Mojave airport is not a spaceport

John Carmack wrote:
Look at:

http://media.armadilloaerospace.com/...BoostedHop.mpg

An actual pressure fed rocket with almost no moving parts doing a
powered landing. It is my considered opinion that this is The Right
Way To Do It. Build a big, simple booster that lofts a high
performance upper stage all the way out of the atmosphere, then
returns to land on the same pad it took off from. At first flance it
sounds like an inefficient staging strategy, since the upper stage
requires nearly SSTO dV, but removing the requirment of boosting
through the atmosphere (optimize only for vaccuum boost and reentry)
does still simplify the problem quite a bit, and the operational and
testing aspects are great.

Not an especially bad idea. Of especially consideration
are, on the plus side, greatly eased aerodynamic
considerations on the launch vehicle as well as perfomance
improvement on of the near-SSTO main engine in vacuum.
On the minus side though there are large gravity losses,
vehicle sizing issues, and characteristic minimum boost
times to deal with. Which make up for some of the
disadvantages. At the minimum it's a workable interim
solution that makes progression toward lean, mean, SSTO
launching machines slightly easier. I guess this would be
a "bigger dumber stage" concept.

Let me expand on some of the issues here. First is
aerodynamics, modern launch vehicles travel at supersonic
and even hypersonic speeds during launch. This places
constraints on payloads and especially payload packaging,
which can steal from raw payload capability by requiring
strong payload attachments and aerodynamic fairings, which
take up mass. This also brings into play a great deal of
vibrational energy, which the payload has to survive once
and only once to get to orbit but never after. Which
requires designing and testing the spacecraft carefully
to make sure it will survive a launch, things that usually
do not come free, or even cheap. However, a "pop-up and
boost" launch vehicle would have a much different flight
profile. Theoretically the portion of the pop-up flight
within the atmosphere could be at rather unimpressive
speeds, such as low mach numbers, or even sub-sonic. At
mach 1 it takes only a few minutes to climb out of the bulk
of the atmosphere. Raw gee forces will almost necessarily
still be a concern, as the pop-up booster still has to
avoid throwing away all its fuel to gravity and the near
SSTO still has to climb into orbit before reentering.
Nevertheless, steady gee forces are much, much easier to
deal with mechanically than vibrations and lurches, and
with liquid propulsion systems on all stages it's possible
for a pop-up booster to give satellites or other cargo an
incredibly gentle ride. This is no small issue, because
engineering fragile spacecraft to be able to survive the
rigors of launch is a huge cost sink, this could
potentially make low-cost satellites much more feasible.

Working only in low pressure or vacuum can provide
substantial performance improvements for a rocket engine.
Roughly about a 10% improvement in Isp for LOX/Kero
engines and up to a 25% improvement in Isp for LOX/LH2
engines. For Kerosene engines, this is enough of an
improvement to bring the delta V/Isp side of the equation
into overlap with the achievable dry mass fraction side,
with a bit of margin as well. Additionally, depending on
how much time the pop-up booster gives the launcher to get
into orbit the necessary thrust to weight ratios could be
lowered as well, allowing for lower engine weight and
easing some throttling issues (if you start off with a
thrust/weight of less than 1 gee then the inevitable
problems of high gees or very deep throttling in a LOX/Kero
booster near burnout become much less of a concern).

With those two factors added together, if we *had* a bigger,
dumber, pop-up booster now we could quite easily put SSTO
capable vehicles on them using very prosaic engineering
(60s vintage). With a few tweaks (such as modern
electronics, more finely honed structures, modern alloys,
and composite structures) we could almost certainly build in
enough margin to add the bits that would make it reusible
(like TPS).

