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-   -   The 100/10/1 Rule. (http://www.spacebanter.com/showthread.php?t=96795)

Henry Spencer March 12th 07 01:47 AM

fun with expendable SSTOs (was The 100/10/1 Rule.)
 
In article ,
Pat Flannery wrote:
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.


...Considering that I've seen propane cylinders for refueling lighters that
have very thin aluminum walls, is it even necessary to chill it?


It's desirable for several reasons.

For the same pressure, wall thickness rises as the tank gets bigger, so
big tanks won't be as thin as those little ones. The vapor pressure of
pure propane(*) is about 8.5atm at room temperature, and a rocket tank
pressurized solely to support structural loads probably needs less than
1atm overpressure, so we're talking an order-of-magnitude difference in
wall thickness.

(* Note also that commercial "propane" is a mix of light hydrocarbons, and
often has considerable butane in it to lower vapor pressure. In fact, my
recollection is that the fluid for those lighters is mostly butane. )

Then too, room-temperature tanks would be bigger, because propane at LOX
temperatures is about 50% denser than at room temperature.

Finally, chilled liquids make pump design easier -- might not even need a
boost pump for the propane -- because they're much less prone to cavitate.
(In fact, it might be worth chilling the LOX below its boiling point too;
Rockwell's X-33 design did that.)
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Jorge R. Frank March 12th 07 01:34 PM

The 100/10/1 Rule.
 
(Henry Spencer) wrote in
:

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.


No doubt about it. But it can be confusing - especially to one with an
orbital mechanics background who hasn't worked vehicle ascent-performance
issues - and especially when you don't label your delta-Vs as "actual" or
"ideal". I understood what you were getting at, but can understand why Herb
didn't. I'm not disputing anything you wrote, just disputing Paul Dietz's
assertion that you'd explained the issue completely.

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.


That works for me too.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Hyper March 12th 07 01:57 PM

The 100/10/1 Rule.
 
On Mar 11, 10:51 pm, Pat Flannery wrote:

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


Is that curable? Or does she turn ballistic from time to time?




Herb Schaltegger March 12th 07 02:55 PM

The 100/10/1 Rule.
 
On Mon, 12 Mar 2007 06:34:56 -0600, Jorge R. Frank wrote
(in article ):

I understood what you were getting at, but can understand why Herb
didn't. I'm not disputing anything you wrote, just disputing Paul Dietz's
assertion that you'd explained the issue completely.


And bearing in mind that my orbital mechanics education is getting VERY rusty
with disuse over these last 10 - 15 years . . . :-)

--
You can run on for a long time,
Sooner or later, God'll cut you down.
~Johnny Cash


Paul E. Black March 12th 07 08:50 PM

The 100/10/1 Rule.
 
On Friday 09 March 2007 22:48, kT wrote:
Pat Flannery wrote:
Even then though you are going to have to do something to store it, as
it will boil off fairly quickly once the sun starts warming the exterior
of the LOX tank.


I'll shade it then, and use it up as fast as possible.


Convert LOX and H2 into water. You could save it easier and longer.
'Course, you'd need lots of electricity to get O from the water. And
it'd be a shame to waste all the energy from making water. Maybe use
the electricity with hydrogen to raise or change your orbit ...

-paul-
--
Paul E. Black )

[email protected] March 12th 07 10:40 PM

fun with expendable SSTOs (was The 100/10/1 Rule.)
 

Henry Spencer wrote:
In article ,
Pat Flannery wrote:
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.


...Considering that I've seen propane cylinders for refueling lighters that
have very thin aluminum walls, is it even necessary to chill it?


It's desirable for several reasons.

For the same pressure, wall thickness rises as the tank gets bigger, so
big tanks won't be as thin as those little ones. The vapor pressure of
pure propane(*) is about 8.5atm at room temperature, and a rocket tank
pressurized solely to support structural loads probably needs less than
1atm overpressure, so we're talking an order-of-magnitude difference in
wall thickness.

(* Note also that commercial "propane" is a mix of light hydrocarbons, and
often has considerable butane in it to lower vapor pressure. In fact, my
recollection is that the fluid for those lighters is mostly butane. )

Then too, room-temperature tanks would be bigger, because propane at LOX
temperatures is about 50% denser than at room temperature.

Finally, chilled liquids make pump design easier -- might not even need a
boost pump for the propane -- because they're much less prone to cavitate.
(In fact, it might be worth chilling the LOX below its boiling point too;
Rockwell's X-33 design did that.)


I'm not sure if I understand what you are saying here. Are you
proposing
to make a rocket where structural strengh is provided by pressure in
the
tanks and where this pressure comes from LOX chilled below its boiling
point? If not, maybe it would be a good idea to point out that we
aren't
talking about baloon tanks anymore or that some other gas is inserted
to provide pressure or what ever.

