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The 100/10/1 Rule.



 
 
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  #191  
Old March 19th 07, 06:11 PM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
Craig Fink
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Posts: 1,858
Default fun with expendable SSTOs (was The 100/10/1 Rule.)

john hare wrote:

"Henry Spencer" wrote in message
...
In article . net,
Craig Fink wrote:

...I bring up "Fluid
Variable Intakes" which fits nicely with his engine and invalidating
certain assumptions of his trade study. His study just swung in a
different
direction.


I don't believe you explained what "Fluid Variable Intakes" are. *It's
not a term I'm familiar with either, although I don't keep up with the
current fads :-) on the airbreathing side.


I don't keep up with the fads to much either. To be honest, I just made the
term up. I thought it best described what it was, a "Fluid Variable
Intake", sounds good to me. If it's not really a physical cooler (cooling
really just a bonus) and not really a physical inlet, and it's variable,
what is it? The concept, I made that up too. It also sounded good to me, it
would essentially let a slightly modified jet aircraft fly straight up exit
the atmosphere at Mach 5, winning the X-Prize. But, like I said, it's hard
to find a good used engine/supersonic "non-government developed" aircraft
these days.

If you have a better name, or know of a different name that it goes by, let
me know. It would be fun reading about it.

It would have needed negative mass to counteract the engine weight
to a degree necessary to match pure rocket stage performance.
I lost interest somewhat in supersonic intakes when it became
clear that there could not be a cost advantage to them.


Someone's selling tickets for short zero gee trips in a modified commercial
airliner for probably $1000, someone else is selling tickets in supersonic
aircraft for $10,000-$20,000, and someone else will soon be selling long
zero gee trips in custom designed rocket planes for $200,000. There is a
whole lot of room between $1000-$20,000 and $200,000 to sell tickets for
long zero gee trips in turbojet/rocket powered aircraft.
  #192  
Old March 19th 07, 09:30 PM posted to sci.space.history,sci.space.policy
john hare
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)


"Craig Fink" wrote in message
hlink.net...
john hare wrote:
Someone's selling tickets for short zero gee trips in a modified
commercial
airliner for probably $1000, someone else is selling tickets in supersonic
aircraft for $10,000-$20,000, and someone else will soon be selling long
zero gee trips in custom designed rocket planes for $200,000. There is a
whole lot of room between $1000-$20,000 and $200,000 to sell tickets for
long zero gee trips in turbojet/rocket powered aircraft.


That is the basic business plan. Except that the short trips you mention
are ~$4,000.00. Airbreathing to safe corridor, rocket boost to mach?,
jump to dark sky and available zero gee, airbreathing back to base.
Two seats with dual controls. Note that 25% rocket fuel can get
a couple of mach numbers for the thrill.

Not that I am currently working on it physically. Time and money
shortages. Same general concept as I tried in 2001.



  #193  
Old March 20th 07, 02:54 AM posted to sci.space.history
Paul F. Dietz
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)

Craig Fink wrote:

From your posting, you still don't get it yet.


We get it, Crai You're asking them to prove a negative,
and rejecting any and all evidence they present.

Frankly, given the demonstrated failure of airbreathing
to make sense in study after study, the idea that it
does make sense has become sufficiently extraordinary
that the onus of proof is on the proponent, not the skeptic.

Paul
  #194  
Old March 20th 07, 08:36 AM posted to sci.space.history
Pat Flannery
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)



Paul F. Dietz wrote:

Frankly, given the demonstrated failure of airbreathing
to make sense in study after study, the idea that it
does make sense has become sufficiently extraordinary
that the onus of proof is on the proponent, not the skeptic.


Instead of dragging the airbreathing engines all the way to orbit, let's
use them as boosters and then jettison them.
I think that may be the idea behind this concept:
http://www.collectspace.com/images/s.../x15model.html
Although this one looks sub-orbital.
Gnom certainly used that tecnique: http://www.astronautix.com/lvs/gnom.htm
With ramjets, as opposed to scramjets you should be able get to around
Mach 5 or 6 prior to engine jettison.
Ramjets can be made cheaply enough to be disposable also.

