Dear lord what a beautiful plane

[kgbsquirrel]

it was genius to use slow motion to create the illusion of speed.

http://youtu.be/a3O2kuEXf8Y 

[birdman]

Shopped, by I like it.



Hey local aero-engineers. I was pondering how to create a super-heavy lift aircraft and I was suddenly curious if the diamond-wing shape (F-22, YF-23) could be applied to something the size of a C-5 or A380?

Doesn't quite work for late subsonic aircraft, where you want an extremely high effective camber, and a supercritical shape to maximize wing loading vs mass.

To put in perspective, a c-5/c-17/787-9 have wing loadings of 120/150/158 lb/sqft, while an f-22 is 77.  The airfoils on large aircraft are designed for max L/D at high subsonic speeds, and maximum lift / structural mass, with little other requirements.  Delta wings are designed for best area-rule limited L/D for sub/trans/supersonic performance--that why variable geometry wings (f-14/b-1) change sweep for different conditions.

Effectively, wing efficiency goes up the more the upper surface can be maintained w/o shocks and with limited boundary layer, which effectively means short chord (width of wing)--the opposite of a delta or cranked arrow.  

See also the Rutan T3, which used two thin chord wings rather than one wide one.

Make sense?

[kgbsquirrel]

Doesn't quite work for late subsonic aircraft, where you want an extremely high effective camber, and a supercritical shape to maximize wing loading vs mass.

To put in perspective, a c-5/c-17/787-9 have wing loadings of 120/150/158 lb/sqft, while an f-22 is 77.  The airfoils on large aircraft are designed for max L/D at high subsonic speeds, and maximum lift / structural mass, with little other requirements.  Delta wings are designed for best area-rule limited L/D for sub/trans/supersonic performance--that why variable geometry wings (f-14/b-1) change sweep for different conditions.

Effectively, wing efficiency goes up the more the upper surface can be maintained w/o shocks and with limited boundary layer, which effectively means short chord (width of wing)--the opposite of a delta or cranked arrow.  

See also the Rutan T3, which used two thin chord wings rather than one wide one.

Make sense?

Actually yeah, it does, but I'm a geek. *shrug* 

I was just envisioning a large super-sonic mother-ship for hefting a shuttle up to altitude. Kind of like the A-12 idea, but without the suckage. Would it help much to take your orbiter past supersonic at 50k' AGL before letting it go?

[Tallpine]

My thought for a "mother ship" was something along the lines of a high swept flying-wing with wing tip stabilizers pointed down.

Basically eliminate the tail altogether, not too unlike some of the hang-glider style ultra-lights, only of course much much larger and jet powered.  Could have an extended nose with canard 

The general idea being the mother ship is high-wing / no tail, and the shuttle is of course low wing, so you could launch the shuttle right off the rails on the back of the mother ship.

Being just a software engineer*, I have no idea if more altitude vs more speed before separation is preferable 


* I did an aero simulator a long time ago, but I've forgotten most of it.  Anyway, it was just designed to test/calibrate electronic landing guidance systems.

[birdman]

Actually yeah, it does, but I'm a geek. *shrug* 

I was just envisioning a large super-sonic mother-ship for hefting a shuttle up to altitude. Kind of like the A-12 idea, but without the suckage. Would it help much to take your orbiter past supersonic at 50k' AGL before letting it go?

Altitude helps more than speed (I've done a few air-launched designs before, and also done some analysis on the NCADE missile).

Think about it this way, to make orbit, you need about 7700m/s at 150km
Going from 15km altitude to 150km altitude is the equivalent of an extra ~700m/s of velocity, so each 3300ft of altitude is about 5m/s of extra velocity
Earths rotation gets you about 500m/s if you launch due east at the equator.
Early on aero-lift buys you roughly 10*L/D m/s for every second you have aero-lift (supersonic/hypersonic L/D of >4 is hard), but you only get that below about 20-25km altitude
Gravity loss costs you ~10*sqrt(V/7700) m/s for every second you are boosting
Drag losses can be pretty significant at lower altitudes, basically, aero lift is only useful for steep climbs, and only for the first 20-40 seconds (ie Pegasus)

So basically, getting above the air matters more than anything else.  Or, to put it another way, going from Mach 0.8 to Mach 3 is less than 1/5th of your fuel (assuming no air, a 300s average ISP, and a single stage with overall mass fraction of 0.93)

It turns out the best possible air-launch when you look at an overall system is a high subsonic launch at 40-50kft with a climb angle of 30-40deg...however, that puts some tough requirements on the platform.

