Author Topic: Jet engine stage  (Read 158683 times)

Offline DarkenedOne

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Jet engine stage
« on: 06/15/2012 02:36 am »
We all know about Stratolaunch, Spaceship two, and the Pegesus system.  All are air launch systems.  They all use commercial airline jet engines.  They all have large wings.  The problem I see is that when your trying to reach high speeds wings only slow you down by creating drag.  It is not inconceivable to create a jet powered system without wings at all.


What I have been thinking about is what if you were to design a system with a jet engine first stage.  Essentially take jet engines and strap them to the side of the stage core which contains the jet fuel.  Thrust vectoring would provide control jet as it does with rockets.  The jet engine stage would take the rocket from the ground, accelerate it, then jettison once it has reached the point where the jet engines can no longer provide assistance.  The jet stage would then come back to earth and land just like any other VTOL system. 

Also rather than using airline jet engines, which have a max speed of Mach 1, a T/W ratio of 5.45, and altitude of something like 13,500m, use fighter jet engines with afterburner.  Moderns engines like the Pratt & Whitney F119 from the F-22 have a  max speed of over Mach 2.25, a T/W ratio of almost 8, and a service ceiling of 19,812 m.  The engine from the Blackbird had a T/W ratio of 6, a max speed of over Mach 3.2, and a service ceiling of 25,900 m. 

     
I would imagine that if such a system would be built it would have significantly less recurring costs than today's systems.  Even less than a VTOL system like the grasshopper SpaceX is trying to build.  The reason is that jet engines are cheap to maintain compared to rocket engines. 
« Last Edit: 06/15/2012 02:38 am by DarkenedOne »

Offline LegendCJS

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Re: Jet engine stage
« Reply #1 on: 06/15/2012 02:58 am »
I imagine that with jet engines being so well understood, mass produced, and cheap for over half a century and unmanned flight dating back to WWI that if this idea was feasible or helpful we would be seeing it in action already, but that is just my speculation.

There is a veritable zoo of threads discussing this topic to look at too.
Remember: if we want this whole space thing to work out we have to optimize for cost!

Offline aero

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Re: Jet engine stage
« Reply #2 on: 06/15/2012 03:15 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT
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Online Lee Jay

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Re: Jet engine stage
« Reply #3 on: 06/15/2012 03:32 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

NOT indeed - you've only got enough engines to hover, not to climb.   ;)

You can get to 10:1 (give or take) T/W on jet engines with afterburners.  Let's say you could produce a GE90-115b with afterburners and run it at above-rated power, and get, say, 85 tons of thrust.  You're still talking about three of them to replace one SSME, and even the regular version costs on the order of $25 million.  Oh, and they can't go supersonic.   :-[

Offline DarkenedOne

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Re: Jet engine stage
« Reply #4 on: 06/15/2012 03:32 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

First of all you would not need 500 tonnes of rocket where 95% of it is fuel if your engines had an ISP of 3000 seconds. 

Second of all at that scale one would build larger engines with higher T/W.

Lastly the benefits in maintainability will likely be more cost effective in the long run.

Offline DarkenedOne

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Re: Jet engine stage
« Reply #5 on: 06/15/2012 03:36 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

NOT indeed - you've only got enough engines to hover, not to climb.   ;)

You can get to 10:1 (give or take) T/W on jet engines with afterburners.  Let's say you could produce a GE90-115b with afterburners and run it at above-rated power, and get, say, 85 tons of thrust.  You're still talking about three of them to replace one SSME, and even the regular version costs on the order of $25 million.  Oh, and they can't go supersonic.   :-[

But they are reusable.  Not in the sense of Shuttle reusable where the engines had to be taken apart, inspected, and reassembled after each flight.  We are talking about airplane reusability where inspections are required after hundreds of flights.

