Author Topic: Air-Launched SSTOs  (Read 68486 times)

Offline CFE

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Re: Air-Launched SSTOs
« Reply #60 on: 01/19/2008 03:16 am »
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kevin-rf - 18/1/2008  6:08 AM

Twin C-5? If your going to make a custom aircraft for air launch might as well do it right and design one from scratch. It was the cost of the flyback (design and manufacturing) that forced NASA down the SRB road with the shuttle. Me thinks designing anything that requires something other than an existing carrier aircraft will doom the project.

The exception being what rutan is working on. Though the SS-1 carrier is finding work on other projects, meaning there was a need for it.

I'm not sure about the twin C-5 idea will cost, relative to a totally new development.  The fact that it was studied for a shuttle carrier aircraft shows that NASA did take it seriously (although I'd have serious misgivings if the center wing was expected to resist all of the torsional loads.)  The NASA airlaunch study that was linked in a previous post seemed favorable to the idea.  The study also looked favorably on air liquification systems, and re-engining existing transports with GE90 engines.

While our goal should be to modify an existing aircraft as minimally as possible, the truth is that existing aircraft aren't a very good solution for the airlaunch problem.  While I'm not very knowledgeable on the subject, there's a lot of concern on the part of the engineers I've spoken with about carrying the orbital vehicle on the back of the mothership (as opposed to slinging it under the belly.)  While this approach worked well for the Enterprise glide tests, I don't know how well it will work on a larger, orbital vehicle that is laden with fuel and payload.  A lot of people were scared off by the collision between D-21 drone and its M-21 mothership.
"Black Zones" never stopped NASA from flying the shuttle.

Offline kfsorensen

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RE: Air-Launched SSTOs
« Reply #61 on: 01/19/2008 04:10 am »
I would tend to agree that an air-launched rocket would be better served with a new aircraft design.  The ability to drop the rocket, rather than to try to separate off the back, is essential and cannot be met in current aircraft (747, 777, A380, etc).

Strange creatures like the dual C5 remind me of the Biblical parable of "old wine in new bottles".

Offline tnphysics

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Re: Air-Launched SSTOs
« Reply #62 on: 01/19/2008 04:18 am »
WK2 can do the job.

Offline CFE

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Re: Air-Launched SSTOs
« Reply #63 on: 01/19/2008 04:23 am »
I would also add that the ability to "eject" the rocket from the aft cargo doors could also be desirable.  BladeRunner baselined a C-141 and QuickReach baselined a C-17.  If an internal-carry concept utilized the volume and payload mass that can be carried by a C-5 or one of the Antonovs (instead of the comparatively-tiny capacity of the C-141 and C-17,) we might really be onto something for an airlaunched orbital vehicle.
"Black Zones" never stopped NASA from flying the shuttle.

Offline kkattula

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Re: Air-Launched SSTOs
« Reply #64 on: 01/19/2008 05:40 am »
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CFE - 17/1/2008  3:01 PM

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kkattula2 - 16/1/2008  9:21 PM

Traditionally first & second stages split delta v fairly evenly. In this 'Assisted SSTO RLV' scenario the first stage is by far the junior partner. It gives whatever help it can, wjile still returning to base in a simple and inexpensive manner.

Are you talking about splitting propulsive delta-v evenly between the stages, or splitting delta-V evenly after the velocity losses have been taken into account?

After losses. First stages absorb most of the losses, second stages provide most of the orbital velocity. Total delta v is similar although often a little higher for the second stage if it uses LOX/LH2 and the first stage is LOX/HC.

Offline kkattula

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Re: Air-Launched SSTOs
« Reply #65 on: 01/19/2008 06:00 am »
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joema - 18/1/2008  7:24 AM

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kkattula2 - 16/1/2008  10:21 PM...
Traditionally first & second stages split delta v fairly evenly. In this 'Assisted SSTO RLV' scenario the first stage is by far the junior partner. It gives whatever help it can, wjile still returning to base in a simple and inexpensive manner.
The impact of TSTO staging velocity on total vehicle weight and program cost was studied extensively during shuttle development. Probably much of it applies to other TSTO concepts.

The dilemma is you get optimum weight and cost with a fairly high staging velocity -- say around 12,000 ft/sec. It varies based on design details, but in general it's fast.