The downsides are worth considering though. First, gravity
losses are going to be big, and pretty much all the
propellant used by the pop-up booster will at least appear
to be entirely wasted in terms of getting to orbit. The
good news is that this is a propellant issue, and propellant
is dirt cheap compared to orbital launch costs, so it's a
non-issue with respect to operations today, but won't be
competitive in the long term when ground launched SSTOs
can operate at low multiples of propellant cost (may we some
day be blessed with such a "problem"). Second, the real
concern is the sheer size of the booster. SSTOs are almost
inevitably rather bulky with high GLOWs. The pop-up booster
has to be big enough to take that beast a couple hundred km
straight up. And that means it has to be absolutely
gargantuan. The thrust on it ought to be quite impressive.
The good news though is that it can be made very low tech,
and metal, or even concrete, and propellants are cheap.


As a concept design, imagine something along the lines of
a Saturn-V first stage with a fixed payload bay and crew
compartment(s) above the tanks, as well as TPS and landing
systems and such like here and there, with slightly fewer,
lower thrust engines. Now imagine this connected to the
mother of all brick sh*t-houses with a small horde of the
most powerful engines in the world on the underside. I'd
imagine the pop-up booster on this beast would have a
"payload bay / shroud" more similar to an aircraft hangar
than anything else. It would be impressive at the very
least.

Earl Colby Pottinger wrote in message ...
(Vincent Cate) :

Idling the engines for most of the way down would mean
you don't have as much trouble with reentry heat, since you
are sort of doing transpiration. Yet you would not need
a huge amount of fuel, since really it was still drag doing
the work on the way down (and your total mass is much less).

Important point, once the engines are warmed up Armadillo seem to have no
problem with restarts. So in coming down most of the trip can be done in
free fall and the engines only fired up once they are close to earth. This
saves on fuel needed.

True. You might choose to run the engines during peak heating just
to reduce the amount of heat that gets to the vehicle.

-- Vince

(Vincent Cate) wrote in message . com...
On the way down your tanks are closer to
empty, so the fixed mass of the engines would shift the
center of mass lower.

For a suborbital passenger vehicle you could put the people
just above the engines and below the tanks so that their
fixed mass was also low. This is not an issue for a
the booster lifting another stage that goes on to orbit.

-- Vince

John Carmack wrote:

JC Why do you keep bringing up wings? Most of the people on this thread
JC are partial to VTVL.

JC A BDB can do a powered landing with little more than software changes
JC and the addition of landing struts (you could even avoid that if you
JC are REALLY confident in your terminal positioning and land on a
JC special ground structure, but I don't reccomend that). Yes, you need
JC to cart some more propellant around, but these are big, easy to
JC fabricate vehicles that can be "made in shipyards", right? Adding
JC some size doesn't cost much, and the operational win would be dramatic
JC compared to a splashdown recovery.

Yes, VTVL (vertical takeoff and vertical landing on dry land without
parachute and without wings) is the cheapest option and probably the
best one if the rocket is sturdy enough to survive hard landing
without catastrophic damage. (Russian landing retrorockets malfunctioned
several times.)

JC The major point of contention is splashdown versus a powered landing.

This issue got more coverage in this thread than it deserves.
Reusability is much more important.

JC We have done some work with big parachutes, and I'm not a fan. Taking
JC a boat out to fish your rocket out of the ocean is going to suck.
JC Landing the booster is feasible, economical, and gives the best
JC operational characteristics, as long as you are willing to accept some
JC limitations on stage trajectory and aspect ratio. I contend that
JC these are worthwhile tradeoffs.

We could go on for a long time if you enjoy this topic.

AN ...The best design for the second stage is my engine cluster... Its
AN description is posted at:
AN http://www.islandone.org/LEOBiblio/S...engine_cluster

JC Qantity and replication are easy in a spreadsheet or CAD program. It
JC is a little more troublesome in the real world. We recently made out
JC lives much, much better by moving from four differentially throttled
JC engines to a single larger engine with jet vanes. At some point mass
JC production effects can kick in, but it isn't in the development stage.

There is huge difference between making the rocket by hand and
making it by a robot.

Before the Civil War guns were made by gunsmiths. Their parts
were not interchangeable because the gunsmiths could not make
identical parts by hand. Making the rockets by hand takes lots
of time, and quality control is difficult -- you have to inspect
every weld because one bad weld can ruin your rocket.