Offline QuantumG

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Re: Jet engine stage
« Reply #6 on: 06/15/2012 03:44 am »
Go design your vehicle and come back when you have something to present. Then we'll discuss it.
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Offline ARD

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Re: Jet engine stage
« Reply #7 on: 06/15/2012 04:11 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

NOT indeed - you've only got enough engines to hover, not to climb.   ;)

You can get to 10:1 (give or take) T/W on jet engines with afterburners.  Let's say you could produce a GE90-115b with afterburners and run it at above-rated power, and get, say, 85 tons of thrust.  You're still talking about three of them to replace one SSME, and even the regular version costs on the order of $25 million.  Oh, and they can't go supersonic.   :-[

But they are reusable.  Not in the sense of Shuttle reusable where the engines had to be taken apart, inspected, and reassembled after each flight.  We are talking about airplane reusability where inspections are required after hundreds of flights.

Is there a physical law that makes jet engines less maintenance-intensive than rockets?  It's not like jets lack complex moving parts.  In fact, one can make the case that jets are more complex than some rocket engines (especially pressure-fed designs). 

Rocket engines are also reusable, or nearly so--some engines ran several times their actual flight burn times while on test stands.  It's recovery that is the difficult part, actually getting the engine back to maintenance workers.  And putting jet engines on the first stage doesn't address the recovery problem any more effectively than having pressure-fed rockets, or even SSMEs, on a winged stage would. 

Offline hkultala

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Re: Jet engine stage
« Reply #8 on: 06/15/2012 07:29 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

First of all you would not need 500 tonnes of rocket where 95% of it is fuel if your engines had an ISP of 3000 seconds. 

going to mach 2.2 at 20km means you still have like 7.5km/s left in order to reach orbit., or going to mach 3.2 at 25km means you still have like 7.2 km/s left to reach orbit.

You still need big rocket second stage with lots of fuel.

So you will need tens of tons of those jet engines.

Quote

Second of all at that scale one would build larger engines with higher T/W.


What makes you think that if you just build a bigger engine, you get magically better T/W?

Quote
Lastly the benefits in maintainability will likely be more cost effective in the long run.

What's the difference in maintainability between jet and rocket engines? where does it come from?


Achieving T/W of >1 with jet engines (needed for wingless flight) just makes the engines too big and expensive. With jet engines, it makes more sense to use wings, which allows using like 3.5 times less thrust, meaning 3.5 times lighter engines (xb-70 has T/W of 0.32, Concorde has T/W of 0.37, sr-71 has T/W of 0.4 )
« Last Edit: 06/15/2012 07:30 am by hkultala »

Offline Jim

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Re: Jet engine stage
« Reply #9 on: 06/15/2012 10:52 am »
We all know about Stratolaunch, Spaceship two, and the Pegesus system.  All are air launch systems.  They all use commercial airline jet engines.  They all have large wings.  The problem I see is that when your trying to reach high speeds wings only slow you down by creating drag.  It is not inconceivable to create a jet powered system without wings at all.


What I have been thinking about is what if you were to design a system with a jet engine first stage.  Essentially take jet engines and strap them to the side of the stage core which contains the jet fuel.  Thrust vectoring would provide control jet as it does with rockets.  The jet engine stage would take the rocket from the ground, accelerate it, then jettison once it has reached the point where the jet engines can no longer provide assistance.  The jet stage would then come back to earth and land just like any other VTOL system. 

Also rather than using airline jet engines, which have a max speed of Mach 1, a T/W ratio of 5.45, and altitude of something like 13,500m, use fighter jet engines with afterburner.  Moderns engines like the Pratt & Whitney F119 from the F-22 have a  max speed of over Mach 2.25, a T/W ratio of almost 8, and a service ceiling of 19,812 m.  The engine from the Blackbird had a T/W ratio of 6, a max speed of over Mach 3.2, and a service ceiling of 25,900 m. 

     
I would imagine that if such a system would be built it would have significantly less recurring costs than today's systems.  Even less than a VTOL system like the grasshopper SpaceX is trying to build.  The reason is that jet engines are cheap to maintain compared to rocket engines. 

many threads on this.  Answer is no.