That means at staging, the 1st stage is a long way off and headed away from the launch site at high hypersonic speed. To get it back requires wings, engines, thermal protection, etc. That in turn increases the cost and complexity.

If you make the 1st stage smaller, it's easier to return but the overall vehicle cost and weight goes up. Same with making the 1st stage bigger for "AOA" recovery.

All those factors drive you toward either SSTO or expendable 1st stage.

But the dilema is that SSTO, especially SSTO RLV is very difficult. Allowing the orbiter to use vacuum optimised nozzles, and drop the delta v by 2 km/s, is a big advantage.

I don't want to make the first stage smaller, it should be big, simple and robust, but not hugely efficient. I don't care if it uses twice the fuel it otherwise might. Fuel is cheap, LOX is dirt cheap. Each first stage should serve multiple orbiters, requiring minimal servicing between flights.

Offline libs0n

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RE: Air-Launched SSTOs
« Reply #66 on: 01/19/2008 06:06 am »
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vanilla - 18/1/2008  12:10 AM

I would tend to agree that an air-launched rocket would be better served with a new aircraft design.  The ability to drop the rocket, rather than to try to separate off the back, is essential and cannot be met in current aircraft (747, 777, A380, etc).

Strange creatures like the dual C5 remind me of the Biblical parable of "old wine in new bottles".

That's the genesis of my earlier idea from this thread, a towed gliding mothership, in that it would allow you to use existing large aircraft as towing vehicles without the need to modify them significantly, save for the towing apparatus.  Imagine, if you will, the payload increase of a WK2 towed by an Antonov-124 (I'm not an aerospace engineer so this hypothetical concept may be wildly unfeasible, but you get the drift).

Offline CFE

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RE: Air-Launched SSTOs
« Reply #67 on: 01/19/2008 06:43 am »
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libs0n - 18/1/2008  12:06 AM

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vanilla - 18/1/2008  12:10 AM

I would tend to agree that an air-launched rocket would be better served with a new aircraft design.  The ability to drop the rocket, rather than to try to separate off the back, is essential and cannot be met in current aircraft (747, 777, A380, etc).

Strange creatures like the dual C5 remind me of the Biblical parable of "old wine in new bottles".

That's the genesis of my earlier idea from this thread, a towed gliding mothership, in that it would allow you to use existing large aircraft as towing vehicles without the need to modify them significantly, save for the towing apparatus.  Imagine, if you will, the payload increase of a WK2 towed by an Antonov-124 (I'm not an aerospace engineer so this hypothetical concept may be wildly unfeasible, but you get the drift).

Towing gives you free velocity, but that's about it.  The wings and landing gear still have to be sized for runway takeoff.  The extra dry mass kills your fuel fraction.  If you're captively-carried,  the wings need only be sized for slightly less than the gross weight after launch at the launch altitude (most airlaunch concepts lose a bit of altitude during the initial phases of the burn.)  The landing gear need only be sized for the dry mass of the craft after landing.
"Black Zones" never stopped NASA from flying the shuttle.

Offline libs0n

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RE: Air-Launched SSTOs
« Reply #68 on: 01/19/2008 07:40 am »
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CFE - 19/1/2008  2:43 AM

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libs0n - 18/1/2008  12:06 AM

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vanilla - 18/1/2008  12:10 AM

I would tend to agree that an air-launched rocket would be better served with a new aircraft design.  The ability to drop the rocket, rather than to try to separate off the back, is essential and cannot be met in current aircraft (747, 777, A380, etc).

Strange creatures like the dual C5 remind me of the Biblical parable of "old wine in new bottles".

That's the genesis of my earlier idea from this thread, a towed gliding mothership, in that it would allow you to use existing large aircraft as towing vehicles without the need to modify them significantly, save for the towing apparatus.  Imagine, if you will, the payload increase of a WK2 towed by an Antonov-124 (I'm not an aerospace engineer so this hypothetical concept may be wildly unfeasible, but you get the drift).

Towing gives you free velocity, but that's about it.  The wings and landing gear still have to be sized for runway takeoff.  The extra dry mass kills your fuel fraction.  If you're captively-carried,  the wings need only be sized for slightly less than the gross weight after launch at the launch altitude (most airlaunch concepts lose a bit of altitude during the initial phases of the burn.)  The landing gear need only be sized for the dry mass of the craft after landing.

