poor man's rocket

I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

Zoltan

Zoltan Szakaly wrote:
I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

There isn't really any point.
You end up with a rocket engine with very, very limited performance,
that's much heavier than standard.
There might be a very limited case for extending the altitude that it
will work at.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
"I meant, have you ploughed the ocean waves at all?" Colon gave him a cunning
look. 'Ah, you can't catch me with that one, sir' he said 'Everyone knows
horses sink' -- Terry Pratchett - Jingo

(dave schneider) wrote in message om...
Ian Stirling wrote in message ...
Zoltan Szakaly wrote:
I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

There isn't really any point.
You end up with a rocket engine with very, very limited performance,
that's much heavier than standard.
There might be a very limited case for extending the altitude that it
will work at.

Throwing away the compressor (and the blades at the other end that
drive it) would give you a more standard rocket engine, where your
oxidizer pumps (or pressurized tank) take care of providing the right
mix, eh?

/dps

The point here would be that you could initially use the air as
oxidizer and later when you are higher and faster you could switch
over to the oxidizer tank without having to use an extra rocket
engine.

The only reason I brought it up is for the purposes of discussion,
like a brainstorming aid.

Zoltan

(Zoltan Szakaly) writes:

(dave schneider) wrote in message om...
Ian Stirling wrote in message ...
Zoltan Szakaly wrote:
I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

There isn't really any point.
You end up with a rocket engine with very, very limited performance,
that's much heavier than standard.
There might be a very limited case for extending the altitude that it
will work at.

Throwing away the compressor (and the blades at the other end that
drive it) would give you a more standard rocket engine, where your
oxidizer pumps (or pressurized tank) take care of providing the right
mix, eh?

The point here would be that you could initially use the air as
oxidizer and later when you are higher and faster you could switch
over to the oxidizer tank without having to use an extra rocket
engine.

As Henry Spencer has pointed out many, many times in this newsgroup,
careful study shows that, since the airborne portion of the the trajectory
is a relatively short portion of the total launch profile, it is more
efficient to build a rocket that can double as a "poor man's jet" than
a jet that can serve as a "poor man's rocket."


The only reason I brought it up is for the purposes of discussion,
like a brainstorming aid.

It has already been "brainstormed" to death. The engineering conclusion is
that, with the limited exception of drop-launching from a carrier aircraft,
airbreathing at BEST provides a MARGINAL advantage that depends sensitively
on one's design assumptions, and that in general it is actually DETRIMENTAL
to overall launch costs. For a discussion of why airbreathing launch is in
general harmful, see http://www.islandone.org/Propulsion/SCRAM-Spencer1.html.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Zoltan Szakaly" wrote:
I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

Despite the hype, rockets are less complex than jet engines, much
less complex. A jet engine for a large aircraft (fighter jet or
jumbo jet) is itself the size of a large automobile, but a rocket
engine with the same thrust performance could easily sit on your
coffee table without straining it much.

Here's an example, the Pratt & Whitney model F100* jet engine,
designed for fighter jets and the engine which powers the F-15
and F-16C (among other aircraft) weighs 1900 kg (4100 lbs) and
outputs 145 kN (32,500 lb-f) maximum thrust. For a thrust to
mass ratio of 78 N/kg. Meanwhile, there's the X-405* LOX /
Kerosene rocket engine, of a decidedly older vintage (it
powered some of the stages of the Vanguard rocket), which weighs
191 kg and outputs 135 kN maximum thrust. For a thrust to mass
ratio of 707 N/kg, nearly 10x that of the jet engine (and this is
for a rocket engine which is over 3 decades older than the jet).

Building rockets with jet engine thrust is no simple task, but
they can be bought at a reasonable price. And, yes, they can be
made without too much difficulty either. More importantly,
building a rocket engine designed not to maximize the thrust to
weight ratio as much as possible but simply to *exceed* the
exceedingly poor thrust to weight ratios (by comparison) of jet
engines is a much easier task. Which is why nobody even thinks
of using jet engines as you describe, it's simply not worth it.

An F-100 jet engine costs around $4.5 million dollars, an
NK-33 rocket engine costs around $2 million dollars but has
*eleven* times the thrust and weighs 1/3 less.



(*) http://www.pratt-whitney.com/3a/html/products_f100.html
(+) http://www.astronautix.com/engines/x405.htm

snip snip ...
An F-100 jet engine costs around $4.5 million dollars, an
NK-33 rocket engine costs around $2 million dollars but has
*eleven* times the thrust and weighs 1/3 less.



(*) http://www.pratt-whitney.com/3a/html/products_f100.html
(+) http://www.astronautix.com/engines/x405.htm

Now we are getting somewhere.

The comparison between the two is like this:

Jet engine:
Isp= 5,000 Thrust to weight: 10

Rocket engine:
Isp= 350 Thrust to weight: 100

There are clearly variations but these are somewhat representative
numbers. The jet engine's weight limits the mass ratio that can be
accomplished. Here I will try to compute performance for both.