Offline DarkenedOne

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Re: Jet engine stage
« Reply #10 on: 06/15/2012 01:49 pm »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

NOT indeed - you've only got enough engines to hover, not to climb.   ;)

You can get to 10:1 (give or take) T/W on jet engines with afterburners.  Let's say you could produce a GE90-115b with afterburners and run it at above-rated power, and get, say, 85 tons of thrust.  You're still talking about three of them to replace one SSME, and even the regular version costs on the order of $25 million.  Oh, and they can't go supersonic.   :-[

But they are reusable.  Not in the sense of Shuttle reusable where the engines had to be taken apart, inspected, and reassembled after each flight.  We are talking about airplane reusability where inspections are required after hundreds of flights.

Is there a physical law that makes jet engines less maintenance-intensive than rockets?  It's not like jets lack complex moving parts.  In fact, one can make the case that jets are more complex than some rocket engines (especially pressure-fed designs). 

Rocket engines are also reusable, or nearly so--some engines ran several times their actual flight burn times while on test stands.  It's recovery that is the difficult part, actually getting the engine back to maintenance workers.  And putting jet engines on the first stage doesn't address the recovery problem any more effectively than having pressure-fed rockets, or even SSMEs, on a winged stage would. 

Jet engines are definitely more complex, however they operate at lower pressures and temperatures than rocket engines. 

Secondly it does help with the recovery problem.  Rocket engines  are more difficult to use in VTOL because because they have low ISP, and therefore have to carry significantly more fuel to make the return trip.
 

Offline DarkenedOne

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Re: Jet engine stage
« Reply #11 on: 06/15/2012 02:49 pm »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

First of all you would not need 500 tonnes of rocket where 95% of it is fuel if your engines had an ISP of 3000 seconds. 

going to mach 2.2 at 20km means you still have like 7.5km/s left in order to reach orbit., or going to mach 3.2 at 25km means you still have like 7.2 km/s left to reach orbit.

You still need big rocket second stage with lots of fuel.

So you will need tens of tons of those jet engines.

From my calculations accelerating to mach 2.2 at 20 km would knock off about 1.3 km/s off the total delta-v.  That is only taking in the altitude and speed into account.  It is actually more than that due to air resistance and gravity drag that I am not taking into account.

A decrease of 1.3 km/s results in a 25% mass decrease to a rocket with an ISP of 450 s.  A decrease of 1.3 km/s results in a 31% decrease for rockets with ISP of 350s. 



Quote
Quote

Second of all at that scale one would build larger engines with higher T/W.


What makes you think that if you just build a bigger engine, you get magically better T/W?


The jet engines we have all been talking about using are aviation jet engines meant for long distance travel.  Even the military jet engines like the ones on the F-22 and the Blackbird have to strike a balance between having lots of power when they need it and having enough fuel efficency to travel thousands of miles.  If we were to make an analogy to cars the airline jet engines would be the ones in your average consumer vehicles, and the military jet engines would be the ones in sports cars. 

If a jet engine was specifically designed for launching rockets than it would be analogous to a drag race car engine.  That is it would be designed for extreme thrust and power for the few minutes it takes the system to reach the upper athmosphere.   

So what I am saying is not necessarily that bigger engines get better T/W, but that engines designed for this purpose would definitely get better T/W.

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Quote
Lastly the benefits in maintainability will likely be more cost effective in the long run.

What's the difference in maintainability between jet and rocket engines? where does it come from?



Its kind of an unfair comparison sense most rocket engines except for those used in space are designed to operate a few times than be thrown away.  If a rocket were designed to operate like a jet engine maybe it would be as maintainable.  All I know is that previous ones like the SSME had to be taken appart and reassembled after each use, which eliminates a large amount of the benefit of them being reusable.

Quote
Achieving T/W of >1 with jet engines (needed for wingless flight) just makes the engines too big and expensive. With jet engines, it makes more sense to use wings, which allows using like 3.5 times less thrust, meaning 3.5 times lighter engines (xb-70 has T/W of 0.32, Concorde has T/W of 0.37, sr-71 has T/W of 0.4 )

Its a trade off.  Wings give you lift, thus allowing you to get off the ground with T/W >1, but as you try to go faster they cause drag, which is why you start to see smaller wings on faster vehicles. 