That's what I'm talking about, captively carrying the payload(the SSTO or TSTO+its payload) on a gliding mothership that is itself towed by an existing large aircraft.  I am specifically not talking about towing the payload(the SSTO) itself.  So what you're getting at is the mothership would still have to be able to carry the fully fueled SSTO on its own and the only advantage it would see is the dropping of the weight of its propulsive capacity if it was towed rather than self-propelled.

Again, I do not have the required expertise or knowledge to determine the exactitude of the possible efficiencies the 3 vehicled towed gliding mothership scenario might offer other than my theory that it may allow the offloading of the wing/landing gear burden from the SSTO to the mothership, and may allow existing large aircraft to serve as the primary mule without major modifications, if it offers any efficiencies at all.  That's why I put my caveat in at the end; ie a WK2 that can carry a 30mt SSTO may not be able to structurally handle an SSTO that was 100mt when being pulled by an AN-124.  

The only other thing I would like to add to this concept is that I recall once reading that being towed doubles the payload capacity of the aircraft being towed, but again I have no way of verifying this, or even what exactly it means in relation to this issue we're discussing.  For instance, perhaps it means, if true, that our hypothetical towed WK2 would be able to carry 60 mt, excluding factors like the increased burden of the payload size doubling on the WK2 itself.

Another thing on my mind: Let's say that a large aircraft carried a rocket internally, as you suggest; what would the outcome of a second large aircraft towing the first be; would it allow for an increase in the mass carried by the first?  That's really the crux of what I'm throwing out there.

Offline libs0n

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RE: Air-Launched SSTOs
« Reply #69 on: 01/19/2008 08:22 am »
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libs0n - 19/1/2008  3:40 AM

The only other thing I would like to add to this concept is that I recall once reading that being towed doubles the payload capacity of the aircraft being towed, but again I have no way of verifying this, or even what exactly it means in relation to this issue we're discussing.  For instance, perhaps it means, if true, that our hypothetical towed WK2 would be able to carry 60 mt, excluding factors like the increased burden of the payload size doubling on the WK2 itself.

Sorry, it's late at night; what I recall once reading is that an aircraft can tow twice what it can normally carry as payload.  So, if an Antonov 124 can carry 150mt of cargo, then it can pull a 300 mt glider.

Offline joema

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RE: Air-Launched SSTOs
« Reply #70 on: 01/19/2008 03:32 pm »
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libs0n - 19/1/2008  2:40 AM....
Again, I do not have the required expertise or knowledge to determine the exactitude of the possible efficiencies the 3 vehicled towed gliding mothership scenario...
Whether you use captive carry & air drop or towing, the performance advantages of subsonic aerial release are small.

This is counter-intuitive, as we mentally picture being "up high" as part way to orbit, or avoiding atmospheric drag.

In reality, most of orbital kinetic energy is in a lateral (not upward) vector.

It turns out the losses from air drag for a booster launched vertically from sea level are very small. This AIAA paper says the Saturn V incurred only 150 feet per second losses from air drag: http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf

It also says: "Surprisingly, a typical straight and level subsonic horizontal air launch such as used by the X-15 does not result in any significant changes in the delta V requirement as compared to a baseline vertical surface launch."

This is illustrated by the X-15 itself. It had many times the energy performance of Rutan's SpaceShipOne, yet wasn't remotely capable of achieving orbit. However the X-15 almost maxed out the B-52 payload capacity.

Therefore a manned orbital air launch vehicle would require a mothership much bigger than a B-52.

t/Space (http://en.wikipedia.org/wiki/T/space) is thinking of having Rutan build a gigantic mothership to air launch a man carrying orbital vehicle - January 2006 Air & Space Magazine.

The required mothership size would be titanic -- gross weight of one million lbs, payload 150 tons (3x a 747-400 freighter), wingspan 320 feet (1.5x a 747). It would the largest aircraft ever constructed.

Since development cost increases with gross vehicle weight, a sufficiently large mothership to air launch a manned orbital vehicle would be very expensive. However despite that cost, it would impart little performance benefit, if subsonically launched.

A high supersonic launch would impart some perf. benefit, but a 1,000,000 lb, Mach 3+ airbreathing launch vehicle is even more expensive. Picture something like an XB-70, but 4x the gross weight.

Offline kfsorensen

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RE: Air-Launched SSTOs
« Reply #71 on: 01/19/2008 04:11 pm »
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joema - 19/1/2008  10:32 AM

Whether you use captive carry & air drop or towing, the performance advantages of subsonic aerial release are small.

