Could a hybrid F-15/SR-71 space plane reach orbit with added PAM rockets?

[physicsCU]

I don't know if this would work or not.

What if you used one engine bay of an F-15 for a scramjet, the other for an SR-71 engine. That takes you to mach 10 and the edge of space. If you strapped PAM rockets to the missle racks on the wings and belly, could you make it to orbit velocity?

Obviously I haven't thought the heat shield through yet, but I am just wondering over the possibilites it opens up.

And obviously there is no possibility for Star Wars fighters, but the maneuvering would be done by maneuver rockets on all control surfaces.

 

[FredGarvin]

Three things immediately come to mind:

1. The F-15's fuselage would have to be greatly modified to go to those speeds and altitudes. Also would you be able to carry enough fuel to get that far?

2. The wing structure and missile mounting scheme would have to be modified to withstand that kind of thrust load.

3. How the heck would you control the thing at hypersonic speeds? The normal control surfaces would probably be non existent after Mach 3 or 4.

Just some initial thoughts. I guess if you strap a large enough engine on anything you can get to where you need to go.

 

[Pengwuino]

Probably with enough motifications... but why the hell would you want to do that?

 

[Francis M]

It's a neat idea but (this question goes to Fred) wouldn't the plane have to be so heavily modified that at some point you'd end up spending as much as if you'd just built an original airframe?

 

[FredGarvin]

It's a neat idea but (this question goes to Fred) wouldn't the plane have to be so heavily modified that at some point you'd end up spending as much as if you'd just built an original airframe?

I agree with you 100% on that thought. There would probably be a list as long as my arm as to why you couldn't use that airframe. I am not a structures type of guy though.

 

[physicsCU]

I figured that the structure needed to be modified, I was just wondering if the propulsion scheme would work out for it. As far as fuel goes, you would need to carry only the fuel needed by the two engines. The PAM's carry their own fuel. And I think that airframe could carry enough to do it, but I am not sure.

But thanks for your input!

 

[DaveC426913]

"If you strapped PAM rockets to the missle racks on the wings and belly, could you make it to orbit velocity?"

The SR-71 and scramjet speeds would require a very streamlined veseel. As soon as you strap on an external set of rockets, you immediately screw up any aerodynamics.

 

[russ_watters]

Well, an F-15 would utterly disintegrate before it got to mach 5 (flip a coin between aerodynamic forces and heating), but the general idea of having two engines for two different flight envelopes is valid.

One of the more difficult problems with supersonic/hypersonic wind tunnel testing is making models that won't melt too fast (a steel model melts in a matter of seconds at mach 4), but still aren't too expensive (steel is cheaper than, say, titanium). I've seen better, but http://www.ae.utexas.edu/research/cfpl/topics/sidra/research.html#Movies are some videos of such research.

 

[Danger]

Even if all of the other problems that folks have pointed out were adequately addressed, I'd be very uncomfortable with the asymmetrical thrust aspect of having incompatible engines side-by-side. One above the other would be okay, but I think that having your centre of thrust offset from the midpoint of the plane would cause trim problems at extremely high speed. I'm not sure, but that's my impression from a pilot's perspective.

 

[physicsCU]

That is an interesting point, Danger.

I do know that military aircraft with two engines can fly with one, but the extreme speeds are not known.

It sounds like there is just not a lot of research into control over mach 3/top speed of the SR-71. That would have to be the first thing addressed in order to make a true space plane work.

 

[FredGarvin]

I was going to mention adverse yaw but I figured if they could get by the airframe problems and control issues, they could work that into the scheme as well. Good call Danger.

I can't imagine what they would have to do to counteract that yaw at mach 5+ when the scramjet kicks in and the conventional engine cuts out.

 

[Pengwuino]

I do know that military aircraft with two engines can fly with one, but the extreme speeds are not known.

I think one huge problem there is that, like commercial jets, once you lose an engine, you either start to roll or you start to yaw to one side.

 

[russ_watters]

I do know that military aircraft with two engines can fly with one, but the extreme speeds are not known.

You may note that the engines of the F-18 are significantly closer together than those of the F-14. The reason is that the F-14 is not a lot of fun to fly on one engine due to the asymetric thrust.

 

[Tom10331]

hey Cu i think you will be better off getting plans for a scram jet or something like that.
Plus what r u going to use as a oxidizer and a fuel?
There is a lot of planing but if u got the money and the parts(and guts)do it!

 

[Intuitive]

I think taking advantage of (Specific Gravity) is the best option to get into space with less effort. (Specific Gravity Ballast), (like Submarines) but with lighter than Air ballast tanks. Everyone is looking for speed to get into space but you don't need to go fast, Just float up, It would take a pretty good material and design effort to accomplish it, Maybe NASA could afford it. It seems to also be the better way to come back to Earth by ballasting Specific Gravity, You could put Scram jets or even the latest Black light Ion engines on it. Just a thought.

 

[Danger]

That's fine for getting something to an altitude where it can be launched with less power and fuel consumption, and it's been done, but a balloon can't get into space by 'floating'. It will inevitably reach a point where the trapped gas inside is of equal density to the atmosphere and quit rising.

 

[russ_watters]

Everyone is looking for speed to get into space but you don't need to go fast, Just float up...

Actually speed is by far the more important consideration (the goal for getting to space is orbit, which for low-earth orbit of 100 miles or so, is 17,500mph), which is why balloons such as what Danger describes are not used. The idea has been kicked around before, but it just doesn't help much to lift a rocket to 100,000 feet, then light it.

 

[kublai]

Think of the Space Shuttle. Consider the huge fuel tanks it takes up and then drops off when they are used up. Consider the shuttle returning using up all it's remaining fuel slowing down to re-enter the atmosphere and coast to the landing site dry of fuel. Consider the shuttle being carried from awest coast landing to Florida on the back of a 747, reminds me of a monkey on an elephants back; it's not much bigger than an F-15.

