Turbofan launch vehicle

This question may belong in the space exploration catagory, but given the wealth of knowledge I've seen on this thread, I thought I'd ask here first:

Modern high-pass turbofan jet engines like the GE90 can produce up to 127,000 lbs of thrust. Strap 8 of these together and you could have a first-stage space launch booster producing, theoretically, a million pounds of thrust between ground level and 60,000 ft., albeit at a subsonic speed.

Expendable rockets of a similar thrust (Saturn I) could put up to 37,000 lbs. in LEO. Recoverable fan jets would seem to be a cheaper option.

What's wrong with this theory?

You answered the question yourself. They can only operate up to about 60,000 feet if that, maybe only 45-50 thousand. The required height to orbit a vehicle is what, 130 miles. The only point for this option is to save a slight amount of fuel. You would still need a marge 1st stage booster to get the job done. I can only think of one option like this, a booster aircraft capable of ramjet speeds which releases a hypersonic vehicle. But I think theres easier ways to get this done than a 2-3 part vehicle. I think landing and takeoff hypersonic vehicles is the next step, and then some type of small rocket engines or ion drives to power it in orbit. Thats just my thoughts.

Train

Point taken. The idea here is to put a rocket second stage with a idealized vacuum nozzle configuration in a position to boost the payload to orbit. From what I've read, most 1st stage separations occur under 120,000 feet at trans-sonic speeds, fanjets would be lower and slower, but reusable.

Once you talk about hypersonic or Sanger type 1st stages the costs soar higher than the vehicle because of the engineering considerations on the airframe.

There has to be another reason this won't work.

I disagree. The costs of development is no doubt higher, but the cost savings in fuel and non reusabvle rockets over time will be just like the shuttle program. The best alternative is to have a vehicle that can have a life of around 1000 missions, take off under own power and land under own power and be ready to fly again in hours notice. I believe we already have vehicles like this that are top secret of course. Rockets are outdated and can only operate with the massive fuel load. Nuclear propulsion and ion drives are thousands of times more efficient and can provide much longer orbit times. Rockets are also more dangerous to operate.

I like your idea of a high altitude release vehicle, but I just dont think its the right thing to do when greater technology is available.

Train

You may be right, but every day sub-sonic birds log millions of passenger miles and the Concorde is a thing of the past.

A great source of information on space,
www.encyclopediaastronautica.com
has a great compilation of ideas from Zubrin's Black Horse to the operational air-launched orbital platforms; there must be a technological obstacle to a recoverable turbojet 1st stage for lower-atmosphere propulsion, former NASA chief O'Keefe hinted at it in his last congressional testimony.

As to what's secret, I hope there is a lot, I suspect there is very little.

There is a problem - horizontal velocity component.

A turbofan cannot impart the needed horizontal velocity component onto the payload to get it to a useful orbital speed - for example the Shuttle travels at Mach 25 and that isnt even in a high earth orbit, its well below that which geosynchronous satellites are at.

There are experimental launch vehicles which involve dropping a rocket from a 747 or similiar vehicle at high altitude, but these are only used for extremely low orbits and thus very shortlife satellites.

There is an operational vehicle called Pegasus which is air launched from a L-1011 and can put up to 500lbs. in a durable low earth orbit.

Again, I'm not talking about the turbofan 1st stage lifting payloads to orbit, just getting wieght off the launch pad and above the thickest portion of the atmosphere so a rocket 2nd stage can work efficiently in a vacuum to accelerate to escape velocity.

It would seem an engine that doesn't need to lift it's own oxidizer but can burn what's in the air around it would make sense, I still suspect there is a reason that won't work but haven't heard it yet.

TurboFans Wont work!!
Firstly if you want to take an aircraft to around say 60,000 feet the thrust would decrease the higher you go, i.e. the higher and faster you go the thrust generated decreases almost parabolicaly and reaches a particular critical value and stays at that value irrespective of the increase in speed. Also if you rig say- six Rolls Royce EJ200's you would probably get insane amount of thrust but also the overall weight of all the six engines would be much higher than a simple rocket motor arrangement. The effective altitude of operation would also decrease and that would put more load on the secondary rocket booster to compensate. Moreover for an high-bypass turbo fan to be effective it should be in the horizontal position as the incoming air provides significant advantage to the bypass thrust and also decreases the load on the main rotor. Here is a representation of Mach No Vs thrust for a FJ44 type engine.


Lastly, the amount of fuel required increases as the altitude increases. This is due to the fact that at higher altitudes the air being rarer inhibits combustion drastically and thus more fuel is required to produce the same amount of thrust as some fuel is wasted due to incomplete combustion. The Specific Fuel combustion ratio of Fuel plays an important role in the efficiency of the engine. The weight of Fuel Tanks (which are properly installed and ‘safe’) causes great load compared to a rocket motor. Also the cost of fuel for these engines inhibits the feasibility of such a endeavor. In a conventional solid rocket motor the fuel used is Nh4ClO4 which is comparatively cheaper than 99% octane.

Thanks, IAF, great info!

Originally posted by Realist05
Point taken. The idea here is to put a rocket second stage with a idealized vacuum nozzle configuration in a position to boost the payload to orbit. From what I've read, most 1st stage separations occur under 120,000 feet at trans-sonic speeds, fanjets would be lower and slower, but reusable.

Once you talk about hypersonic or Sanger type 1st stages the costs soar higher than the vehicle because of the engineering considerations on the airframe.

There has to be another reason this won't work.

A major reason why, you could say for simplisticity, a space shuttle doesn't strap on turbo-fans is because of the same reason why you want to minimize surface area in flight, Shock-waves. Inside the engine the air is also traveling through faster or close to the speed of sound, so shockwaves are created actually inside the engine, on the compressor blades, everywhere. As you can imagine this is not good.