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
Quote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTNOT 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.
Quote from: Lee Jay on 06/15/2012 03:32 amQuote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTNOT 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.
Quote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTFirst 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.
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.
Quote from: DarkenedOne on 06/15/2012 03:36 amQuote from: Lee Jay on 06/15/2012 03:32 amQuote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTNOT 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.
Quote from: DarkenedOne on 06/15/2012 03:32 amQuote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTFirst 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.
QuoteSecond 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?
QuoteLastly 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 )
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.
Quote from: aero on 06/15/2012 03:15 amHmm ... 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 ....NOTFirst 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.
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.
Go design your vehicle and come back when you have something to present. Then we'll discuss it.
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.
many threads on this. Answer is no.
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.
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.
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.
People have considered doing this in various studies over nearly 50 years of spaceflight, its never been considered practical and never will be.
There is a reason why this has not been done before.