Jet Engines as 1st Stage cluster

[rocketwatcher2001]

Not to sound overly picky, but Rockets are Jets, atleast they are in the jet engine family. If I understand you correctly, what you are asking is whether or not it would be better for the launch vehicle to save a little weight by using atmospheric O2 for the first 50-100 thousand feet.<br /><br />The short answer is NO, LOX has so much more oxidizing ability than any practical amount of air/O2 that you could squeeze though an atmospheric intake. There is no practical way of feeding enough air/O2 to the combustion chamber to burn the huge amount of fuel required to provide the huge amount of thrust to lift a big launch vehicle. <div class="Discussion_UserSignature"> </div>

 

[propforce]

Thank you for that concise to the point explanation <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[scottb50]

Having turbofan engines that operate from lift off to near Max-Q then shut down, then restart at lower altitudes on the return of a first stage would be very feasible. If nothing else their thrust would offset the weight of the engines, structure and fuel.<br /><br />Once the upper stage is released and the first stage descends to 25,000 feet or so, they are restarted for a return to base. <br /><br />If your premise is taking them to orbit then I would say that's not a very good idea at all. <div class="Discussion_UserSignature"> </div>

 

[rocketwatcher2001]

Scott-<br />If you already have LOX, why would you want a gas-turbine generator? A low power rocket would provide all the power that a gas turbine engine could without any additional stuff, like another type of engine and everything that goes with it. <div class="Discussion_UserSignature"> </div>

 

[barrykirk]

First, this is for first stage only.... Certainly these engines would never shut down. As soon as the thrust to weight ratio drops to below 1, the jet engines, or the jet engine stage/platform drops off for a flyback.<br /><br />So, a legitimate question is this.<br /><br />Which weighs less?<br /><br />1) frame including a jet engine and all associated hardware and fuel to run it for say 2 or 3 minutes.<br /><br />2) frame including a rocket engine and all associated hardware and fuel and LOX to run it for say 2 or 3 minutes.<br /><br />Both frames should have the same thrust output.<br /><br />If the choice 2 weighs less, than I would say that most of the merit for a VOTOL jet engine stage goes away.

 

[spacelifejunkie]

"Jet Engines as a 1st Stage Cluster"<br /><br />Isn't this basically what Burt Rutan is doing with Spaceship One/Two? The jet engine stage is the White Knight lifting vehicle with the rocket kept off until reaching altitude. t/Space has an interesting design using a Burt Rutan White Knight like lifting airplane to drop a rocket into the vertical position before turning on. The rocket/jet combination is being tried and these two companies may have the only plausible ideas on actually making it work. The only exception to my previous statement might be the perfection of a pulsed detonation engine which gives rocket like thrust while using less fuel than even typical jet engine. Nice! We'll wait and see on that development. I think GE and Pratt & Whitney are due to have a working PDE soon.<br /><br />SLJ

 

[scottb50]

f you already have LOX, why would you want a gas-turbine generator?>><br /><br />I would use turbo-fans for the fly-back first stage mainly, even a low power rocket wouldn't be much good for that. <br /><br />As far as using it for approach and landing you might as well use it to offset it's mass during initial launch. If you use turbofans they will only be good to Max-Q, or so, but you need to lower the acceleration anyway and that would maybe give the main engines a higher idle capability. <br /><br />If you figure your return vehicle at about 200,000 pounds, or so, a couple of 50,000 pound thrust turbofans would work pretty well. You couldn't take off, but you don't need to anyway. Primarily they would allow a stabilized approach a go-around capability and cross-range cruise, depending on fuel capacity restraints. <div class="Discussion_UserSignature"> </div>

 

[davf]

But wait! We can build a complex control system to handle that sort of contingency and call it KORD! <img src="/images/icons/wink.gif" />

 

[rocketwatcher2001]

<font color="yellow">I would use turbo-fans for the fly-back first stage mainly, even a low power rocket wouldn't be much good for that.</font><br /><br />Turbofans are more efficent, sure, but I can't imagine it being worth the added weight/complexity. A low pwered rocket would be a lot simpler/lighter because it has less parts, less drag because of no intakes, and less complex because a hypersonic/supersonic/transsonic/subsonic air intake sounds like a alot more of a headache than a few extra gallons of LOX which is required to be onboard anyway. <div class="Discussion_UserSignature"> </div>

 

[barrykirk]

The Burt Rutan and t/space ideas were the motivating factor for me starting this thread in the first place.<br /><br />So, how much lift can an airplane and or lifting body develop? <br /><br />How much thrust is required to generate that much lift? <br /><br />How high in the atmosphere can that much lift be developed?<br /><br />The idea is to start the rocket stages at high altitude and moderate velocity to reduce the total energy required by the rocket stages to get to orbit.<br /><br />Is this a cost effective idea?<br /><br />I don't know.