Pat
  #195  
Old March 20th 07, 01:15 PM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
Craig Fink
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Posts: 1,858
Default Fluid Variable Intakes ( fun with expendable SSTOs ...)

Fluid Variable Intakes

For Acceleration type mission profiles with Turbojet, Ramjet, and Scramjet.
Fluid Variable Intakes fix many of the difficult problems associated with
these engines, making it possible to use a single fixed inlets or a much
less complex inlet. As the aircraft passes the maximum design Mach Number
for the inlet, a fluid is used to compensate for the changing area ratio
between the beginning of the inlet and the sonic throat. Cryogenic Oxygen /
Gaseous Oxygen being the fluid, also enriches the air taken in by the
engine with Oxygen, enhancing the performance when it is needed to continue
to accelerating.

It reduces the complexity of the associated inlet hardware, replace complex
mechanical moving aerodynamic surfaces with a fluid distribution system.
It's also possible to preheat the Cryogenic Oxygen to gasify it at a
relatively high pressure to dynamically bring throat to sonic conditions,
reducing the Oxygen flow rate. This allows the Oxygen flow rate to be
regulated to maintain engine operating conditions, statically added mass
(dumping) vs dynamically added mass (thrusting) Oxygen in the inlet.

It cools the air, radically reducing compressor inlet temperatures.

Since Oxygen flow rate is related to Mach number, performance is maintained
as the aircraft continues to climb and accelerate. Essentially, it slowly
converts the engine from an airbreather into a rocket smoothly varying the
performance during acceleration from an Specific Fuel Consumption (ISP in
rocket terms) greater than 2000 down to something less than 500 at the end
of the acceleration.

Normally for turbojets the added Oxygen would be a problem for turbine inlet
temperatures. But this can be compensated for by changing the mixture ratio
in the combustion chamber. Switching from a fuel lean mixture ratio
combustion usually used for endurance/range type missions, to a fuel rich
mixture. Essentially going over the high temperature hump, to an extremely
fuel rich condition. This also adds mass flow through the turbine, spinning
the compressor and turbine up.

The fuel rich turbine exhaust can then be mixed with the bypass air as an
afterburner. The bypass air also enriched with Oxygen. Hydrogen, with
double the energy per weight of any other fuel would probably be the best
fuel for such an acceleration type mission. It also would simplify
combustion, ensuring complete combustion.

Fluid Variable Intakes smoothly transition an airbreathing engine into a
rocket engine and allows an aircraft continue accelerating quite a bit
past what would normally be considered it's maximum speed.

Probably not very useful for endurance/range type mission, but extremely
useful for acceleration type mission, and going to Orbit in the Atmosphere.

--
Craig Fink
Courtesy E-Mail Welcome @
--

Henry Spencer wrote:

I don't believe you explained what "Fluid Variable Intakes" are. *It's not
a term I'm familiar with either,



  #196  
Old March 21st 07, 01:11 PM posted to sci.space.policy,sci.space.shuttle,sci.space.history,sci.space.station
Craig Fink
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Posts: 1,858
Default Fluid Variable Intakes ( fun with expendable SSTOs ...)

Hi Derek,

I have a choice, how to respond to your post. The response can be negative
or positive. Which shall it be. Hummm, lets go with positive.

From my point of view: ;-) Interesting how in these threads, Henry invokes
religion as an argument for non-Atmospheric Flight to Orbit. The
connotation of "religion" negative and with a negative rate of change right
now. Then another, and another, till the choirs joins in. You should really
check before chiming in about God and another Man's religious belief. If
you must know, I'm Christian, of the variety that believes Physics is
simply Man's description of God's work. Math being the language that it's
spoken in. To understand Physics is to understand God's work, and therefore
it is the duty of Man to try the best he can to relieve himself of his own
ignorance.

Ignorance, also has a bad connotation, when it really shouldn't. For we are
all born ignorant, not knowing, and begin our own personal journey to
relieve ourselves of our own ignorance. No one can do this for us.