Offline Jim

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Re: Jet engine stage
« Reply #12 on: 06/15/2012 03:06 pm »

If a jet engine was specifically designed for launching rockets than it would be analogous to a drag race car engine.  That is it would be designed for extreme thrust and power for the few minutes it takes the system to reach the upper athmosphere.   


And that is where your idea falls apart. That is where the jet engine becomes a rocket engine.    The SSME is two jet engines with a common afterburner. 

Using jet engines just up to 20km is a waste also.
« Last Edit: 06/15/2012 03:10 pm by Jim »

Offline Nomadd

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Re: Jet engine stage
« Reply #13 on: 06/15/2012 05:05 pm »
 What's SpaceX spending on each M1D? $2 million? More thrust than the biggest jet engine and much smaller size and weight at 1/10 the price of a GE90-115. You'd save the weight of some oxygen in the first parts of the flight, but pay for it with a much heavier structure toward the end and during recovery. And have $200 million in engines instead of $20 million.
 Stuff like Stratolauncher isn't really about efficiency. It's about flexibility. If you want to put more weight up, it's a lot easier just to make a bigger rocket than the enormous complexity and expense of making one with jets.
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Offline vulture4

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Re: Jet engine stage
« Reply #14 on: 06/17/2012 07:39 pm »
Rockets have much higher T/W ratio but higher fuel consumption, so are limited to <10 min total burn time. Jets can run for hours but are heavier and only efficient within a narrow range of speed and altitude.

A rocket that takes off vertically is above jet altitudes in 3 minutes so there is no way a jet can compete. Jets are only feasible when the first stage is an aircraft with wings and aerodynamic lift. The aircraft and the rocket-propelled upper stage can separate either at subsonic or supersonic speed. All current airlaunch concepts separate at subsonic speed (Pegasus, etc) because the advantage of supersonic separation is modest (altitude is more important in the atmosphere), because there are no operational supersonic aircraft large enough to carry an orbital upper stage with a useful payload, and because separation from a large payload at that speed can result in a collision, as occurred with an SR-71 derivative intended to launch a ramjet-powered drone. That said, the Skylon and tge German Sanger staged rocketplane concept are still worth considering.


Offline tnphysics

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Re: Jet engine stage
« Reply #15 on: 06/18/2012 03:10 am »
With ordinary jet engines limited to mach 3-ish...forget it.

However, not all jet engines have this limitation. By running the combuster fuel-rich (rather than air-rich), using a high pressure ratio in the bypass air, and burning the fuel-rich combustion products with the bypass air, mach 6 can be achieved. This speed is high enough for ignition of a scramjet-and those can reach mach 15. You are now 2/3 of orbital velocity-without having to light a single rocket.

Online FinalFrontier

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Re: Jet engine stage
« Reply #16 on: 06/18/2012 03:16 am »
Hmm ... My rocket will mass about 500 tonnes, and I am looking for first stage ... Jet engine, T/W 5 or 6. Need 500 tonnes of thrust, Lets see ... Oh wow, I only need 100 tonnes of Jet engines ....

NOT

First of all you would not need 500 tonnes of rocket where 95% of it is fuel if your engines had an ISP of 3000 seconds. 

Second of all at that scale one would build larger engines with higher T/W.

Lastly the benefits in maintainability will likely be more cost effective in the long run.
\

1. First of all you would not get the mach numbers or T/W you needed out of going bigger, that is not possible with jet engines, not on the scale needed.

2. They would have a lower T/W and lower mach number courtesy of larger turbo-machinery, so that falls flat.


People have considered doing this in various studies over nearly 50 years of spaceflight, its never been considered practical and never will be.
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Online FinalFrontier

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Re: Jet engine stage
« Reply #17 on: 06/18/2012 03:20 am »
With ordinary jet engines limited to mach 3-ish...forget it.