 

[rocketwatcher2001]

Until there is a breakthrough in materials science, big rocket powered launch vehicles, lifting off nearly straight up until they clear the bulk of the atmosphere before pitching over nearly level with the horizon to build up orbital velocity is the most practical way of launing big and heavy payloads into space. <br /><br />Now, what I'd really like to see developed is a mach 5, 1,000,000 lb gross weight launch aircraft(a little bigger than a new 747) that also doubles as a trans-Atlantic frieghter/passenger hauler, cruising at 120,000 ft, with a 100,000 lb orbiter(half the size of today's Space Shuttle) launched from the top of it. The only thing holding this up is we don't know how to make materials that we can make these kinds of machines out of. <div class="Discussion_UserSignature"> </div>

 

[stex6109]

Not only is is feasible, but it is also very cost effective in the long run. Though this idea is nothing new and has been floating around since jet engines became mainstream in the 60's. Turbofans are the generally accepted way too go, do to the obvious reasons. They are much larger than turbojets and therefore produce more aerodynamic drag, but this is not an issue as the idea is for the the jet-powered first stage to travel rather slow compared to rocket powered first stages. The idea is simply to use the jet-powered stage to slowly but steadily lift the upper stagestages up and out of the thicker portion of the atmosphere. Separation word likely occur around 40,000 feet, at relatively slow vertical speeds. In essence giving to upper stages a 40,000 foot starting launch pad. The entire first stage can be completely recovered and reused with little to no maintainence. This method is generally understaood to be used solely for small economic launchers, (under 1500 lbs. to LEO) and is not suitable for heavy-lift vehicles. A single turbo-fan for instance could easily lift a fully fueled Falcon 1. An example being the GE90-115B turbofan from General Electric, which produces 115,000 lbs thrust at sea level and only weighs 18,000 lbs. The first stage of the fully-fueled Falcon 1 only produces about 75,000 lbs. thrust. At the writing of this post there is currently and small launcher being devoloped that will hopefully see flight-time in the coming decade.<br /><br />Early precursor work is being done on advancing the concept. More info can be found here.<br /><br />http://www.geae.com/aboutgeae/presscenter/other/other_20020829.html

 

[rocketwatcher2001]

I can't imagine a turbofan powered lifting launch platform being practical in any way. If you want to use turbofans to get you to 40,000 ft, the Orbital Science's L1011 launched Pegasus looks like the way to go. The problem with that is it can't put anything bigger than a breadbox in orbit.<br /><br />The link you provided was cool, thanks. I'm very excited about the mach 4 engines that are under development, but they are nothing like the big turbofans on today's modern airliners. The GE90-115B turbofan you mentioned has a fan diameter of 115 inches, that's what the 115 means. That's about 20 feet in diameter by the time it's cowled up, and that just gives you about 10% of the amount power that a SRB of the same diameter gives you, not to mention that the shrouded turbofan is limited to subsonic flight. The mach 4 engines will probably provide a lot less power, but will be much smaller, and may have a future as a flyback booster, but I can't see the advantage of launching vertically. It seems to me that a mach 4 aircraft acting as the first stage is the way to go. Sort of like the L1011/Pegasus but bigger, and faster. <div class="Discussion_UserSignature"> </div>

 

[dobbins]

The primary advantage of air launch is gaining altitude before launching the rocket avoids the aerodynamic stresses it would encounter at high speed in the lower atmosphere. This means you can design a rocket with a lighter airframe allowing for increased payload. You lose this advantage by going supersonic.<br /><br />Ideally what you want in an air launch system is to gain as much altitude as possible rather than speed. Air speed at launch is a nice little bonus like the Earth's rotational speed at latitudes closer to the equator, but the most important point is getting the lowest possible aerodynamic stresses on the rocket so you can have the lightest airframe possible. A launch at 600 MPH at 60,000 feet is better than one going faster at a lower altitude.<br /><br />Aircraft are capable of carrying a heavier load aloft than a Pegasus, NASA uses a 747 to ferry an unloaded Shuttle cross country, and that is a considerable weight. A C-5 can carry an even heavier load as can a Russian An-225. None of these are optimized for a mission of air launch, a plane that was purpose designed to do this should be able to perform this task far better than any existing aircraft.<br /><br />To get full advantage out of air launch you need a system, a plane that is designed for the task and a rocket that is designed for air launch. With that system you have in effect a reusable first stage, and one that doesn't require any exotic and expensive technology to maintain. You can even farm maintenance out to someone like an Airline until you reach the point where you have a launch rate high enough to make it economically viable to have your own maintenance facility.<br />

 

[rocketwatcher2001]

<font color="yellow">The primary advantage of air launch is gaining altitude before launching the rocket avoids the aerodynamic stresses it would encounter at high speed in the lower atmosphere. This means you can design a rocket with a lighter airframe allowing for increased payload. You lose this advantage by going supersonic.</font><br /><br />The weight savings are minimal, and hardly worth it, compared to the cost and complxity of a subsonic air launched system/vehicle of any real payload capacity. The main weight of any launch vehicle is the fuel/oxidizer, not the airframe. Using a high altitude, subsonic first stage launch vehicle seems to me like dropping a dollar to pick up a dime. <div class="Discussion_UserSignature"> </div>