Given real platforms, near horizontal at high subsonic and as high as you can get (trading altitude for speed) is the best.

Tallpine is right, a high subsonic flying wing with a low-wing daughter-craft would be ideal aerodynamically, however, in terms of cost, its prohibitive compared to a big dumb booster, and really prohibitive compared to what spaceX wants to do with re-usability.

Effectively, fuel and tankage doesn't cost anything, its the other bits and the NRE that kills cost--so adding a second system (the airplane) always loses out to just making the first stage bigger--unless the airplane is ass cheap (used L1011 for Pegasus) and it allows you to then use readily available, already designed/built motors (Pegasus).

[Tallpine]

birdman, what do you think about the British Skylon ?

[birdman]

birdman, what do you think about the British Skylon ?

Dumb.  Same with HOTOL, the big huge one I think bezos is developing, and all the others.
Remind me sometime to tell you a story about them trying to buy Merlin's.

Given that we know how cheap BDB's can be made, and even in the most optimistic estimates the reusable air launched stuff doesn't come close, it just doesn't make sense.

[Tallpine]

Dumb.  Same with HOTOL, the big huge one I think bezos is developing, and all the others.
Remind me sometime to tell you a story about them trying to buy Merlin's.

Given that we know how cheap BDB's can be made, and even in the most optimistic estimates the reusable air launched stuff doesn't come close, it just doesn't make sense.

Hmmm    Seems like a big re-usable fuel tank.

The jet/rocket engines seem overly complicated, though.

[birdman]

Hmmm    Seems like a big re-usable fuel tank.

The jet/rocket engines seem overly complicated, though.

Its the reusable nature of large things with wings that makes things costly.  heat shielding the ass end, not hard...lifting re-entry, hard.  And complex engines are bad....its all about cost now.
The reason Merlin's are cheap is they are pretty much the simplest, can't go wrong, type liquid engine there is.

[kgbsquirrel]

Altitude helps more than speed (I've done a few air-launched designs before, and also done some analysis on the NCADE missile).

Think about it this way, to make orbit, you need about 7700m/s at 150km
Going from 15km altitude to 150km altitude is the equivalent of an extra ~700m/s of velocity, so each 3300ft of altitude is about 5m/s of extra velocity
Earths rotation gets you about 500m/s if you launch due east at the equator.
Early on aero-lift buys you roughly 10*L/D m/s for every second you have aero-lift (supersonic/hypersonic L/D of >4 is hard), but you only get that below about 20-25km altitude
Gravity loss costs you ~10*sqrt(V/7700) m/s for every second you are boosting
Drag losses can be pretty significant at lower altitudes, basically, aero lift is only useful for steep climbs, and only for the first 20-40 seconds (ie Pegasus)

So basically, getting above the air matters more than anything else.  Or, to put it another way, going from Mach 0.8 to Mach 3 is less than 1/5th of your fuel (assuming no air, a 300s average ISP, and a single stage with overall mass fraction of 0.93)

It turns out the best possible air-launch when you look at an overall system is a high subsonic launch at 40-50kft with a climb angle of 30-40deg...however, that puts some tough requirements on the platform.

Given real platforms, near horizontal at high subsonic and as high as you can get (trading altitude for speed) is the best.

Tallpine is right, a high subsonic flying wing with a low-wing daughter-craft would be ideal aerodynamically, however, in terms of cost, its prohibitive compared to a big dumb booster, and really prohibitive compared to what spaceX wants to do with re-usability.

Effectively, fuel and tankage doesn't cost anything, its the other bits and the NRE that kills cost--so adding a second system (the airplane) always loses out to just making the first stage bigger--unless the airplane is ass cheap (used L1011 for Pegasus) and it allows you to then use readily available, already designed/built motors (Pegasus).

