If you think, that turbine engines for lifters are unrealistic :D

[falconek]

Check this DARPA proposition:

It works great in KSP, and turns out that it might work well in real life

[KerikBalm]

except in KSP, the F-15 would go all the way to orbit....

If done that way in KSP, the F-15 would despawn as soon as the 1st stage is 2.5 km away.... oh well...

[GoSlash27]

This has nothing to do with turbine engine lifters in KSP. You could recreate this proposal more realistically by using basic jets.

Best,

-Slashy

[regex]

If you think, that turbine engines for lifters are unrealistic

Unrealistic in KSP for sure, nothing like in the proposal.

It's "been done" before; air-launching goes back to anti-satellite weapons, although those missiles didn't really boost into orbit, just to intercept. It's also important to note that the payload of that system is 100lbs. so the missile itself doesn't need to be all that big. Launching stuff like, say, Dawn via any sort of jet system is pretty unrealistic.

[problemecium]

This has nothing to do with turbine engine lifters in KSP. You could recreate this proposal more realistically by using basic jets.

The basic jets ARE turbine engines; they're just not "turbo"-jet engines. Anything with a spinny fan thing in it to push air around is a turbine engine.

[GoSlash27]

The basic jets ARE turbine engines; they're just not "turbo"-jet engines. Anything with a spinny fan thing in it to push air around is a turbine engine.

Thanks, I'm aware of that and it seems you've missed the point

Best,

-Slashy

[Renegrade]

except in KSP, the F-15 would go all the way to orbit....

^ This. Hehe.

This has nothing to do with turbine engine lifters in KSP. You could recreate this proposal more realistically by using basic jets.

I did a series of experiments, and determined that it would be a viable system in KSP, if you could overcome the launch-despawn problem that Kerik mentions (say via one of those recovery mods)... easier though just to stick the basics on the rocket itself and SSTO your way to orbit

It's "been done" before; air-launching goes back to anti-satellite weapons, although those missiles didn't really boost into orbit, just to intercept.

Don't forget the White Knight flying launch platform - also sub-orbital like an ASAT system.

(Reminds me of the Bell X-1 - launched from a piston-engined propeller-driven B-29~)

[SRV Ron]

The advantage of a recoverable heavy lift aircraft is to get the rest of the space craft up to 40,000+ feet at either subsonic or even to supersonic speed. Being well above the thickest part of the atmosphere, The rest of the space craft could be easily capable of placing its payload into orbit and returning.

Best if an aircraft is designed for that purpose as opposed to bolting on a payload to an existing one as was done by the Air Force during their X program.

[falconek]

This has nothing to do with turbine engine lifters in KSP. You could recreate this proposal more realistically by using basic jets.

Best,

-Slashy

By saying "turbine engines" I meant turbine engines. That means both basic jets and "advanced jets".

- - - Updated - - -

Anyway you guys go into too much details. What I wanted to show you is a space ship, that uses turbine engines for it's first stage (which is doable in KSP). I really don't care if this F-15 will despawn, or crash into the ground or explode or go straight to the orbit. The point is it's function as an air-breathing first stage.

Edited February 9, 2015 by falconek

[Streetwind]

For what it's worth, KSP's basic jet engine is somewhat in line with a modern RL turbojet, allowing supersonic flight up to around roughly mach 2.5 before the thrust drops off to impractical levels.

What KSP calls "turbojet" is actually far closer to an air-turboramjet - which is what you get when you rig a closed-cycle, full bypass turbojet to blast directly into a ram combustion chamber. It's an insanely complicated engine class, requiring three different fuels at once to run, which is why it's not really been used outside of feasibility studies. But it does give you the performance profile that you see from KSP turbojets. First low-ish but still acceptable thrust on the runway thanks to the active jet component, then steadily increasing thrust as the aircraft goes supersonic and the ramjet gets going for real, until finally bottoming out between mach 5 and 6 - the practical limit for ramjet engines IRL.

As for the F-15 as an airbreathing first stage, it will likely have the same issues as all other air-launched rockets: height is largely irrelevant in getting to orbit. The F-15 can maybe push between mach 2 and 3 or so? But orbital velocity for Earth is mach 25. Meaning that if a ground launched rocket needs about 10 km/s dV to get all the way into orbit, then this F-15 launched rocket will still need somewhere between 8 and 9 km/s dV. You save very little at all, and considering how small the rocket is, it's probably not staged like the ground launched rocket would be, which puts it at an additional disadvantage. It needs to be built close to what would be a rocket SSTO, and thus its payload is utterly miniscule.

I'm going to watch the video once I get home and see if these points are addressed there, but for now I remain really skeptical.

Edited February 9, 2015 by Streetwind

[regex]

The point is it's function as an air-breathing first stage.

I don't think anyone ever, in KSP history, claimed that the concept of lifting a rocket to a suitable height for air launch in order to save delta-V using air-breathing engines was unrealistic (and if they did, they were shown the error of their claim).