 

[dobbins]

There isn't a magic bullet that is going to solve the problem of high launch costs. It's going to have to be nickel and dimed to death. That means a lot of "hardly worth it" things are going to have to be tracked down and taken care of. 10 dollars per kg here, 20 dollars per kg there, until the small savings add up to bigger reductions.<br />

 

[barrykirk]

The other advantage of air drop or jet assist or jet ascent to launch altititude is that rocket engines lose a considerable amount of efficiency at sea level. (The ISP is considerably lower).<br /><br />You can optimize a rocket to be more efficient at sea level, but it loses efficiency in a vacuum.<br /><br />By starting your rocket at 60,000 feet, the pressure should be low enough that you can use a vacuum optimized rocket engine. So that is another bonus for launching at altitude.<br /><br />So, so far I've seen.<br /><br />1) Lighter airframe (because less air drag stress)<br />2) More efficient engine design possible.<br />3) Slightly less energy required to reach orbit because of higher starting point.<br />4) Less energy lost due to air drag.<br /><br />Remember that if the energy required to reach orbit goes down by just a little, the payload goes up.

 

[rocketwatcher2001]

<font color="yellow">It's going to have to be nickel and dimed to death. That means a lot of "hardly worth it" things are going to have to be tracked down and taken care of. 10 dollars per kg here, 20 dollars per kg there, until the small savings add up to bigger reductions.</font><br /><br />I respectfully disagree. I think it's going to take a revolutionary step, rather than many evolutionary steps. Much like the 707 was a step over the Stratocruiser. If we are going to do airlaunch, hypersonic is the way to go. Sure the spacecraft needs a beefier airframe, but it will still weigh much less because the extra fuel needed to launch from subsonic 40,000 ft. is going to weigh 20 times (my guess) what the slightly beefier airframe will weigh. <div class="Discussion_UserSignature"> </div>

 

[dobbins]

5.) If flown due east a slightly higher speed advantage similar to launch at a lower latitude.<br />6.) Elimination of Acoustic stresses, the effects of the noise from the rocket being reflected off the ground back at the rocket.<br />7.) Able to launch when weather at the ground would have forced a delay.<br />8.) Better flight profiles by eliminating the purely vertical part of the early flight.<br />9.) Easier to achieve a desired inclination by flying to proper latitude.<br />10.) Less ground support infrastructure needed, can use an existing airport instead of needing launch pads.<br />

 

[rocketwatcher2001]

Please don't get me wrong, I think airlaunch is the way to go, but not subsonic, 40,000 ft. At SRB seperation, the space shuttle is traveling at mach 5, and is at appox 200,000 ft, if you could build an airplane to launch a slightly smaller orbiter, at the same speed and altitude, the whole spacecraft, including fuel to orbit, could be carried by a 747 size airplane. Of corse the trick is to make a 747 size airplane fly at mach 5...... If you can figure that one out, I'll buy you lots of beer. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

I'm pretty sure that no aeroplane could fly at 200000ft, particularly with a heavy rocket strapped to it. There's next to no oxygen for the engines, and virtually no air for lift.

 

[dobbins]

You can not ignore service costs. The Shuttle's weakest point is the service costs are very high to prepare it for the next mission. A subsonic aircraft used as a launcher will have service costs similar to those of a Jet airliner. The service costs of a supersonic launcher will be far higher. A subsonic launcher can have it's service farmed out to someone like an airline that has a volume of work high enough to drive costs down until such time as when there is a high enough launch rate to justify having a dedicated service facility for the space launchers. A Supersonic launcher's service can't be farmed out, and that will cause the service costs to soar until a high launch rate can be achieved.<br /><br />A second or third generation supersonic air launch system might work if the high launch rates needed to make servicing it economically viable exist when it's developed, but the first generation is going to have to be subsonic to hold down service costs.<br />

 

[rocketwatcher2001]

Actually what I was thinking was a scramjet dash and coast to that altitude. Some of the early Shuttle proposals come to mind. But you're right 200,000 ft is probably asking too much, but 150,000 seems do-able.....well, sort of. If we can accept that a 747 size airplane can fly at mach 5, than 150,000 ft doesn't seem so crazy either. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>

 

[rocketwatcher2001]

<font color="yellow">A second or third generation supersonic air launch system might work if the high launch rates needed to make servicing it economically viable exist when it's developed, but the first generation is going to have to be subsonic to hold down service costs.</font><br /><br />And we will be stuck with Micky Mouse payloads. Is this a chicken or the egg situation? <div class="Discussion_UserSignature"> </div>

 

[dobbins]

"Is this a chicken or the egg situation?"<br /><br />Yes it is. Those chickens and eggs keep haunting us. <br /><br />The STS was susposed to be a giant leap that would drive down launch costs. Instead it's high service costs increased launch costs.<br /><br />If we can get a reduction in costs for payloads below, say 5000 kg, with a better air launch system than Pegasus then we will be on the way to killing the chicken. <br />