Air breathing Engines

Henry And no airbreathing engine is very efficient at very high speed,
Henry for fundamental reasons. (It necessarily takes very large amounts
Henry of energy to further accelerate air which is already going past at
Henry very high speed.)

I agree that airbreathing isn't great, but it's the low T/W and low
L/D that's the problem, which come from the problem of collecting and
compressing all the voluminous air. You've said this before. Energy
requirements for accelerating fast air are not the issue.

Here are the numbers for a propane airbreather accelerating air going
by at V1 to V2. I got these by assuming propane-LOX has a Ve=3300
m/s, and adding the same energy into propane-air. This basically
assumes a perfectly expanded exhaust, which assumes a pretty high
compression ratio, which is not going to happen.

V1 V2 | Thrust/mass propellant Isp
0 1705 | 29604 m/s 3020
500 1772 | 22586 2304
1000 1962 | 17703 1806
1500 2242 | 14383 1467
2000 2584 | 12140 1238
2500 2966 | 10591 1080
3000 3374 | 9493 968
3500 3801 | 8726 890
4000 4241 | 8184 835
4500 4689 | 7781 793
5000 5144 | 7500 765

Here's my math:

204kg of propane-lox gives 3300*204 = 673200 N*s impulse
and is 0.5*204*3300^2 = 1.11e9 J

Suppose we have air going by at v1 m/s. We add propane:
C3H8 + 5O2 + 20N2 = 3CO2 + 4H20 + 20N2
44 + (5*32=160) + 20*28 = 764

720 kg goes from v1 to v2
44 kg goes from 0 to v2

0.5*44*v2^2 + 0.5*720*(v2^2-v1^2) = 1.11e9
22*v2^2 + 360*v2^2 - 360*v1^2 = 1.11e9
382*v2^2 = 1.11e9 + 360*V1^2

v2 = sqrt( 2.9e6 + 360/382*v1^2 )

Thrust is 44*V2 + 720*(V2-V1)
Thrust per mass is V2 + 720/44*(V2-V1)

On 9 Jan 2004 07:57:30 GMT, James Logajan wrote:

Depends on the objective. If one's intent is a SSTO _and_ SSTE (Single
Stage to Earth) RLV then I'd expect a designer would not want to deal with
the problems of de-orbiting a behemoth.

Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

--
"Sore wa himitsu desu."
To reply by email, remove
the small snack from address.
http://www.esatclear.ie/~rwallace

Russell Wallace wrote:

On 9 Jan 2004 07:57:30 GMT, James Logajan wrote:

Depends on the objective. If one's intent is a SSTO _and_ SSTE (Single
Stage to Earth) RLV then I'd expect a designer would not want to deal with
the problems of de-orbiting a behemoth.

Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

Greatly simplified operations, coming from not having to pick up and
return (or replace) all the pieces, refurbush where needed, and
re-intergrate them. Full checkout of one set of engines before launch
commit, rather than count on successful staging, and ignition at
altitude of 'upper stage' engines. Assorted other things we unspokenly
take for granted where aircraft are involved. Few vehicles of *any* kind
weigh less than their payloads...

The above should not be taken to assume I have a problem with a small
winged TSTO, though....


--

You know what to remove, to reply....

(Russell Wallace) wrote in message ...

Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

No SRBs falling away needing a ship to putter out into the ocean and
pick them up.

No throwing away of millions of dollars of precision-assembled
external fuel tank.

No need to disassemble the SRBs, refuel section by section, and
carefully restack.

No need to hoist the 30 to 100-ton components into the air in a very
expensive "vehicle assembly building" and bolt them together.

The point of SSTOs never involves weight efficiency - you'd use
multi-stage designs if you want to put a lot of weight into orbit
relative to the upper stage mass. (Well, Orion SSTOs kinda buck that
trend, but Orion SSTOs have issues of their own.) The point of SSTOs
is COST. Only 1 vehicle to dust off, reload, refuel, and relaunch.