Alain Fournier


Henry Spencer March 13th 07 12:59 AM

fun with expendable SSTOs (was The 100/10/1 Rule.)
 
In article m,
wrote:
(In fact, it might be worth chilling the LOX below its boiling point too;
Rockwell's X-33 design did that.)


I'm not sure if I understand what you are saying here. Are you
proposing to make a rocket where structural strengh is provided by
pressure in the tanks and where this pressure comes from LOX chilled
below its boiling point?


Not quite. The pressure comes from the tank pressurization, just like it
does with any other balloon-tank scheme. The one blemish is that you
can't easily pressurize subcooled liquids with their own vapor -- you have
to pressurize with something else, unless maybe you can put an insulating
barrier between gas and liquid. Pressurizing with something else is often
done anyway, for other reasons, but here it's pretty much mandatory,
especially for the propane, which is rather drastically subcooled.

(For the oxygen the subcooling is not so dramatic, given LOX's narrow
temperature range. It's also worth noting that most existing LOX-using
rockets effectively run their LOX *slightly* subcooled -- it's boiling at
countdown pressure, but then they go to a higher tank pressure for flight,
which raises the boiling point. It generally doesn't have time to warm up
to the new boiling point before it's burned.)

The orthodox approach is to pressurize with helium, which is light and
inert but does suffer from being relatively expensive and in limited
supply. Some experimenting with alternatives would be in order.

You might be able to pressurize the propane with methane, which
conveniently is also liquid at about LOX temperatures. (You could almost
certainly pressurize propane with hydrogen, but that's a pain to store.)
Some of the methane will dissolve, but that's an issue for pressurizing
with propellant vapor too, and it can be controlled well enough.

Another option, for both propellants, is neon -- still not exactly cheap,
but the supply is essentially unlimited (unlike helium, it's made from
air, as a byproduct of LOX and LN2 production) and it's inert and has a
very low boiling point.

If you were careful to minimize turbulence in the gas, you could probably
pressurize mildly-subcooled LOX with GOX, if you threw in a bit of neon
first: you'd get a layer of cold neon gas on top of the LOX, with warmer
GOX above it. An experiment or two would be needed -- neon *is* somewhat
soluble in LOX, but then, both Delta II and Soyuz pressurize LOX with GN2,
which is very soluble in LOX... Some GOX would get through the neon
layer, so you'd have a layer of warm LOX at the top of the liquid, but
that again is a common issue -- most propellant-vapor pressurization
systems heat the pressurant beyond boiling to reduce its density.

Whether a neon buffer layer would suffice to let you pressurize propane
with propane is less obvious. Maybe.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Pat Flannery March 13th 07 12:13 PM

fun with expendable SSTOs (was The 100/10/1 Rule.)
 


wrote:
I'm not sure if I understand what you are saying here. Are you
proposing
to make a rocket where structural strengh is provided by pressure in
the
tanks and where this pressure comes from LOX chilled below its boiling
point? If not, maybe it would be a good idea to point out that we
aren't
talking about baloon tanks anymore or that some other gas is inserted
to provide pressure or what ever.


The LOX still boils off to some extent due to contact with the tank
walls, which are warmer than it is.
you simply put a pressure relief valve on the tank set to the amount of
internal pressure you want in it, and it vents the LOX if the pressure
goes above that value in the LOX tank.
These used to give trouble in the early days, as they could frost up and
stick, causing the LOX tank to over-pressurize and rupture.

Pat

Proponent March 13th 07 12:23 PM

The 100/10/1 Rule.
 
Pat Flannery wrote:
Proponent wrote:
Is it possible that with the booster engines attached all the way to
orbit, the vehicle could fly a somewhat more efficient trajectory,
therefore boosting the payload a bit? On the other hand, the booster
engines might have to be shut down anyway in order to keep the
acceleration from damaging the structure.

I assume they carefully worked out to the second when the boosters
became a net deficit to the ascent, and jettisoned them at that point.


No doubt you are correct that the separation of the booster half-stage
was timed to optimize the performance of the actual stage-and-a-half
vehicle. And we agree that performance would have suffered
substantially under the constraint that the booster half-stage not be
dropped. I'm just pointing out that re-optimizing the flight program
under the no-staging constraint would probably result in performance
slightly better than flying the without staging under original stage-
and-a-half program.


Pat Flannery March 13th 07 12:29 PM

fun with expendable SSTOs (was The 100/10/1 Rule.)
 


Henry Spencer wrote:


The orthodox approach is to pressurize with helium, which is light and
inert but does suffer from being relatively expensive and in limited
supply. Some experimenting with alternatives would be in order.


Was Atlas helium pressurized prior to launch, or was the LOX boil-off used?
I assume the latter, as it was venting pretty much right up till launch.
In fact, in this shot it looks like it's venting after lifting off:
http://users.skynet.be/space-link/Pictures/Atlas3.jpg

Pat


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