However, not all jet engines have this limitation. By running the combuster fuel-rich (rather than air-rich), using a high pressure ratio in the bypass air, and burning the fuel-rich combustion products with the bypass air, mach 6 can be achieved. This speed is high enough for ignition of a scramjet-and those can reach mach 15. You are now 2/3 of orbital velocity-without having to light a single rocket.

Works for scram-jet/jet combo stage but it is not possible to use it on a large enough scale for what is proposed in the OP. Might work for a small sat/microsat launcher but that is about it.

Also, an "all jet" stage as proposed in OP cannot generate the T or T/W needed. You make it bigger to get more thrust, T/W+Mach top end drop, you go smaller for more mach, you lose thrust. And as I said above, combo's are limited in size as well for similar reasons.


There is a reason why this has not been done before.
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Offline RanulfC

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Re: Jet engine stage
« Reply #18 on: 06/18/2012 05:56 pm »
Well it's nice to see the subject being brought up again though I wonder what set this one off? :)

Go design your vehicle and come back when you have something to present. Then we'll discuss it.
Ok, "I" didn't but I DID post a design and got very little "feedback" on it that I recall. Overall the concept was sound and the economics worked out. Boeing just didn't decide to proceed with the vehicle...

Oh and then there is "Charon" from Blue Origins, from what I read it has four (4) RR-Olympus engines on it which makes a nice initial platform for testing.

Is there a physical law that makes jet engines less maintenance-intensive than rockets?  It's not like jets lack complex moving parts.  In fact, one can make the case that jets are more complex than some rocket engines (especially pressure-fed designs). 

Rocket engines are also reusable, or nearly so--some engines ran several times their actual flight burn times while on test stands.  It's recovery that is the difficult part, actually getting the engine back to maintenance workers.  And putting jet engines on the first stage doesn't address the recovery problem any more effectively than having pressure-fed rockets, or even SSMEs, on a winged stage would. 
Ya, you MIGHT want to check into that information before you rely TOO much on it...

The TOP rocket engines are rated in SECONDS of operation, the maximum I know of is a bench-test RL-10 which ran for for something like 30 minutes before it crapped out.

The WORST jet engine in the world is rated in HOURS of operation, with the majority able to operate for hundreds of thousands of hours with little or no major maintenance. And the maintenance itself is much easier and less expensive to perform.

Recovery and reuse is actually enhanced with a jet since it can easily fly back to a vertical landing as proposed for rocket stages, but unlike those rockets and their associated stages a jet powered version is much easier to maintain and quicker to turn-around than a rocket version.

Even using jet engines as replacements for the current versions of launch assist SRBs such as the GEM series would allow a quicker and less expensive turn around as well as higher flight rates.

Unfortunatly THAT is the sticking point at the moment. There is neither incentive nor interest in gaining higher flight rates.

Achieving T/W of >1 with jet engines (needed for wingless flight) just makes the engines too big and expensive. With jet engines, it makes more sense to use wings, which allows using like 3.5 times less thrust, meaning 3.5 times lighter engines (xb-70 has T/W of 0.32, Concorde has T/W of 0.37, sr-71 has T/W of 0.4 )
Actually VTVL with jet engines is quite possible and has been examined extensivly in the past.

I don't know what rocket you're designing that mass' "500-tonnes" without a first stage (1,102,311lbs?) but it doesn't sound like it would be designed for such operations and what you WANT is a vehicle that is.

Wings are nice for some things and Air-Launch-To-Orbit has some major advantages over fixed launch sites that make it both operationally and economically attractive. But I'll note again there isn't anyone seriously looking at ANY of the reusable concepts because of the lack of flight requirements.

As for T/W there are a number of engines available with postive numbers and they are not so expensive as you'd think. More so as the militaries of the world retire high-hour engines which become available on the open market.

The standard Olympus engine as used on the "Charon" have a total thrust output of around 11,000lbs (for a total of 44,000lbs) while the after-burning modesl can reach over 38,000lbs.