Most of the Agena rocket engine was made by a robot -- coolant
passages were drilled in a monolithic slab of aluminum alloy.
My engine cluster has similar design -- it can be made by a
milling robot. The robot is cheaper than the rocket plumber,
and the engine is so sturdy that it may survive the hard VTVL
landing. You do not have to worry about weak welds -- there are
very few of them. Fabrication quality is determined by your CAD
drawing rather than by the robot. Even if you make just one
rocket, it is cheaper to make it by the milling robot than by
hand. I do not know if standard robots can make the narrow
injection nozzle holes, but they can certainly make all the
other holes.

JC You can't design a high aspect ratio vehicle, but again, it doesn't
JC really matter for a booster stage. Make it squat, and let the upper
JC stage be a sphere if it wants to. Go ahead and be highly non-optimal
JC in the aerodynamics and staging fraction if it gets you good
JC operability. A booster like this would be a cargo elevator to 100km
JC or so. Up and down on the hour if you wanted to.

Can you comment on stacking the engines sideways?
(http://www.islandone.org/LEOBiblio/SPBI1010.JPG)
I believe that having lots of engines covering
large area of the rocket is a good idea because
it improves thrust, specific impulse, or both.

JC Most of the fundamental complexity of a rocket stage is independent of
JC stage performance. Lots more stages will give lots more problems.
JC Pushing performance requirements to the edge can easily give even more
JC problems, which is why I'm not an advocate of a completely SSTO
JC design, but two stages is going to be both more reliable and easier to
JC develop and test than more stages.

I have seen lots of comments about problems caused by large
number of stages, but I do not understand these problems.
It seems to me that when you stack identical rocket stages
like Lego blocks, your only problem is designing the
explosive bolts which hold the stages together.

JC An upper stage from us would probably use 98% peroxide and kerosene.

Other good choices are H2O2/RP-1 and H2O2/propylene.
They are easy to store, but, to the best of my knowledge,
their critical pressures and critical temperatures are high.
If the coolant pressure is lower than its critical pressure,
bubbles may form in the coolant.

If you use oxygen/methane instead of H2O2/RP-1, the payload
mass will increase by about 50% and it will be easier to
cool the engine. Methane can be extracted from natural gas,
so it is cheap and easily available. Propellant tanks holding
liquid oxygen and liquid methane are covered with natural
thermal insulation in the form of frost. Oxygen and methane
have similar boiling point temperatures, so there are no
problems with a propellant freezing in a pipe.

JC Probably still pressure fed, but at a tank pressure of only
JC 100 psi or less, which doesn't hurt it much in vacuum operation.

100 psi = 6.9 bars
That is pretty low pressure. (Russian RD-170 engine has the chamber
pressure of 245 bars). Low pressure means high mass ratio, but the
engine must have large exhaust nozzle exit area to produce high
thrust and high specific impulse. Again, the engine cluster looks
like the right choice.

JC A gas-and-go RLV would be a huge advance even if it used TEN TIMES
JC the propellant that a conventional rocket used for a given amount of
JC propellant.

I agree.

"Christopher M. Jones" wrote:

CMJ...However, a "pop-up and boost" launch vehicle would have
CMJ a much different flight profile. Theoretically the
CMJ portion of the pop-up flight within the atmosphere could
CMJ be at rather unimpressive speeds, such as low mach numbers,
CMJ or even sub-sonic. At mach 1 it takes only a few minutes
CMJ to climb out of the bulk of the atmosphere. Raw gee forces
CMJ will almost necessarily still be a concern, as the pop-up
CMJ booster still has to avoid throwing away all its fuel to
CMJ gravity and the near SSTO still has to climb into orbit
CMJ before reentering. Nevertheless, steady gee forces are much,
CMJ much easier to deal with mechanically than vibrations and
CMJ lurches, and with liquid propulsion systems on all stages
CMJ it's possible for a pop-up booster to give satellites or
CMJ other cargo an incredibly gentle ride...

True.