*scratches chin* But you wouldn't really need a whole new aircraft. Here's a crazy thought. Take one of those 747's that was converted to shuttle ferry. Now, since it doesn't need to be transcontinental anymore, it just has to get to altitude with it's payload, add four or six more engines (two more between the current four, another attached to the hull just aft of the wing). Lift, thrust, and enough fuel load to get the job done. And it comes back home to do it all again.

[birdman]

*scratches chin* But you wouldn't really need a whole new aircraft. Here's a crazy thought. Take one of those 747's that was converted to shuttle ferry. Now, since it doesn't need to be transcontinental anymore, it just has to get to altitude with it's payload, add four or six more engines (two more between the current four, another attached to the hull just aft of the wing). Lift, thrust, and enough fuel load to get the job done. And it comes back home to do it all again.

Its not engines, its lift.  Engines just make bigger wing more usable, or gain a small bit of altitude.

Besides, they have already made twin-pods for a 747 (its parked at the top-gear airfield in the background for example)

But still, that doesn't help much.  The maximum payload for a 747 is about 100-125tons,  rea

A Pegasus XL carried 950 lbs and had a gross mass of ~50,000lbs, so a scaled up version coud theoretically lift 3800lbs to orbit from a 757, and weigh 200,000lbs

Now, a Minotaur IV can put 3800lbs into orbit, and has a gross mass of 190,000lbs.

So what does the air launch buy you?

[birdman]

Better example:
Pegasus XL is 980lbs payload / 51000 gross, air launched
Minotaur I (Minuteman 1st, 2nd stage, Pegasus XL 2nd/3rd stage) is 1,300lbs to LEO, and 81,000lbs gross from the ground.

So on a scaled basis, the air launch buys you about 20-25% payload vs gross weight, but that advantage disappears once you get to even slightly larger systems (Minotaur IV where the payload to gross weight fraction is the same or better than Pegaus XL)

Now, if you applied more recent tech, and a liquid fuel booster, you could probably get the payload:gross ratio to about 1:20 for the upper stage, meaning for a 747 carry-able 200,000lb vehicle (but expendable) you could get close to 10,000lbs.  Such a vehicle would require about 1/2 the engines of a Falcon-9, and 1/3rd the tankage, but that means its effective cost would be about 1/2 a falcon9, -not- counting the carrier aircraft...for about 1/2 the payload to orbit.

Add re-usability for everything but a small upper stage, and the mass fraction would decrease, and you would likely only get 1:30 or worse (remember, the shuttle orbiter is about 1:3 payload:gross not including tankage, and 1:40 including the whole stack...(the ET is probably the highest mass fraction item in existence that has to bear reasonable loads...it is almost 97% fuel/ox by weight at liftoff)

It turns out the key to SSTO and DSTO (dual stage, which counts either a reusable rocket first stage or an air launch) isn't tankage, we have that, its whole-trajectory optimized engines.

For example, looking purely at stage mass fraction, the Titan-2 first stage which has a gross mass of 121,200kg, and a dry mass of 4300kg (28.2:1 mass ratio), a 258s sea level ISP (and with no modification, a vacuum ISP of approx 295+s) would be capable of a (neglecting drag and gravity loss) of 8800m/s.  with a burn time of 158 seconds, a T/W at takeoff of 1.6:1 is only shy about 1000m/s of achieving orbit even including  estimated drag losses even with zero payload...however, that is due to the fact its engines can't throttle significantly, so it would be accelerating too fast after about 20-30 seconds and would be killed by drag.

If you could throttle the engines, it would be only about 500m/s shy.
If the engines had extended nozzles to increase the vacuum ISP to 310-315s, while decreasing sea level ISP by about 5%, it could achieve orbit with a small (<1000lb Pegasus type) payload.

So the issue isn't technology (Merlin is throttle-able, and has better ISP, and obviously even 1960's tech can build a light enough stage) its that single stage is inefficient.  After all, when one adds the Titan-2 second stage (a mere 28,000kg more) the proven payload to orbit without any mods is close to 8000lbs (3600kg) or a payload to gross mass ratio of 40:1, better even than a Pegasus.