More fuel efficent and modern the Pratt-&-Whitney F100 engine since retired from the F-16/F-15 runs between 17,800lbs (dry) to 29,160lbs with after-burner. You can double these numbers by the way with a simple water and/or LOX injection system to densify the air at the intake.

As an added benfit to that pre-compressor injection tends to "fool" the engine into thinking its operating at a lower altitude and Mach so for most of the above example engines which normally top out at around Mach-2 and 80,000ft they can still operate at positive values above 100,000ft and Mach-4.

The hardest part is vehicle design as air-breathing system needs to pay strict attention to intake paths and shock interactions at high Mach speeds.

many threads on this.  Answer is no.
Ahh Jim, always so verbose :)

First part is correct, second part is opinion, mostly yours :)
That ACTUAL answer is "it-depends" on a lot of the assumptions and design decisions one is making as well as what your trying to acomplish.

Working the numbers for a Falcon-9-ish first stage you don't come up with the same parameters as the current one or the proposed "Boost-Back" VTVL stage so saying it would be "cheaper" is misleading.

The current Falcon-9 stages the first stage around Mach-10 at much higher than 100,000ft so using an air-breathing first stage won't get the needed performance for a direct replacment. The entire vehicle design would need to be finesed to come up with an optimal solution.

Darkened one; I suggest you simply take the Falcon-9 Second stage and payload and figure from that point.

If a jet engine was specifically designed for launching rockets than it would be analogous to a drag race car engine.  That is it would be designed for extreme thrust and power for the few minutes it takes the system to reach the upper athmosphere.   
Well it's NOT the jet that's going to have to be specifically "designed" but the intake and exhaust system as well as how that all interacts with the rocket vehicle itself. It's why simply "strapping-on" jet engines (the easiest and least expensive method) is only really useful to about Mach-2 and about 50,000ft (@15.4km) because after that the whole vehicle has to be redesigned around the jet engines.

Your most use for such type engines is about the first 60-90 seconds of flight from the pad to Mach-2 and 50Kft, after which the rocket engine throttles up and the engine pods are jettisioned for recovery.
(Which if anyone is interested pretty much consists of guided para-foil recovery, visual inspection for damage and then they are ready to go)

F100s were 'baselined' for the original concept and the whole "pod" including recovery gear, fuel, avionics and engine totalled around 6000lbs. And of course each 'pod' generates a little over 29,000lbs thrust...

And that is where your idea falls apart. That is where the jet engine becomes a rocket engine. The SSME is two jet engines with a common afterburner.
Not even close but i'm sure you know this. The SSME is NOT a "jet" engine as it has to carry ALL it's propellant load on top of everything else. The whole point to a jet engine is that it does NOT carry any oxydizer, or at least a lot less than an equivelent rocket engine.

What he's talking about is "tweeks" that can be done to enhance jet engine performance for the short (60-90 seconds) that it has to operate at peak performance during a launch sequence. I've already mentioned water/LOX injection, (the LOX though is more for stabilizing the afterburner at high altitude rather than combustion augmentation) which would get a doubling of thrust from a standing start. (So the above example29,000lbs becomes around 58,000lbs static) This would increase the jet-pod mass to around 8,000 to 9,000lbs with the injection system and water tank.

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Using jet engines just up to 20km is a waste also.
Jim? 20km is only about 66,000ft airbreathing is NOT a "waste" at all unless you insist on thinking in only "rocket" terms. (Which is the only basis for the insistance that jet-engines for space launch won't work :) )

What's SpaceX spending on each M1D? $2 million? More thrust than the biggest jet engine and much smaller size and weight at 1/10 the price of a GE90-115. You'd save the weight of some oxygen in the first parts of the flight, but pay for it with a much heavier structure toward the end and during recovery. And have $200 million in engines instead of $20 million.
Hey that's quite the observation! Space-X is spending $2-million dollers per engine and then throwing them away. So for 10 two-minute uses they have spent $20-million dollars. A used, high-time F100 was quoted at around $5-10-million dollars which means they still have around 800+ HOURS of lifetime in them. So by using a jet engine you just pointed out they could be spending HALF of what they do now and doing so with less maintenance and per-engine expenses!