(Allen Meece) wrote in message ...
Well, If oxidiser is 1/4 to 1/3 of the weight you're trying to get off
the
ground, it makes sense to burn the oxygen in the air.
No it doesn't, not when you consider the extra technical problems and the
extra engine mass.
Well, the theory out of the AFRL is that PDE combined cycle air-breathing
rockets will have *less* mass than rockets with turbopumps and *big* LOX tanks.
[air breathing rockets will still need tanks above the atmosphere but no *big*
pumps because the injection occurs at low chamber pressure]
Also, less fuel mass means less horsepower required which means less engine
weight. It's very hard to see why Henry says it's fine to go big and heavy if
it's cheap. Weight's never cheap in the space game.

In case some of you people haven't got the point about LOX yet, look
at this diagram of the Ariane 5 core booster stage for an example:
http://www.esa.int/export/esaLA/ASEZ...unchers_1.html

Do you still think eliminating part of that LOX tank will give you
much benefit? The LH2 tank is the largest dead weight and you want to
make it bigger by saving on oxidizer...

(Russell Wallace) wrote in message ...
On 9 Jan 2004 07:57:30 GMT, James Logajan wrote:

Depends on the objective. If one's intent is a SSTO _and_ SSTE (Single
Stage to Earth) RLV then I'd expect a designer would not want to deal with
the problems of de-orbiting a behemoth.

Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

There is much confusion regarding terms. First: You can have an
expendable or a reusable SSTO vehicle.

For e.g. the expendable concept of the SSTO Shuttle Tank with RS-68s
underneath. Per se, the only advantages I can see in SSTO is that
since you only have one stage you most likely have less parts and
complexity, so it is cheaper and more reliable all else being equal.

The usual reasons pointed for SSTO being cheap only apply to reusable
SSTO vehicles. Take a DC-Y like vehicle as an example. Since the whole
vehicle is reused, including the most expensive parts (i.e. the
engines and control electronics), you get maximum reusability. You
could also make a completely reusable TSTO vehicle. But a TSTO vehicle
will likely have a higher turnaround time and higher maintenance costs
(more parts to inspect, you need to mate vehicles again before
launch). In an ideal reusable SSTO vehicle you would just refuel and
launch again.

FWIW I wasn't that fond of the DC-Y concept. It just seems more
natural for me if a VTVL vehicle launches and lands in an upright
position. This way you can even use the engine exaust as an heat
shield and you have less control complexity. I blame that on the
military requirement for high crossrange on reentry.

"Russell Wallace" wrote:

Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

Faster turnaround time? Modern aircraft can refuel and re-launch in hours
or even minutes. Preparing the space shuttle to re-launch takes weeks and
lots of special equipment.

- Xerxes

(Russell Wallace) wrote in message ...
Can someone remind me why SSTO is considered a good idea? The Space
Shuttle spends ~75% of its (fairly impressive) lift capacity on
hauling dead weight to orbit that just gets immediately deorbited
again; for an SSTO the figure would presumably be even worse. What's
the corresponding advantage?

The Shuttle is not an SSTO and only just barely reusible.
Actually, the cost of running the Shuttle as an expendable
vehicle would probably be fairly similar to the costs of
the Shuttle now. It requires about as much work to
refurbish the Orbiter's pieces and assemble the Shuttle
stack as it would to build a fresh rocket to launch the
same mass. Interestingly, if you include the Orbiter
mass, the cost per kg to orbit for the Shuttle system
is a bit lower than for other launch systems in the US.
So really they are saving money, but not much, and
only if the Orbiter is absolutely, justifiably needed
for each and every use of the Shuttle as a launcher.

Now, back to SSTOs and RLVs, which are actually separate
topics even though they are often taken together. SSTO
has the potential to make launch cheaper by making the
launch vehicle less complex. Thus, testing, engineering,
manufacturing, etc. is all reduced in cost, in theory,
and all you have to pay for is extra metal and propellant,
which are both cheap compared to the labor that goes into
aerospace equipment. Think of SSTO as a simpler desing
more amenable to mass production. Like a plastic cased,
digital watch instead of a fine pocket watch with
umpteen jillion fine parts that needs to be assembled
with care (and expense). Think of an SSTO launcher as
a Model-T. Maybe not the best vehicle in the class, but
cheap to build, and it gets the job done. RLVs are a
different stab at the launch cost equation. With
expendables and refurbishables there is a hell of a lot
of labor needed for each and every flight. Metal needs
to be bent, parts need to be machined, stuff needs to
be stripped down and thoroughly inspected, etc. But
with a true RLV the per-flight needs are very few.
Waggle the control surfaces, kick the tires, fill up
the tank, and then launch again. It's like a car or an
airplane, there isn't a whole lot of labor involved in
"turnaround" between trips. That reduces costs
tremendously, but only if you make enough trips. For
fairly straightforward reasons it's most likely that
the best RLVs will need to be SSTOs, though there are
other possibities.

