What about the possiblility of having a first stage that uses a couple of conventional jet engines strapped to the side of the core rocket. The idea would be that for the first minute or two of flight the jet would provide thrust. In an atmosphere, the ISP of a jet far exceeds any other chemical rocket and it can do it with good thrust to weight ratio. The down side is that after leaving the atmosphere, the jet is just dead weight. But by that point, it's already paid for itself. It can be jetisoned and recovered with the rest of the first stage.<br /><br />Any thoughts?
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a jet-rocket combination might work for vertical take-off but I don't think it would ever be as efficienta s a pure rocket.<br />However, if your first stage is airplane-like, you could use a rocket to gain necessary speed during take-off and then just jet engines for a large portion of the ascent...maybe even in-flight refuelling. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>
IIRC, jet engines don't develop full thrust until incoming air flow is able to produce a certain pressure ratio across the turbine blades. Lofting a large mass straight up seems unlikely, horizontal take-off is going to require an airfoil of some kind.<br /><br />And now you have a complicated mess.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
100,000 pounds of thrust is a pretty big jet engine. Lofting a 4,000,000 pound vehicle straight up is going to take 40 of them just to hover. You want to accelerate at 2 gees, make it 120 of them.<br /><br />Scarcely an elegant engineering solution.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
You got me curious about something I had forgotten, so I looked up what the thrust for the J58 was/is - about 34,000 pounds.<br /><br />Sorry, I know its a tangent...<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
Which got me curious about one other engine, the YJ-93 (the Valkarie) - it checked in with 28,000 pounds.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
The '6-pack' on the Valkyrie was pretty heady stuff in its' day. Plane could still do Mach 3 with 1 out.<br /><br />Early design concept called for borated fuel for more power. Emissions were staggering and it was never implemented.<br /><br />XB-70 was a sexy, sexy looking aircraft. Surprisingly (to me) had lots of stainless steel in the skin. (iron is just not what comes to mind when you first think about stuff to make an airplane out of.)<br /><br />Largest movable wing panels until (I think) B-1. The outer 1/3 of each wing could fold down 65 degrees to capture the sonic boom energy. Movable wing section larger than entire B-58 wing.<br /><br />My favorite plane. Saw the sole survivor in Dayton a few years ago, looks fast standing still.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
Steel is generally used because of one condition - heat.<br /><br />Some initial designs for the Blackbird used steel - before they made the decision to go to Titanium.<br /><br />I agree about the look of the Valkrie by the way, especially with the wingtips in the down configuration. It just looks like a futuristic plane - even today.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
pretty much everytghing designed in the 50s looks futuristic today 
we're going back to the stone age. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>
we're going back to the stone age. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>The Ford Nucleon concept car! <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
The General Electric GE-4 engines that were to be used on the Boeing 2707 SST put out 60,000lbs of thrust. I think they made it a fair ways through the development stage (ie: were being run on test stands). <br /><br />Instead of using a jet engine AND a rocket engine... why not just develop the combined cycle engine further? Whatever happened to that work?
I suspect there has not been a market for it, but I have no real data (or useful knowlege) on that.<br /><br />(Yeah yeah, I can hear it now, "But Wayne, how is that different from anything you talk about???")<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
I have wondered about this myself. There was once a research version of the F-104 that had a small rocket engine as well. I believe it got above 100,000 ft. So, say if you're a bit of a nutcase, what would happen if you combined a jet fighter from the sixties or seventies (there are a few in private ownership) with a couple of rocket motors (either solids or hybrids) slung on the (modifed) hard points, say 2,000lbs apiece. Climb to the mid-40's, hit the afterburner(s), then fire the rockets. You would need some attitude control, and some thermal protection, not to mention Air Force high altitude suits, but I wonder if you could break 60 miles? Any engineers want to comment?
There were in fact attitude control jets in the Starfighters you mention.<br /><br />General Yeager had one of his harrowing moments in one of those when he couldn't get it to pitch over to get a restart on his jet engine, and ended up in a flat spin that he could not recover.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
Yes, I read Yeager's story on that too. Not enough air for the control surfaces to work, but too much for the reaction control to overcome.
I love reading Yeager!<br /><br />You gotta love someone who tells his readers that one of the most useful skills for someone in wartime is the ability to fall asleep on command. I know if sounds like a joke - but it is valuable, and damn hard.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
He got that puppy down in one piece - I am sure I would have been crying and sucking my thumb....<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
<i>Which got me curious about one other engine, the YJ-93 (the Valkarie) - it checked in with 28,000 pounds.</i><br /><br />Kinda emphasises the difference between pure jet and turbo fan, the RR Trent engine in the 777 is rated up to 110,000lbs thrust!