That IS what your point was, wasn't it? :)

You seem to think that jet-engine = heavier structure somehow, well I suppose since the idea IS to recover the engine for reuse then YES it would be a bit heavier.

The F100 weighs in (dry) at 3,740lbs while the Merlin-1 is about 1,380lbs (again dry) which makes it about 1/3rd of the weight.

Not bad though I'd point out that while the F100 is still usable over and over again for a couple of hundered HOURS the Merlin-1 is still less than 30-minutes of lifetime ASSUMING reuse. Something to keep in mind.

But an engine is not a full system, what does it take to operate a Merlin-1 for its full run? Well it takes (at a minimum) around 3000lbs and 6000lbs of tank, structure and support for a SINGLE engine. Probably towards the lower end with multiple engines which can share some of the assets.

But to that you have to add somewhere between 47,000lb to 90,000lb of propellant which brings your "structure" for that single engine to somewhere between 50,000lbs to up to 96,000lbs or more.

Ok what about our jet engine? Well as noted we need the 3,740lb engine, then we'll need at least 1000lbs of fuel for operation (assume for the moment "non-powered" recovery as per the Merlin in a Falcon booster) which means we'll still need tankage, recovery, gear, support structure and thrust and thrust transfer structure.

In 1995 Boeing engineers calculated the total required weight of this equipment which includes module structure, landing legs, parachute, fuel tanks, mounting structure, interface structure, thrust and transfer structure and avionics to be a little over 1300lbs. This of course is for a single engine you can actually save a lot by intergrating multiple engines into a single thrust structure and airframe but for our comparision purposes this will do.

Now you'd be right to point out that the single Merlin has a thrust of 125,000lbs while my jet pod only has a thrust of a mere 29,000lbs meaning I'd have to "gang" five (5) of these pods onto a (for example) Falcon-1 to achieve lift off and flight similar to the installed single Merlin engine. Of course my 5 pods generate a total of 145,000lbs of take off thrust and deliver the whole Falcon-1 vehicle to Mach-2, 50,000ft, at an angle of around 32 degrees to the hoizon WITH a full propellant load. Which translates roughly to around an increase of about a third in overall payload to orbit for the vehicle. And I've only "used" about 90-seconds of the hundreds of hours I have left on my engines.

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Stuff like Stratolauncher isn't really about efficiency. It's about flexibility. If you want to put more weight up, it's a lot easier just to make a bigger rocket than the enormous complexity and expense of making one with jets.
No it's not JUST about efficiency but it's still a factor because the oft cited "complexity and expense" of a jet powered stage is one of those "well-known-facts" that is more myth than reality.

Rockets have much higher T/W ratio but higher fuel consumption, so are limited to <10 min total burn time. Jets can run for hours but are heavier and only efficient within a narrow range of speed and altitude.
Ahhh, someone who "gets" it I see...

One issue is that jets are indeed only 'efficent' over a narrow range and speed but so to are rocket engines in many senses. A rocket optimized for sea-level does not perform as well at high altitude and vice-versa as is well known. But there are ways to tweak that performance and the same holds true to jet engines. It becomes a study in trade-offs for benifits.

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A rocket that takes off vertically is above jet altitudes in 3 minutes so there is no way a jet can compete.
Try closer to a minute or less :) But jets CAN be used up to speeds in excess of Mach-4 (turbojets) Mach-8 (ramjet) or even speeds in excess of Mach-12 (scramjets if they can get them to work consistantly) and altitudes of up to 200,000ft or more. It just depends on how much trouble you want to go to.

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Jets are only feasible when the first stage is an aircraft with wings and aerodynamic lift.
No though you can lift a greater amount of "rocket" with wings you don't NEED them to be feasible or economic. One of the hardest things for people to wrap their minds around is with jets you actually get closer to "aircraft" operations which means more flights per DAY rather than looking towards finally getting rocket operations to a few times a MONTH.