CMJ The pop-up booster has to be big enough to take that
CMJ beast a couple hundred km straight up.

False. When the altitude is increased by 20 kilometers,
atmospheric pressure drops about 10 times. You can
ignore atmospheric drag above the altitude of 40 km.

The precise formula for atmospheric pressure is:
p = B(exp(-MgY/RT)), Whe

p = pressure at elevation Y
B = pressure at elevation zero
exp = natural exponent
M = molecular mass, or mass in kg per mol
(M = 0.0288 kg/mol for dry air)
g = acceleration due to gravity = 9.8 m/s^2
R = gas constant = 8.314 J/(mol*K)
T = absolute temperature (in Kelvins) = about 273 K

(Vincent Cate) wrote in message . com...
Assuming you are not moving any fins or changing shape to
change your center of drag, you would have to do this by
changing your center of mass. You do have a lot more fuel
mass when you are on the way up, so your center of mass
can be higher. On the way down your tanks are closer to
empty, so the fixed mass of the engines would shift the
center of mass lower. If you transition between a high
center of mass and a low center of mass when you are above
most of the atmosphere, things could work out. Is this
what Armadillo is planning?

Our vehicles should be aerodynamically stable in decent, but need to
be actively controlled on ascent. All of our short flights will be
pwoered both up and down, but when we start doing our waivered flights
we will sneak our way into having the engines shut down on descent.
The actual algorithm is that once the timed boost phase completes, the
vehicle adjusts the engine until the vehicle is seeing a given
acceleration. The last test was very conservative, with the minimum
acceleration set at +0.5G, or half a G of decelleration. If we just
shtu the throttle down, propellant wouold fly away from the tank
outlet, and we would lose control. The test flight didn't get going
fast enough to show it, but if the vehicle went high enough,
eventually the aerodynamic drag on the way down would provide the
minimum acceleration and the engine would throttle itself down. It is
our intention to feel this transition out very gently, because we
don't know how bad the vehicle will oscillate under only aerodynamic
loads. We might wind up sticking some DC-X style strakes on the nose
to give it a bit better descent stability.


Idling the engines for most of the way down would mean
you don't have as much trouble with reentry heat, since you
are sort of doing transpiration. Yet you would not need
a huge amount of fuel, since really it was still drag doing
the work on the way down (and your total mass is much less).


Reentry heat for a suborbital vehicle is pretty much a non-issue. The
engines get much hotter while operating than the relatively mild
suborbital reentry heat pulse. The base of the vehicle is just going
to have a layer of RTV over everything.

John Carmack
www.armadilloaerospace.com

"johnhare" wrote in message
...

I may be in disagreement with you about the nearly SSTO
performance requirement. MR for SSTO seems to be about 16
(Lox/Kero) from the ground, and 10 from the vacuum altitude
you deliver to. Going from 6.25% dry mass including payload to
10% dry mass including payload is a major gain in margins. Even
without the mass savings on lighter engine and tank mass
percentage, 37.5% of the upper stage dry mass becomes
available to increase payload. I was convinced several years
ago by Len Cormiers' Space Van booster concepts.

Me too, assisted SSTO where you stage just above the atmosphere such
that your lower stage can still easily return to the launch site, (this
is critical), seems like such an ideal way of doing things. These are
two very different regimes prompting two very different optimal
solutions. Trying to do it all in one SSTO will just result in a
compromised design, which does not really gain you anything.

If there was such a cheap reusable assist stage commercially available,
then even very small orbital vehicles, (less than 100kg), could be
developed at almost the hobbyist level. There would no longer be an
aerodynamically imposed small scale limit on orbital vehicles.
Development of small orbital vehicles could be directly within the means
of individuals, a very low cost Prize on that basis could be real
interesting.

There are a number of different approaches to the assist stage. I have
come to quite dislike the straight aircraft approach, not to deride
Scaled Composites, (they are getting the results), but I suspect it has
cost them 5-10 times as much as it should. This is telling, they used
to be the pin up boys of low cost development, but were they really? I
fear that they opted for the design that they did not because it was the
right one, but because it was the only one they knew how to make.