Rocket fuel is pretty cheap, and if all you needed
between launches was to refuel then orbital launch
would be rediculously inexpensive. Let's say that
you had an RLV that cost 3x more than today's
launchers to build (which would make it tremendously
expensive) but could fly 30 flights before needing
to be decomissioned. Then, your per flight cost
would be 1/10 of today's launchers plus the cost
of fuel (which is marginal in comparison). It's
all about amortization, spreading out overhead costs.
More flights per vehicle means the vehicle cost gets
spread out over more flights. That's RLVs. More
vehicles means the design, tooling, engineering,
and other overhead costs gets spread out over more
vehicles. That's SSTOs. The great advantage is
that it can cost a lot MORE to design, develop, and
build these vehicles but it would still be cheaper
because of the amortization of costs.

Think about today's automobiles. They are produced in
large volumes and have long lives. It costs billions
of dollars to develop each new version of a car, and
there are probably many, many billions of total
development dollars behind any given car model on the
road today. It costs billions of dollars to build
and run the factories that make cars. But they make
millions of cars and those cars run for hundreds of
thousands of miles. Even though billions of dollars
were spent in the process, the cost per mile for a
car is low, around the same level as the cost of
fuel (cents per mile from the cost of the vehicle and
cents per mile from the cost of the fuel). Now
imagine if auto makers just built a handful of cars
each year and those cars would only last for one
trip. The cost of automobile travel would be
dominated by the development and manufacturing costs
and would be horrendously expensive. That's how we
build launch vehicles today and that's why using
them is so expensive.

Mike Miller wrote:
The point of SSTOs never involves weight efficiency - you'd use
multi-stage designs if you want to put a lot of weight into orbit
relative to the upper stage mass. (Well, Orion SSTOs kinda buck that
trend, but Orion SSTOs have issues of their own.) The point of SSTOs
is COST. Only 1 vehicle to dust off, reload, refuel, and relaunch.

More to the point; only one vehicle to design. That means you only need
half the R&D staff (in principle), this saves lots of money, since the
up-front capital costs are half the cost of launch.

OTOH, you have a riskier program; and that increases the cost of
borrowing money.

(quasarstrider) wrote in message . com...
(Allen Meece) wrote in message ...
Well, If oxidiser is 1/4 to 1/3 of the weight you're trying to get off
the
ground, it makes sense to burn the oxygen in the air.
No it doesn't, not when you consider the extra technical problems and the
extra engine mass.
Well, the theory out of the AFRL is that PDE combined cycle air-breathing
rockets will have *less* mass than rockets with turbopumps and *big* LOX tanks.
[air breathing rockets will still need tanks above the atmosphere but no *big*
pumps because the injection occurs at low chamber pressure]
Also, less fuel mass means less horsepower required which means less engine
weight. It's very hard to see why Henry says it's fine to go big and heavy if
it's cheap. Weight's never cheap in the space game.

In case some of you people haven't got the point about LOX yet, look
at this diagram of the Ariane 5 core booster stage for an example:
http://www.esa.int/export/esaLA/ASEZ...unchers_1.html

Do you still think eliminating part of that LOX tank will give you
much benefit? The LH2 tank is the largest dead weight and you want to
make it bigger by saving on oxidizer...


We would not just be eliminating the weight of the oxygen tank, we
would be eliminating the weight of the oxygen. The mixture ratio being
6:1 we would be eliminating 83% of the weight of the stage. In fact
the engine would have to be much smaller because of the less weight,
so we would be eliminating the weight of the engine and the weight of
the hydrogen, and the weight of the tanks and the thrust structure.
Basically we would be eliminating the weight of the whole first stage.
We could just use an air breathing engine in the second stage and do
away with the first stage altogether.

Zoltan