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The aircraft and the rocket-propelled upper stage can separate either at subsonic or supersonic speed. All current airlaunch concepts separate at subsonic speed (Pegasus, etc) because the advantage of supersonic separation is modest (altitude is more important in the atmosphere),
Correction there, speed IS the most important aspect of Air Launch and supersonic speed is greatly better than subsonic. In order following that are Launch Angle and Altitude as important aspects. You go on:
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..because there are no operational supersonic aircraft large enough to carry an orbital upper stage with a useful payload, and because separation from a large payload at that speed can result in a collision, as occurred with an SR-71 derivative intended to launch a ramjet-powered drone.
The biggest issue with supersonic launch is cost. Supersonic platforms cost a lot to develop and maintain where as subsonic, high payload aircraft are common and have a very widespread and robust support infrastructure to call up for reduced operations costs.

NOTE: The SR-71/D-21 collision is highly overplayed as a "negative" in fact supersonic seperation is well proven and safe as a procedure. The ONLY reason the drone collided with the mother ship is a malfunction on-board commanded a "Dive" moments after seperation and there was no collision avoidance equipment on board the drone.

As to subsonic versus supersonic if one can't have speed then THE next most important aspect is the launch angle when the LV is fired. Pegasus and most other plans have a very shallow or no 'angle' above the horizon at launch and they therefore pay a large penalty in order to mover from horizontal to a more vertical angle. T-Space/AirLaunch got around this issue with the Trapeze-and-Lanyard system while from what I've read StratoLaunch may be looking at igniting the rocket motor and using that to pull up to a high AOA (Angle Of Attack) prior to seperation. In either case this is going to have a large impact on total payload to orbit over horizontal, low AOA Air Launch To Orbit (ALTO) concepts.

People have considered doing this in various studies over nearly 50 years of spaceflight, its never been considered practical and never will be.
You forgot to add the rest of that statement:
"As long as current launch rates, and space access requirements remain low"

The majority of studies have pointed out that at ANY increase of the need for access to space OR higher flight rates it makes both economic and practical sense to undertake more work in intergrating air breathing systems into launch systems. It makes practical, realistic, economic, and realiable space launch much easier to achieve. But until the "need" is actually there current launch systems will remain the only "viable" option.

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There is a reason why this has not been done before.
That reason would be simply because "rocket" folks won't look at jets as practical, and most "jet" folks won't look at anything that doesn't have wings as practical. Nothing in the science or engineering has shown it to be more difficult and complex than many of the concepts advanced by both "proponets" of versions of their own "pure" systems. :)

The problem (as shown in this thread) is that "rocket" folks insist on huge payloads (500-tonnes? Really? What the hell for? What's the "payload" fraction in that much mass?) and anyone who consideres "jets" insists on wings and all the associated hardware those concepts include and neither will "bother" to seek any compromise.
(Why should they after all? As in politics compromise seems to have become a dirty word :) )

The original Boeing concept put around 6,000lbs into LEO a figure I suspect could be bettered today. The idea behind Jet-Engine-Launch-Assist-Concept (JELAC) put up around the same but could be much more expanded by the addition of ramjet operations.

All with "Off-The-Shelf" engines and hardware...

You can probably expand even more with scramjets but I personally don't see air-breathing past around Mach-8 as being that much more helpful given the amount of work that needs to be done to keep a vehicle together and useful at higher speeds inside the atmosphere. (YMMV :) )

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Jim

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Re: Jet engine stage
« Reply #19 on: 06/18/2012 06:54 pm »
FYI, rocket engines are jet engines.  Jet engine does not mean airbreathing.  So many other things wrong in the above post to enumerate, but M-12/D-21 incident among them. And using the Falcon 1 as a example, use a Falcon 9 instead and see where things come out.
« Last Edit: 06/18/2012 07:00 pm by Jim »

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