One of the things that I am greatly impressed with Armadillo over, in
addition to their proper low cost development approach, is their pure
nothing but rocket mentality. This keeps weights and costs low,
especially during development, optional extras that provide greater
efficiency at greater complexity can come later when the market is ready
to pay for it.

In the long term the straight VTVL rocket assist stage will be very fuel
hungry, gravity losses dominate so this probably favours aerodynamic
augmentation. I do still quite like VTVL, speed is useful and the white
knight takes something like an hour to get to height, and not very high
at that. I suspect that we need to go much higher than subsonic air
breathing engines allows. Although I expect the assist stage might
fully aerodynamically shield the upper stage, supersonic aerodynamic
vehicles tend to cost, I am unsure of this approach.

So we have the pure VTVL rocket and I am guessing you are favoring this
type of VTVL vehicle that instead uses your very high T/W air breathing
engine and perhaps goes supersonic, the more I think about this, the
more I like it, no wings. This might almost reach a 100km, can you do
it? I suppose I still favour what is effectively a very refined rocket
powered paraglider. This is probably still the cheapest to develop and
operate in the short term, but it is a bit slow and limited in height.
I will continue to investigate various more refined hybrid type
solutions that might overcome some of these weaknesses, I am less sure
of this than I used to be, assuming you can do it.

I think our goals might be similar with a slight difference in
methods and means :-), not to mention real world experience. I
hope to start closing the gap on the last two real soon now, same
as I did last year, and the year before that....


And me too. :-)

I am making some progress on the tethered wing thing, though initial
commercialization will be elsewhere, (where the money is). I am hoping
that maybe five years from now I will be able to make one for rocket
landing. Hopefully for around 1% landing weight I can build one that
provides a glide rate of around five, if desired, with soft vertical
landing, perhaps without the need for any landing gear. This should be
lighter than any of the alternatives, it could also be easily powered.

Pete.

"Pete Lynn" wrote in message
...
"johnhare" wrote in message
...


If there was such a cheap reusable assist stage commercially available,
then even very small orbital vehicles, (less than 100kg), could be
developed at almost the hobbyist level. There would no longer be an
aerodynamically imposed small scale limit on orbital vehicles.
Development of small orbital vehicles could be directly within the means
of individuals, a very low cost Prize on that basis could be real
interesting.

There are a number of different approaches to the assist stage. I have
come to quite dislike the straight aircraft approach, not to deride
Scaled Composites, (they are getting the results), but I suspect it has
cost them 5-10 times as much as it should. This is telling, they used
to be the pin up boys of low cost development, but were they really? I
fear that they opted for the design that they did not because it was the
right one, but because it was the only one they knew how to make.

I happen to favor the straight aircraft aproach for regulatory reasons.
Most of the possible launch sites within reasonable commute distance
of my house have valuable property within a few miles. Given the
difficulty of proving that you are not going to cause damage, I prefer
the option of flying 20-50 miles from a runway to a salt water launch
position.

One of the things that I am greatly impressed with Armadillo over, in
addition to their proper low cost development approach, is their pure
nothing but rocket mentality. This keeps weights and costs low,
especially during development, optional extras that provide greater
efficiency at greater complexity can come later when the market is ready
to pay for it.

In the long term the straight VTVL rocket assist stage will be very fuel
hungry, gravity losses dominate so this probably favours aerodynamic
augmentation. I do still quite like VTVL, speed is useful and the white
knight takes something like an hour to get to height, and not very high
at that. I suspect that we need to go much higher than subsonic air
breathing engines allows. Although I expect the assist stage might
fully aerodynamically shield the upper stage, supersonic aerodynamic
vehicles tend to cost, I am unsure of this approach.

The pure rocket VTVL outperforms HTHL by a good margin. Air
augmentation is very iffy for VTVL, requiring 30+ to 1 T/W ratios
to compete.

