would bolt / dock
together to launch more expensive cargoes. Then NASA doesn't need to
do orbital propellant transfer, which they believe to be impossible.

In article ,
Jorge R. Frank wrote:
So a dense-propellant SSTO doesn't really need less delta-V to reach orbit,
but you use a smaller delta-V term when modelling one using the rocket
equation.

Depends on whether you think of delta-V as the actual change in velocity,
or as the change the vehicle could achieve in ideal conditions -- that is,
a measure of vehicle performance or required vehicle performance.

The latter is often the more useful view. Even in a fairly idealized
situation, not all of the vehicle-performance delta-V actually shows up
as final orbital velocity, because some of it has to go to achieve the
necessary altitude.

So I would phrase it a bit differently. The dense-propellant SSTO has to
achieve the same orbital velocity, but its gravity losses are lower (as
are its drag losses, although that's less important), so the total delta-V
the vehicle must deliver (equal to the velocity it could achieve in
drag-free gravity-free space) is lower.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Rand Simberg wrote:
And at worst, it kills a moose,

My sister was once bitten by a SSTO vehicle. ;-)

or a polar bear (whose numbers are
apparently increasing).

http://www.telegraph.co.uk/news/main...9/wpolar09.xml

Per the headline, I blame global warming.


What will make this interesting is that if the polar ice all goes away,
the polar bear may re-evolve brown coloration over a period of many
generations, as its white coloration will be a net deficit to
camouflaging it in its environment.

Pat

Henry Spencer wrote:
And far more people have regretted
making a launcher a bit too small than have ever regretted making it
slightly too big.


That's certainly true, in fact if you make it 3 tons, in fairly short
order you will find most of your commercial payloads will weigh 3 tons,
as your customers size their payloads to lift capability.
Next, they will add a second stage. :-D


The oxidizer is LOX -- cheap and dense. The fuel is probably propane --
slightly better performance than kerosene, less tendency to leave oily
residues and otherwise misbehave, and it's still liquid and quite dense at
LOX temperatures.


Hank Hill fully agrees with this fuel choice. :-)
There was some speculation that the "Aurora" (or whatever the thing was
if it existed) might have used propane due to its greater density than
LH2 to cut down propellant tank size.
Considering that I've seen propane cylinders for refueling lighters that
have very thin aluminum walls, is it even necessary to chill it?

Pat

Pat Flannery wrote:

:
:
:Rand Simberg wrote:
: And at worst, it kills a moose,
:
:My sister was once bitten by a SSTO vehicle. ;-)

I hear SSTO vehicle bites are nasti...