I'm sorry, but that's just not true.  The advantages of air launch are numerous, and they mainly come about by avoiding losses in the ascent trajectory.  See that presentation I attached.

The first and biggest loss avoidance is gravity loss.  When you've got a rocket pointed straight up, 1.0 g of its thrust-to-weight is pure loss.  By starting at altitude, you avoid the gravity losses incurred in getting to that altitude.

Next is engine pressure loss.  By starting at altitude, atmospheric pressure is about 1/4th of what it was at sea level.  That means more thrust (which also helps alleviate gravity loss) and higher Isp.  It also means that nozzles can be optimized much closer to vacuum expansion, and that engine chamber pressure can be lower without incurring as much performance penalty.  LH2/LOX expander cycle engines, which are very safe, become attractive.

Drag losses, as you note, are not terribly significant, but they are another factor as well.

In short, air-launch ISN'T about getting a lot of your lateral orbital velocity from the carrier aircraft--it's about skipping over major loss mechanisms in the early part of the ascent trajectory.

Offline libs0n

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RE: Air-Launched SSTOs
« Reply #72 on: 01/19/2008 04:21 pm »
Thank you for your post, joema, but I am aware that air launching does not greatly reduce the delta-v change a launch vehicle must execute.  By efficiencies, I directly mean increasing the carrying capacity of existing heavy cargo aircraft, so that they can be better utilized in an air launch situation without going to the expense of building a new very large aircraft; or allowing their existing capacity to be used without major modifications.  For instance, the AN-225 can carry 250 metric tonnes; if that notion I posted above holds true, then it would be able to tow a 500 mt glider.  If it doesn't, it can still tow 250 metric tonnes.  Thats a lot of existing capacity to take advantage of, without talking about constructing a new very large aircraft.

So, when I talk about the advantages of tow launch, I am specifically confining the area of inquiry to the advantage of utilizing existing heavy cargo aircraft in such an air launch scenario with little modification, or even possibly greater utilization of their capabilities.  I concur that the greater expense of air launch for the benefits it provides, especially when we start talking about the construction of a new giant aircraft, may make it a marginal endeavour when compared with land launch or sea launch; I am not advocating a launch method, I'm just joining this thread on its terms with my idea that might benefit it.

Offline jongoff

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RE: Air-Launched SSTOs
« Reply #73 on: 01/19/2008 06:00 pm »
Joema,
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It also says: "Surprisingly, a typical straight and level subsonic horizontal air launch such as used by the X-15 does not result in any significant changes in the delta V requirement as compared to a baseline vertical surface launch."

That's interesting, cause I've seen papers talking about as much as a 1000m/s difference.  It's not just the drag losses, it's also gravity losses that are decreased.  And you get a tiny bit more starting velocity (~250m/s).  When you're dealing with an exponential function, dropping the delta-V requirement by about 1/9 is a big deal.

Quote
This is illustrated by the X-15 itself. It had many times the energy performance of Rutan's SpaceShipOne, yet wasn't remotely capable of achieving orbit. However the X-15 almost maxed out the B-52 payload capacity.

Therefore a manned orbital air launch vehicle would require a mothership much bigger than a B-52.

Not necessarily.  Remember, X-15 was built early in the development of rocket vehicles.  Materials, propulsion capabilities, and propellant combinations have all advanced enormously since those flights.  If you don't understand the details, you can't make a valid comparison.

~Jon

Offline Sid454

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Re: Air-Launched SSTOs
« Reply #74 on: 01/19/2008 06:38 pm »
I wonder if the airbus A380 also be could converted into a launch vehicle platform?
It's cargo specs are close to the AN225 and it's in production just design something like the old soviet MAKS shuttle to ride on back of it.
BTW on air launch a lot of fuel is used by a rocket during those first ew thousand feet and you can have a nozzle design that is altitude optimized and a much lighter escape system since your vehicle is already high enough for parachutes and gliding to be used.
Another good point with air launching is you can fly to a better location for launch then on the ground or if weather is bad just fly to a new location.

Offline libs0n

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Re: Air-Launched SSTOs
« Reply #75 on: 01/19/2008 06:54 pm »
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Sid454 - 19/1/2008  2:38 PM

I wonder if the airbus A380 also be could converted into a launch vehicle platform?