The TL:DR version:
SSTO is inefficient compared to DSTO
DSTO costs more, but not in ratio to payload (effectively, 2x the cost for 4-5x the payload)

SSTO only makes any sense if its truly turn-around re-usable, but even then, a DSTO with the same re-usability is vastly more efficient (why spaceX is working the grasshopper concept for first stage re-use, which is the bulk of the cost)

And air-launch is fundamentally limited by carrier aircraft payload--the advantage of air launch is basically a 10-25% lighter gross weight, but the BIGGEST aircraft can only carry a rocket big enough for the small-medium size launch vehicles.

Sure, people have looked at super-large aircraft as a carrier to loft falcon-9 size payload carrying vehicles, but given the A/C development cost is basically equal to SpaceX's -entire- capital investment...what's the point?  It won't be any cheaper, and far more can go wrong...especially since you can't do the full-throttle run-up before launch that spaceX can, which has proven to be -really- important, and likely saved at least one launch from failure.

[birdman]

Also, to put in perspective, mass production is -key-
The Titan-2 had a recurring cost of $3.2M in 1969 (about $15M today) in quantity of 150+
That's an effective price to orbit of $3000/lb in today's prices...about the same as SpaceX, but not including the capitalization cost.  However, as SpaceX has shown, if you can keep NRE below $500M, and spread it out, you can still very much compete with those costs.

[Tallpine]

Okay ... so the optimum might be a recoverable first stage, and a re-usable aerodynamic second stage ? 

And I guess it makes a difference whether you are trying to boost crew or cargo.  Seems like a mistake to mix them on the same launch.

We ought to have a crew carrying "shuttle" that can move people to/from earth orbit on a routine (and emergency!) basis.  Cargo you can blast up there most any old way.  The vast majority ain't coming back anyway.

Actually, wouldn't it be ideal if you could have a way to re-hab a non-returnable second stage in orbit for some other purpose, such as the hull of a space station module 

[RocketMan]

Besides, they have already made twin-pods for a 747 (its parked at the top-gear airfield in the background for example)

Fake pods on a non-flyable 747, done for a movie.  But the point is valid.
Twin pods on the B-52.

[birdman]

Okay ... so the optimum might be a recoverable first stage, and a re-usable aerodynamic second stage ? 

And I guess it makes a difference whether you are trying to boost crew or cargo.  Seems like a mistake to mix them on the same launch.

We ought to have a crew carrying "shuttle" that can move people to/from earth orbit on a routine (and emergency!) basis.  Cargo you can blast up there most any old way.  The vast majority ain't coming back anyway.

Actually, wouldn't it be ideal if you could have a way to re-hab a non-returnable second stage in orbit for some other purpose, such as the hull of a space station module 

Yes, cargo and people should go separate.  People carrier should be quasi-reusable, as it has expensive stuff
Cargo carrier, re-usability isn't all that useful.

If reusable, the LOWER states are the important ones, as they have more/larger engines, larger tanks, etc, so they dominate the cost of non-reusable spacecraft (ie spaceX). 

As for refurb on orbit of upper stage tankage, yes!  People have proposed this numerous times, with emphasis on the shuttle ET, as it is 70,000lbs of HUGE  volume that is pressure rated and is lifted to 99.5% orbital velocity anyway.

As for current ones, the falcon-9 second stage is also good size.  The issue then becomes how to bet clean it out (RP-1 is icky, and the lox tank isn't that big) and the cost of adding access stuff, and the labor of conversion.

[kgbsquirrel]

So what does the air launch buy you?

Airflow to prime a scramjet? 

[birdman]

Airflow to prime a scramjet? 

Cheaper to rocket boost.  Besides, scramjets need to be supersonic before they can start.

[kgbsquirrel]

Cheaper to rocket boost.  Besides, scramjets need to be supersonic before they can start.

Pft. XB-70.



*is waiting for birdman to start hitting him with a rolled up newspaper*

[birdman]

Pft. XB-70.



*is waiting for birdman to start hitting him with a rolled up newspaper*

Ah, my favorite plane.  Good kgbsquirrel! (Stealthily rolls up newspaper)