The main gripe about doing it in KSP is how completely overpowered and unrealistic jet engines are. For one, they have way too much thrust. For two, they have roughly 16x the isp they should have based on how intake air is used within the engine code (you basically get afterburner-level thrust for cruising fuel consumption). For three, KSP's current aerodynamic/atmospheric/drag model is, to put it mildly, dumb. All of those things combine to make anyone who knows just not care at all about people doing massive air-breathing engine launchers in KSP. Seen it before, you're not doing anything new.

I, personally, do like seeing airplane launches in KSP, however, mainly because the payload is generally fairly reasonable and more in line with realistic launches.

[GoSlash27]

By saying "turbine engines" I meant turbine engines. That means both basic jets and "advanced jets".

- - - Updated - - -

Anyway you guys go into too much details. What I wanted to show you is a space ship, that uses turbine engines for it's first stage (which is doable in KSP). I really don't care if this F-15 will despawn, or crash into the ground or explode or go straight to the orbit. The point is it's function as an air-breathing first stage.

There's a pretty big difference between launching a rocket from a jet and flying a jet up to a near- orbital trajectory right out into space.

Nobody's arguing that launching a rocket from a jet platform is unrealistic and this doesn't make KSP's "turbojet" realistic.

Best,

-Slashy

[magnemoe]

Unrealistic in KSP for sure, nothing like in the proposal.

It's "been done" before; air-launching goes back to anti-satellite weapons, although those missiles didn't really boost into orbit, just to intercept. It's also important to note that the payload of that system is 100lbs. so the missile itself doesn't need to be all that big. Launching stuff like, say, Dawn via any sort of jet system is pretty unrealistic.

This, the F15 works not so much because speed and attitude, its just an faction of that you need however drag is far larger on tiny rockets, and here the plane get 90% of the drag.

Doing the same on Kerbin and an F15 would take you to 1/3 of orbital speed and 1/4 of the attitude, so the F15 works better than most KSP first stages without SRB.

[Jouni]

As for the F-15 as an airbreathing first stage, it will likely have the same issues as all other air-launched rockets: height is largely irrelevant in getting to orbit. The F-15 can maybe push between mach 2 and 3 or so? But orbital velocity for Earth is mach 25. Meaning that if a ground launched rocket needs about 10 km/s dV to get all the way into orbit, then this F-15 launched rocket will still need somewhere between 8 and 9 km/s dV. You save very little at all, and considering how small the rocket is, it's probably not staged like the ground launched rocket would be, which puts it at an additional disadvantage. It needs to be built close to what would be a rocket SSTO, and thus its payload is utterly miniscule.

The real issue is cost, not delta-v. As far as I understand, the idea is to reduce launch costs by reusing existing infrastructure. You just take any F-15E already in service, attach a large missile, and launch from any airfield (or even from a highway or a large parking lot) on 24 hours notice.

The rocket itself is kind of interesting. It's apparently going to use monopropellant and what we call bamboo staging.

http://en.wikipedia.org/wiki/Airborne_Launch_Assist_Space_Access

[Streetwind]

Wait what? A bamboo staged rocket in real life?

Okay, now I *really* need to see that video

[KerikBalm]

Its not (at least they don't show it that way), but the way they have the engines mounted, you'd think it was at first glance...

[Renegrade]

As for the F-15 as an airbreathing first stage, it will likely have the same issues as all other air-launched rockets: height is largely irrelevant in getting to orbit. The F-15 can maybe push between mach 2 and 3 or so? But orbital velocity for Earth is mach 25. Meaning that if a ground launched rocket needs about 10 km/s dV to get all the way into orbit, then this F-15 launched rocket will still need somewhere between 8 and 9 km/s dV. You save very little at all, and considering how small the rocket is, it's probably not staged like the ground launched rocket would be, which puts it at an additional disadvantage. It needs to be built close to what would be a rocket SSTO, and thus its payload is utterly miniscule.

Well, don't forget that the initial part of a traditional launch is basically straight up, minus a degree or three, to help climb above the atmosphere (that adds about 1500m/sec to a Saturn-V launch). An air-launch can help reduce those delta-v costs, by taking the rocket up to the thinner part of the atmosphere to start with, so it can be launched closer to horizontal, with less gravity AND aero drag.

Plus all rocket engines have better specific impulse at lower atmospheric pressure... can get away with smaller/fewer engines up in the rarified air than at ground level.

Once you add that small initial velocity to the cake, it basically becomes the icing. Oh also don't forget the Oberth effect!

Jouni's right about the cost, too. Not only is it using established equipment with high re-usability indexes, but also you're cutting delta-v from the bottom-most stage.. the biggest, heaviest, and most expensive bit...and if you do it well enough, you could delete the entire first stage, reducing the complexity. For example, the Saturn V first stage is only about 3500m/sec, and pretty much half of that disappears to gravity - half of what remains would vanish to the initial velocity. Crank in vacuum or near-vacuum specific impulse, and you have a good shot at completely eliminating that stage.