The moral of this is that you must find a fuel with a lot more energy per pound than what is now available. Now, most of the fuel is spent getting the fuel up to speed!

 

[Cyrus]

Consider the shuttle being carried from awest coast landing to Florida on the back of a 747, reminds me of a monkey on an elephants back; it's not much bigger than an F-15.

? Have you ever stood next to a shuttle orbiter? that thing is MUCH BIGGER than an F-15. Its nearly 4-5 stories tall at the tail fin. Its monsterous.

 

[Ki Man]

whoa. very original idea. plausible but not practical though. your taking pieces of aircraft that had to be engineered from the ground up to be unique/groundbreaking and meshing them together.

u know how much fuel it takes for an SR-71 to operate? sr 71 is HUGE compared to an F-15. it makes an F-22 look like a midget. your taking a SCRAM jet and a RAM jet and putting them in the engine bays of an F-15.

Even if the engines fit and the fuel works, an F-15 isn't aerodynamically designed to go anywhere near mach 3-5 let alone 10. you'd have to redesign the nose, wings, air intake, stabilizer fin, and heat/friction management systems from the ground up

 

[Ki Man]

Think of the Space Shuttle. Consider the huge fuel tanks it takes up and then drops off when they are used up. Consider the shuttle returning using up all it's remaining fuel slowing down to re-enter the atmosphere and coast to the landing site dry of fuel. Consider the shuttle being carried from awest coast landing to Florida on the back of a 747, reminds me of a monkey on an elephants back; it's not much bigger than an F-15.

The moral of this is that you must find a fuel with a lot more energy per pound than what is now available. Now, most of the fuel is spent getting the fuel up to speed!

wth are you talking about. elephant on a monkeys back? wth? have you seen how huge it is? this is a picture of a shuttle on a 747's back, this is an F-15. compare. F-15 must hold 1-2 people for a 2-8 hour mission. Space shuttle is an Earth orbiter/space lab/sattilite deployer that must take a crew of 7 into space, feed and support those crew members, and serve as a laboratory or atmosphereic research for 5-17 days. how the heck is that not much of a difference?

shuttles dimensions:
length: 122.27 feet
height: 56.67 feet
wingspan: 78.06 feet

F-15's dimensions
Length: 63.8 feet
Height: 18.5 feet
Wing span: 42.8 feet

 

[Tom10331]

Hey how about get a carrier plane (with yours on the bottom)that will carry you to a certain altitude that you can lift of and get into orbit. Just like The SpaceShipone.

 

[russ_watters]

As I said before, this approach will save only a tiny (insignificant) fraction of the energy required to get to orbit. You guys aren't grasping the magnitude of the difference in energy between 9 miles of altitude at 500mph and 100 miles of altitude at 17,500mph. Put another way, if you wanted to use this method to launch the space shuttle, you would not even save enough energy to lose the solid boosters.

I think part of the problem is a misunderstandng of what SpaceShipOne accomplished compared to what the Space Shuttle does. Ie, very little.

 

[DaveC426913]

"...9 miles of altitude at 500mph..."
Didn't SpaceShip One reach 100 miles?

Anyway, your point is well-made. It's not the altitude, it's the speed. There's the rub.

 

[russ_watters]

"...9 miles of altitude at 500mph..."
Didn't SpaceShip One reach 100 miles?

Anyway, your point is well-made. It's not the altitude, it's the speed. There's the rub.

[67 miles, iirc] The carrier plane did not - that's what I was talking about in that post: using the carrier plane as the first stage. Having a carrier plane does not enable a SpaceShipOne type craft to come anywhere close to orbit. There are actually several "levels" of performance being discussed here:

1. Carrier plane 9: 9 miles and 500mph
2. SpaceShipOne: 70 miles and 3,500mph
3. Space Shuttle: 100 miles and 17,500mph.

If the goal is Space Shuttle type performance using a carrier plane (Tom's post), the carrier plane does not provide an effective first stage. Tom may have also misunderstood what SpaceShipOne actually accomplished: his post implies he thought SpaceShipOne reached orbit. It did not.

 

[Tom10331]

[67 miles, iirc]If the goal is Space Shuttle type performance using a carrier plane (Tom's post), the carrier plane does not provide an effective first stage. Tom may have also misunderstood what SpaceShipOne actually accomplished: his post implies he thought SpaceShipOne reached orbit. It did not.

But the X prize was who to ever make to LEO(low Earth orbit/100miles above the earth)wasnt it?

 

[russ_watters]

No, it wasn't. http://www.scaled.com/projects/tierone/faq.htm

The goal is to get to 100 kilometers or about 62 miles up. This altitude was established by the X-Prize foundation as a target to stimulate commercial interest in the technology to achieve it.

SS1 actually achieved 67mi, iirc, but it is important to remember that this is a ballistic path - so it isn't going 3,500mph when it gets to the top of it's arc.

 

[Francis M]

That is an interesting point, Danger.

I do know that military aircraft with two engines can fly with one, but the extreme speeds are not known.

It sounds like there is just not a lot of research into control over mach 3/top speed of the SR-71. That would have to be the first thing addressed in order to make a true space plane work.

Try looking up aircraft like the X-15. That may help with more info on controllability at mach 3+ as well as at the edge of space. Might also help with types of aerodynamic and heat stresses encountered at those speeds.

I think it was testing these issues at about the same time frame as Project Mercury and Gemini

 

[FredGarvin]

It was the late 50's to late 60's timeframe. I am not sure if any of that trickled over to Gemini or Mercury. It definitely made it's way to lifting bodies and the shuttle program though.