If there is something you don't understand (ignorance) about the concept of
Fluid Variable Intakes, just ask. Your more likely to get a response if you
ask nicely, positive than negative.

Jeff Findley, is just making comments about one example of an application of
Fluid Variable Intakes, not all applications. His comments are for the most
part true. In an application with compressors and turbines, a complicated
device that required a large effort to perfect, it's most likely true. Is
it more, or less. I don't know.

His comments about the lack of papers is most likely true, as Henry and John
have both made similar comments. But, not surprising, all new concepts
start out this way. Maybe Jeff, if he has the knowledge and background,
might write one of the dozens of AIAA papers to come. That is, if it truely
is a good concept/technique.

Looking at a much simpler application that would require much less work than
incorporating it with compressors and turbines would be a ramjet. A much
simpler engine, pure fluid manipulation, just adding a little bit more
fluid manipulation. Anywhere from trivial to massive, depending on the
approach and who's doing it.

Surely you've clicked and gone here by now
http://www.grc.nasa.gov/WWW/BGH/shorth.html
here,
http://exploration.grc.nasa.gov/educ...et/shortr.html
and here,
http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html
Looking for Ramjets, problems with, and when the region of usefulness ends
and why. Ah, here;
http://www.grc.nasa.gov/WWW/K-12/airplane/lowhyper.html
.... For aircraft speeds which are much greater than the speed of sound, the
aircraft is said to be hypersonic. Typical speeds for hypersonic aircraft
are greater than 3000 mph and Mach number M greater than five, M 5. We
are going to define a high hypersonic regime at M 10 to account for
re-entry aerodynamics. The chief characteristic of hypersonic aerodynamics
is that the temperature of the flow is so great that the chemistry of the
diatomic molecules of the air must be considered. At low hypersonic speeds,
the molecular bonds vibrate, which changes the magnitude of the forces
generated by the air on the aircraft. At high hypersonic speeds, the
molecules break apart ... Because of the pressure losses associated with
the terminal shock of the inlet, a ramjet has very limited performance
beyond Mach 5 ...

High temperatures, vibrating bonds of diatomic molecules breaking apart.
Ramjets are limited to Mach 5. So this is the point where the first ounce
of LOX needs to be added, to go past Mach 5 with a ramjet. Super-cold
liquid Oxygen, a huge heat sink, better use it wisely and add just enough
to keep the vibrating bonds from vibrating too much or breaking apart. Add
a little more Mdot (Mass flow rate) at Mach 6, even more Mdot at Mach 7,
quite a bit more at Mach 10, all to keep vibrating bonds of diatomic
molecules from vibrating too much and/or breaking apart. Extending the
useful range of a simple ramjet well past Mach 5 and probably past Mach 10.
Just how far? Sounds like a fun Trade Study.

Acceleration mission requirements are significantly different than
Range/Endurance mission requirements, which can lead to a significantly
different solution. Almost all in the atmospheric hypersonic flight studies
and research has been for Range/Endurance mission type profile. Even the
X-43A hypersonic research aircraft, they were very proud to have used only
fuel.

Adding the Oxidizer into the equations adds a whole new dimension that is
well suited for the Acceleration mission profile for many many reasons,
making Atmospheric Flight to Orbit not only possible, but with superior
performance over exo-Atmospheric Ascent.

The fun part to me, is the challenge of how to fit all the pieces of the
puzzle to do this.

--
Craig Fink
Courtesy E-Mail Welcome @
--

Derek Lyons wrote:

"Jeff Findley" wrote:

This isn't the sort of thing you can do with out *a lot* of detailed
aerodynamic, combustion, and thermal analysis. Not to mention how much
wind
tunnel time this sort of research would consume. This is the sort of
thing that can easily burn a billion dollars worth of research grants and
never
fly real hardware. Remember NASP?