If I start with a 100 ton take off mass, the rocket would have a 150
tons of thrust and a 1.5 ton engine. The payload, fuel tanks,
structure would limit the mass fraction to between 0.88 and 0.95 so
the velocity increment achieved would be between 7,280 and 10,300
m/sec

The air breathing engine based vehicle would have a mass fraction of
between 0.73 and 0.8 and the velocity increment would be 64,222 to
78,942 m/sec

Clearly the air breathing engine wins. Of course I assumed no air drag
here.

I want to come clear here, the reason I am so interested in this
because I have an engine that has a high Isp (over 4,000) and is light
weight and cheap so it can achieve a thrust to weight of 100. (I
realize that thrust is force and weight is mass, I am using the terms
loosely here, I am dividing pounds by pounds) I am working on the
development of a flying car but I am hoping to also use the engine as
a first stage engine to put payloads into orbit.

I think the air breather wins no matter what somebody just needs to
have enough balls to build one and fly it.

Zoltan

"Henry Spencer" wrote in message
...
In article ,
Zoltan Szakaly wrote:

I want to come clear here, the reason I am so interested in this
because I have an engine that has a high Isp (over 4,000) and is light
weight and cheap so it can achieve a thrust to weight of 100.

Do let us know when you patent it or otherwise publish a detailed
description of how it works. Until then, I'm afraid we must view this
claim with great skepticism.
--
I made an earlier claim like this if you divide by 4 and add a mach limit of
~3.

MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Penguinista wrote:
snip
Vital point here. A jet to orbit must use a higher drag path than is
typical for a rocket to orbit. Also, that ISP figure on the jet is for
a modest speed. At higher speeds the jet engine is much less efficient.
And then there's hypersonic heating, which is easily worse than high
drag reentry. Still, a jet powered first stage dropping of ~mach 3 could
make sense.

SR71 has a fuel load of ~35000Kg, and a dry weight of ~40000Kg.
It flies at mach 3, for 3 hours (the page I was looking at is not completely
clear if this is the total flying time)
That's around 3.2Kg/sec fuel flow.
Assuming an average weight of 55000Kg, and a L/D of 10, that's a thrust of
5500Kg needed, or 55000N, implying an ISP of around 1700, significantly
lower than the ~3000 quoted earlier.


Funky fact (that I have not checked) , the SR71 used a touch of afterburner
when refuelling, as it had to fly at such a steep angle to go slow enough
to keep pace with the tanker.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
Among a mans many good possessions, A good command of speech has no equal.

Ian Stirling writes:

Gordon D. Pusch wrote:
(Zoltan Szakaly) writes:

(dave schneider) wrote in message om...
Ian Stirling wrote in message ...
Zoltan Szakaly wrote:
I wonder if I could feed an off the shelf jet engine with fuel and
oxidizer at a fuel rich mixture ratio and later add the rest of the
oxidizer to the exhaust afterburner.

I think this might work like a cheap rocket engine. Like when you try
to throw something together with off the shelf components, quick
dirty, cheap.

There isn't really any point.
You end up with a rocket engine with very, very limited performance,
that's much heavier than standard.
There might be a very limited case for extending the altitude that it
will work at.

Throwing away the compressor (and the blades at the other end that
drive it) would give you a more standard rocket engine, where your
oxidizer pumps (or pressurized tank) take care of providing the right
mix, eh?

The point here would be that you could initially use the air as
oxidizer and later when you are higher and faster you could switch
over to the oxidizer tank without having to use an extra rocket
engine.

As Henry Spencer has pointed out many, many times in this newsgroup,
careful study shows that, since the airborne portion of the the trajectory
is a relatively short portion of the total launch profile, it is more
efficient to build a rocket that can double as a "poor man's jet" than
a jet that can serve as a "poor man's rocket."

At the current state of the art.
Given dramatic advances (light shape changing refractory materials
that can operate at 2-3000C) this may change.

I =VERY= strongly doubt that this is going to buy an order of magnitude
improvement in the characteristically ****-poor thrust-to-mass ratio
of air-breathing engines, nor is it likely to significantly decrease
the "Air Breather's Burden" term in the equation Henry Spencer cites
on http://www.islandone.org/Propulsion/SCRAM-Spencer1.html.
Air-breathing launch requires implausibly good hypersonic lift-to-drag
ratios and implausibly large accelerations given an air-breathing engine.
Air-breathers are good for cruise, but they suck badly at acceleration!


Trying to do this now though is akin to the Wright brothers attempting a
VTOL jet aircraft as their first vehicle.

I submit that it is more nearly akin to Langley attempting to power a VTOL
aircraft with a coal-fired steam engine...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Ian Stirling" wrote:
Funky fact (that I have not checked) , the SR71 used a touch of afterburner
when refuelling, as it had to fly at such a steep angle to go slow enough
to keep pace with the tanker.

Another funky fact, the SR-71 was designed to operate at such
high speeds and high temperatures that it needed extraordinarily
large expansion joints for its components, which is one reason
why it leaked fuel when it was cold on the ground. Which in
turn was why they tended to take off with minimal fuel and
fuel up in the air.