So we have the pure VTVL rocket and I am guessing you are favoring this
type of VTVL vehicle that instead uses your very high T/W air breathing
engine and perhaps goes supersonic, the more I think about this, the
more I like it, no wings. This might almost reach a 100km, can you do
it? I suppose I still favour what is effectively a very refined rocket
powered paraglider. This is probably still the cheapest to develop and
operate in the short term, but it is a bit slow and limited in height.
I will continue to investigate various more refined hybrid type
solutions that might overcome some of these weaknesses, I am less sure
of this than I used to be, assuming you can do it.

My projected high T/W air breathing engine is only useful if the group
has pre decided to use wings and wheels. I grit my teeth and try to work
around that but so far I have not been able to get the numbers to close for
anything else. The point of my concept is to eliminate as much parasite
engine mass as possible during the real rocket acceleration.

Andrew Nowicki wrote in message ...
Yes, VTVL (vertical takeoff and vertical landing on dry land without
parachute and without wings) is the cheapest option and probably the
best one if the rocket is sturdy enough to survive hard landing
without catastrophic damage. (Russian landing retrorockets malfunctioned
several times.)

The Russians see hard landings on Soyus sometimes because they are
using solid rockets. A proper throttling liquid engine should
reliably set down quite softly once the system is fully operational.
If it fails the landing in some way, it probably won't be just a
slightly bigger bump... Not that I advise making fragile vehicles,
but VTVL should allow the most optimized structures of any recovery
mode if you chose to push it.

JC Qantity and replication are easy in a spreadsheet or CAD program. It
JC is a little more troublesome in the real world. We recently made out
JC lives much, much better by moving from four differentially throttled
JC engines to a single larger engine with jet vanes. At some point mass
JC production effects can kick in, but it isn't in the development stage.

There is huge difference between making the rocket by hand and
making it by a robot.

Before the Civil War guns were made by gunsmiths. Their parts
were not interchangeable because the gunsmiths could not make
identical parts by hand. Making the rockets by hand takes lots
of time, and quality control is difficult -- you have to inspect
every weld because one bad weld can ruin your rocket.

Most of the Agena rocket engine was made by a robot -- coolant
passages were drilled in a monolithic slab of aluminum alloy.
My engine cluster has similar design -- it can be made by a
milling robot. The robot is cheaper than the rocket plumber,
and the engine is so sturdy that it may survive the hard VTVL
landing. You do not have to worry about weak welds -- there are
very few of them. Fabrication quality is determined by your CAD
drawing rather than by the robot. Even if you make just one
rocket, it is cheaper to make it by the milling robot than by
hand. I do not know if standard robots can make the narrow
injection nozzle holes, but they can certainly make all the
other holes.

I am a big believer in CNC machining, and we often have large batches
of engine parts made at once. With CNC, the complexity of a single
monolithic part is indeed of little relevence. The problems are still
in the interconnection of the parts. All of our old engine shells
were CNC machined, and it was no more effort to get a dozen of them
than it was to get one of them. On the other hand, it was still
nearly four times as much work to assemble catalyst packs for four of
them, bolt them together, plumb them up, fix the leaks, and so on. We
always wound up with one engine that was somewhat weaker than the
others, and related problems.

Even the best modern machining centers still aren't "part printers",
and you have to think carefully about which manufacturing processes
you use. We do far more manual welding than CNC machining, and the
large structural parts are made by a separate metal rolling shop. The
gun drilled nozzle on the Agena was a very tiny part of the total work
that went into the stage, let alone the vehicle. There have been
improvements since then (filament winding is vastly better in many
ways than classic tank welding), but there is so much work actually in
the details that aren't seen from a high level design view that it is
still nowhere close to "building by robot". I have spent days doing
nothing but making cables.

I do the CNC milling for Armadillo, at least when we are working in
aluminum. There are a many, many times when we slap something
together at the shop with the band saw, drill press, and welder that I
realize would have taken me several times longer to make on the CNC
mill, even if I had the stock on hand. Sure, it's great when I need
to go back and make more copies of it, but for building a prototype, I
wouldn't trade our welders for the best CNC robot in the world.