Yes, I remember NASP. I worked on the NASP program and developed a closed
loop first stage guidance for the Space Shuttle. Really neat algorithm :-)
What a closed loop first stage guidance for the Space Shuttle and NASP have
in common, ????, your going to have to ask a manager about the convoluted
logic. I was young, and really just happy to have the opportunity to
develop it. The algorithm I named BIG, which used a small (2 pages of code)
3-dof trajectory simulation called SIMP, written by a coworker.

And NASP was easier than an
accelerator since it would have been optimized for supersonic cruise.


Not only that, but it takes the sole 'advantage' of an airbreather
(that one need not pack along the excess mass of oxidizer) - and
tosses it into the crapper.

So, where's the papers? Certainly someone who's done the work has
produced
a few dozen AIAA papers, right? I'm sure we'd all be interested in the
detailed analysis. Until then, liquid fueled rocket engines are the
preferred off the shelf solution to getting to orbit.


Such a stunning triumph of religious fervor needs not such mundane
things as actual proof.

D.



  #197  
Old March 26th 07, 03:42 AM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
Mr Jim
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)

"Henry Spencer" wrote in message
...

... 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? From my readings it appears the classic Atlas
folks had to throw plenty of $ at the problems to solve them (granted they
had completely different requirements, what with the operational USAF silo
basing, for the missles). Von Braun's folks reportedly hated the
pressure-stabilized approach, apparently judging it just couldn't fit with
their culture of protracted stacked checkout. It seems the latest Ariane
model requires pressure stabilization in the cyrogenic stage's LH2 tank even
during handling, which I thought was pretty interesting. Would you chose
steel, like Atlas, or Aluminum alloy?

... 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?

... 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? I've
used those for dewatering bilges and compartments on naval vessels and it's
pretty cool how fast such a simple rig can pump water. Is it bad to have yet
more high-velocity/high-pressure fluid piping running between pairs of
pumps? Seems like asking for trouble, what with all the vibration and flow
you're already having to cope with...

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.


Henry, do aerospike engines have to be carefully integrated with the
particular airframe? If so, is that a problem? Would such an engine use
differential throttling to control pitch and yaw? How about roll control
during boost? Wouldn't your engine layout lend itself to a tripropellant
arrangement? That is, set up some chambers to burn LOX/LH2, start them at
the appropriate time (ground?) and draw from an LH2 tank up on top of the
stack (sorry to stretch out your nice high-bending resistance airframe and
weigh it down w/vacumn-jacketed LH2 lines...). The propane tank would be
smaller, but the LOX tank might have to get bigger. Shut down the LOX/Prop
chambers when the Propane's gone and press on to orbit. I know you don't
like that fluffy LH2 but I couldn't resist ... :-D

J


  #198  
Old March 27th 07, 07:25 PM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
Henry Spencer
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)

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.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
  #199  
Old March 28th 07, 03:25 AM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
Fred J. McCall
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Posts: 5,736
Default fun with expendable SSTOs (was The 100/10/1 Rule.)

(Henry Spencer) wrote:

:In article ,
:Mr Jim wrote:
:
: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).

I (vaguely) recall someone (name escapes me) on the newsgroup some
years ago bringing up the idea that steel might actually be cheaper
and wouldn't add that much weight. Some of the savings come from the
steel being easier to work with. He seemed to have a good case. Has
this changed?

:
: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.

Injection vectoring was the first thing that occurred to me here.

--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
  #200  
Old April 1st 07, 05:18 PM posted to sci.space.history,sci.space.policy,sci.space.station,sci.space.shuttle
kT
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Default fun with expendable SSTOs (was The 100/10/1 Rule.)

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%.)

(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.


This is where hydrogen shines over the 'lesser fuels'.

With the lesser fuels, you just barely make it to orbit, and any fuel
you do have left over, you waste to deorbit the booster to then burn up
in the atmosphere, which is nearly 90% of your usable payload mass,
already delivered to 100 percent of orbital velocity. That's just nuts.

With hydrogen, you get there, and then some, with plenty to spare.

Lesser fuels make the hydrogen 100/1 rule look good.

--
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