><i>Largest movable wing panels until (I think) B-1. The outer 1/3 of each wing could fold down 65 degrees to capture the sonic boom energy.</i><p>They weren't exactly capturing the sonic boom energy, they were more surfing along the shockwave front: trans- and supersonic airflow is a strange phenomenon. The Concorde, for example, actually got most of it's thrust at supersonic speeds from the airflow through and around the engine inlets. They used reheat to get supersonic and then throttled down and just used the engines to <i>stay</i> supersonic!</p>
<font color="yellow">"it can do it with good thrust to weight ratio. "</font><br /><br />IIRC the best military jet engines have T/W little over 10. Compare that to rocket engines, ~50 for hydrogen and over 100 for hydrocarbons. In this light the jet's ratio is wanting, but then again this is apples to oranges comparison. The jet doesn't need LOX at all so there goes 3/4 - 6/7 of your first stage propellant weight. Rocket engines sea level Isp is usually somewhere between 300-350s (liquids) while jets are in 3000s neighbourhood, so there shrinks your fuel requirement too.<br /><br />Clustered jet stage is doable if you accept the fact that most of the stage's weight is engines and structure. You would want to try to tweak the T/W higher though. Perhaps there's a chance for running the engine briefly past usual redline without damaging it too much. Shorter total engine life is acceptable because stage flights would last only a few minutes and number of flights per stage is initially probably less than hundred. Using cryogenic fluid (methane) might offer chances for increased cooling capability. I've read somewhere that you could also spray some water to the inlet. While the water gets vaporized it cools the engine, adds to the total massflow and helps converting the heat to thrust. And there might be some auxilliary devices that could be stripped away to loose weight, like generators, starter devices and such. There isn't a cabin full of people needing light and other electrically driven amenities, just use batteries to drive flight avionics and external equipment to start the engines. My own intuitive wishful thinking hopes that these might enable T/W somewhere between 15-20.<br /><br />Anyway I think the engines would be cheap enough only if you do them as above, take ordinary jet sold in great numbers and do the tweaking yourself. Asking P&W, GE, Rolls Royce etc to develop one for you would skyrocket (pun! <img src="/images/icons/smile.gif" />) the cost bec
<font color="yellow">"would the thrust be significant enough to make any difference?"</font><br /><br />If the jet would only be for taxing back to launch site then it would be just additional mass and drag during ascent. IMO it would make much more sense to counter that mass and drag (plus giving some additional thrust) using Isp of thousands (the jet itself) than Isp of hundreds (the rocket). At least as high on the way up as the jet could operate.
Thank you Tap_Sa, you said it better than I could have.<br /><br />When I made the original post, I hadn't thought about the idea of using the jets for the flyback portion. I was still thinking about parachutes for the flyback. Land it on land to keep the salt water out.<br /><br />Part of the idea behind this was the fact that you might not want to get the velocity too high before going through Max-Q. So for this type of first stage, a lower thrust to weight ratio would help by reducing velocity at Max-Q. But with the high ISP of a jet engine and no need for LOX on the first stage, the gravity losses won't matter that much if you take your time getting up to altitude.<br /><br />I just threw this idea out to see what people would come up with.<br /><br />The other alternative is to have a central core 1st stage of LOX/LH2 with just barely enough thrust to get off the ground. The jet engines would be mounted around the core to augment the thrust and reduce the gravity losses. By the time you run out of air and jettison the jet engines, you've used up some of your LOX/LH2 and the core is lighter. The LOX/LH2 won't be seeing such horrendous gravity loses at that point.<br />
Rocketdyne was developing a combined cycle engine a few years back. NASP and HOTOL were two applications that come to mind for combined cycle engines.<br /><br />Basically, it is a hybrid engine that uses oxygen from the atmosphere, like a 'jet' engine, at lower altitudes and then switches to internally carried oxidizer (like a rocket) at higher altitudes. <br /><br />Big turbofans like Trent and the GENx put out a lot of power but their thrust velocity is low and their large diameter means they produce a lot of drag (compared to a turbojet). The most power turbojet that was developed was the GE4 (60,000lbs) and it had the hallmarks of what you would be looking for: high thrust to weight (for a turbojet), a small cross-section (compared to the thrust) and higher exhaust velocity than a high bypass turbofan. I don't think it's unreasonable to suggest that growth version may have added another 20% or 25% onto that max. thrust rating. Even so, it would only be 75,000lbs AND you still have to haul up both it and the rocket engines. <br /><br />One other issue with the jet engines is that unlike a rocket engine which typically start in under 10 seconds, you would need perhaps 30 seconds or more to start a large turboject. Starting multiple large turbojets simultaneously would make for a cool challenge. <br /><br />So I think perhaps a combined cycle engine might be the best alternative to what you are suggesting.
<i>"... Rocketdyne was developing a combined cycle engine a few years back...."</i><br /><br />So was Pratt & Whitney, who now owns Rocketdyne, and Aerojet. The turbine-based combined cycle is probably best left to supersonic missiles and military jets for the forseeable future. They only work well in the presence of air.<br /> <div class="Discussion_UserSignature"> </div>
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