JC You can't design a high aspect ratio vehicle, but again, it doesn't
JC really matter for a booster stage. Make it squat, and let the upper
JC stage be a sphere if it wants to. Go ahead and be highly non-optimal
JC in the aerodynamics and staging fraction if it gets you good
JC operability. A booster like this would be a cargo elevator to 100km
JC or so. Up and down on the hour if you wanted to.

Can you comment on stacking the engines sideways?
(http://www.islandone.org/LEOBiblio/SPBI1010.JPG)
I believe that having lots of engines covering
large area of the rocket is a good idea because
it improves thrust, specific impulse, or both.

We are actually building a micro-nozzle plate, so you may have some
real pictures to point to in a month or so. The reason we are looking
at it is to save vehicle height over a single nozzle and reduce issues
with flow separation from overexpanded nozzles -- lots of little
nozzles will not all separate in the same direction.

For the same total throat area and expansion ratio, multiple small
nozzles will likely offer slightly worse performance, but they may
allow you to package a higher expansion ratio than you could
otherwise. You will certainly need to be able to throttle at least
banks of the engines separately for steering, and you will need either
dedicated roll engines, or a cant to some of the main engines for roll
control.


JC Most of the fundamental complexity of a rocket stage is independent of
JC stage performance. Lots more stages will give lots more problems.
JC Pushing performance requirements to the edge can easily give even more
JC problems, which is why I'm not an advocate of a completely SSTO
JC design, but two stages is going to be both more reliable and easier to
JC develop and test than more stages.

I have seen lots of comments about problems caused by large
number of stages, but I do not understand these problems.
It seems to me that when you stack identical rocket stages
like Lego blocks, your only problem is designing the
explosive bolts which hold the stages together.

You still have to build them, and any given chance of failure in the
parts will add up. Obviously two identical stages are easier than two
different stages, but it isn't at all obvious that six identical
stages are easier than two different stages.

JC An upper stage from us would probably use 98% peroxide and kerosene.

Other good choices are H2O2/RP-1 and H2O2/propylene.
They are easy to store, but, to the best of my knowledge,
their critical pressures and critical temperatures are high.
If the coolant pressure is lower than its critical pressure,
bubbles may form in the coolant.

Peroxide is a very good coolant, and you have lots of it compared to a
fuel, so for a decent sized engine, you won't even get the peroxide to
the boiling point.

JC Probably still pressure fed, but at a tank pressure of only
JC 100 psi or less, which doesn't hurt it much in vacuum operation.

100 psi = 6.9 bars
That is pretty low pressure. (Russian RD-170 engine has the chamber
pressure of 245 bars). Low pressure means high mass ratio, but the
engine must have large exhaust nozzle exit area to produce high
thrust and high specific impulse. Again, the engine cluster looks
like the right choice.

That ties in with the low aspect ratio booster. A big fat upper stage
can sit on top of it. It may wind up with something like a giant
aerospike covering the entire bottom of a spherical or flatter tank.

John Carmack
www.armadilloaerospace.com

In article ,
"johnhare" wrote:

There are a number of different approaches to the assist stage. I have
come to quite dislike the straight aircraft approach, not to deride
Scaled Composites, (they are getting the results), but I suspect it has
cost them 5-10 times as much as it should. This is telling, they used
to be the pin up boys of low cost development, but were they really? I
fear that they opted for the design that they did not because it was the
right one, but because it was the only one they knew how to make.

I happen to favor the straight aircraft aproach for regulatory reasons.
Most of the possible launch sites within reasonable commute distance
of my house have valuable property within a few miles. Given the
difficulty of proving that you are not going to cause damage, I prefer
the option of flying 20-50 miles from a runway to a salt water launch
position.

Another alternative to consider is to make the first stage a
high-altitude balloon or airship. These can go a little more than 10 km
altitude, which admittedly is a far cry from 100 km, but is still a lot
closer to vacuum than launching from the ground.

Of course this is what the folks at JP Aerospace have been saying for
years.

Best,
- Joe

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