(The only difference between KSP and the WhiteKnight2/F-15E is that the KSP version would carry the payload into space, the pilot would EVA, and give the last 0.5m/sec with a physical tap to put it in orbit~)

(Why do I keep on using the Saturn V as an example? It's the one I know best, quite simply. A smaller rocket would have less gravity drag losses (the Saturn V chugs through something like half the first stage fuel ... clearing the tower), but it would have an equivalent amount of aero losses, so kinda six of one, half dozen of another, or a jet the size of Quebec the third....)

[ShawnPhillips]

Something like this has been done in KSP, and by Robbaz of all people.

[regex]

Something like this has been done in KSP, and by Robbaz of all people.

Not to say that Robbaz is terrible, but this is more impressive than pretty much anything done in stock:

[KerikBalm]

I wonder if multiplayer isn't the solution to this air launch business... if someone takes active control of the craft after release, they should both be able to avoid the physics despawn....

Sometimes I've considered trying to make a space plane that launches a payload from 30km, which gives me until the time it descends to 23 km to get the other craft into orbit....

But now the recovery value of the first stage is so low if I don't get it all the way to orbit and land it back at KSC... that it doesn't seem worth it to deal with the hassle of designing it... and then the limitation about switching craft in the atmosphere (even if you are high enough for them to not despawn).

If that limitation wasn't there, I could send my first rocket on a suborbital trajectory... try to turn around my plane and get it above 23k m again and ascending (on its way 180 degrees back to KSC), switch back to my suborbital rocket, and have it complere its orbital insertion, then switch back to my plane before it despawns.

[Laie]

Well, don't forget that the initial part of a traditional launch is basically straight up, minus a degree or three, to help climb above the atmosphere (that adds about 1500m/sec to a Saturn-V launch).

And also keep in mind that relative to size, bigger rockets suffer less from drag. If you only want to launch a tiny payload, bypassing the better part of the atmosphere is much more worthwhile; and also a lot easier to do with available planes.

[NathanKell]

If you saved as much from air-launching in KSP as you did in real life, you'd need...

wait for it, amazing savings!

...

...

4200m/s.

Yeah, that 300m/s is totally worth air-launching for.

[Sirrobert]

So completely unrelated to the discussions, but it is to the video:

What the hell is that green sack you see when the rocket seperates from the plane? http://youtu.be/BOaJWoVLhAc?t=1m2s

[Itsdavyjones]

So completely unrelated to the discussions, but it is to the video:

What the hell is that green sack you see when the rocket separates from the plane? http://youtu.be/BOaJWoVLhAc?t=1m2s

I would expect it to be a drogue chute to aid in the separation of the rocket and the aircraft.

[Renegrade]

And also keep in mind that relative to size, bigger rockets suffer less from drag. If you only want to launch a tiny payload, bypassing the better part of the atmosphere is much more worthwhile; and also a lot easier to do with available planes.

Fully agreed there!

If you saved as much from air-launching in KSP as you did in real life, you'd need...

wait for it, amazing savings!

...

...

4200m/s.

Yeah, that 300m/s is totally worth air-launching for.

Judging by the numbers you're using, I'm guessing that's a pure stock launch (ie no FAR, right?). Given that the soup-o-sphere/stock-o-sphere is about as thin as molten lead, my off the cuff answer would be that the savings are bigger than an Earth launch, not smaller. Keep in mind that if Kerbin were airless, the launch cost would be ~2500m/s....and it's generally 4600m/s from KSC's pad.

I concocted a weird little test vehicle that puts about 1200kg into orbit for about 2080 dv in a pure-stock (plus info mods) environment... or a savings of about 2522.

It required a TurboJet to get to around Mach 2 unfortunately (the original BasicJet one was being pouty about getting above 400m/sec), and it still didn't quite reach the target speed of 740m/s, so I let it fly a bit higher than the 12km that the Wiki article mentioned (partly because of "ohcrap it's time to decouple!" pilot error too. Actually, let's be honest and say I was staring at the speed and didn't notice altitude until I was shooting past 15k like an idiot).

(Please excuse my poor Gimp skills. Left: vessel before launch. Center: shortly after decoupling, tumbling end over and, and recovering. Right: In orbit. The turbojet bit has almost no fuel in it, and is *assumed* to be an aircraft carrying the rocket that flies back. In actuality, it just tumbles away and is deleted by KSP, as is seen in the far left of the picture under the staging icons)

NB: I did no scaling of the parameters. The document says a launch altitude of 12km and an F-15E Strike Eagle (max speed 740m/sec). If you scaled that back to uhh... 1.09km (12/11 - that's 1.13 in squad terms) and 223m/s (740/sqrt(11)), then that would be a lot closer to 300.

Of course given #lolturbojets, I decided to do another launch of that exact same craft, and instead of trying to cut early to match the parameters, I gave the TurboJet free rein and it reached a 120x110 orbit for a cost of a whopping 92 delta-v.