I know that the wing they designed for it is oversized/overdesigned so that it might be used on a future variant even larger than the A380, so the prospective air launcher should definitely consider it as the basis for their carrier aircraft if they decided to go with an entirely new vehicle.  I think their freight variant, which has been canceled but would presumably be relaunched once Airbus gets their act together, has a greater payload and range than the An-124, so could be considered an option in the air launch architecture I've outlined previously.  Not sure how the freight version of the 747-400 stacks up in all this.

Offline libs0n

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Re: Air-Launched SSTOs
« Reply #76 on: 01/19/2008 07:05 pm »
I forgot about another method of air launch: balloon launch.  Remember that dude who wanted to balloon launch his X-Prize entry?

edit: While I'm throwing out ideas there's also mid air refueling of a launch vehicle: I forget the name, perhaps it was Black Knight, but Mitchell Burnside Clapp proposed a horizontal take off and landing vehicle that would take on oxidiser from a tanker in an aerial operation and then perform its burn.  Supposedly the advantage was that the vehicle in air would be able to handle a greater load being placed upon it in mid flight than during takeoff, and that the landing gear would not have to be sized for the weight of the fully fueled vehicle.  I think Clapp joined Rocketplane and this is the origin of their Pioneer spacecraft, although I have not kept apprised of developments there.

I wonder if perhaps taking on oxidiser in mid air could be worked into my previously proposed towed gliding mothership concept, although it would certainly be tricky with the ever present tow line.

Offline joema

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RE: Air-Launched SSTOs
« Reply #77 on: 01/20/2008 01:42 pm »
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libs0n - 19/1/2008  11:21 AM
...I am aware that air launching does not greatly reduce the delta-v change a launch vehicle must execute....I concur that the greater expense of air launch for the benefits it provides, especially when we start talking about the construction of a new giant aircraft, may make it a marginal endeavour when compared with land launch or sea launch...
You are correct -- subsonic air launch has little delta-v benefits. A crude comparison of potential energy at orbital altitude vs kinetic energy at orbital velocity illustrates this:

The potential energy of a 10,000 kg vehicle raised vertically to 160 km orbital altitude is:

PE = m * g * h, where:
PE = energy in joules
m = mass in kg
g = gravitational acceleration (9.8 m/s/s)

PE = 10000 kg * 9.8 * 160000 m
PE = 15.7E9 joules

Compare this to the kinetic energy (lateral velocity) required for orbit at 160 km, which is about 8,046 m/sec.

KE = 1/2 * m * v^2, where:
KE = energy in joules
m = mass in kg
v = velocity in m/sec

KE = 1/2 * 10000 * 8046^2
KE = 323.7E9 joules

So airbreathing subsonic mother ship (or balloon) which could somehow reach orbital altitude would only save about 15.7E9 J/ 323.7E9 J = 5% from an energy standpoint. This doesn't include gravity and air drag savings, but as already discussed they are quite small.

This rough calculation is consistent with the delta-V savings mentioned in this article on the air-launched Quick Reach I: about 3.5%, or 1000 ft/sec out of 28,500 ft/sec:

http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=awst&id=news/102405p1.xml

You're also correct if an existing launch vehicle can be used that mostly eliminates development cost, possibly justifying the small performance advantage of air launch.

In theory an An-225 could lift a fully-fueled Atlas D (or equiv payload) to 40,000 ft. The problem is this saves little delta V, and it entails additional complexity and risk vs a ground launch.

If you fly to the equator, you pick up some rotational delta V, also nozzle can be optimized for higher attitude.

However you're basically just saving a modest amount of propellant and tankage space, both of which are dirt cheap. LOX is about $0.01 (one penny) per pound, RP-1 is about $0.40 per pound, and LH2 is about $0.90 per pound. The liquid propellant costs for a shuttle launch are only about $12,000 for LOX and $206,000 for LH2.

Air launch seems more useful for smaller, unmanned applications (esp. military). The Vought ASAT used air launch: http://www.astronautix.com/lvs/asat.htm

The Pegusus XL uses air launch. However this illustrates the limits of existing "normal" motherships. It weighs 23,130 kg, roughly equal to an X-15, which is about the max external payload of a B-52. The orbital payload capacity is 443 kg: http://en.wikipedia.org/wiki/Pegasus_rocket

A manned orbiter with significant payload would require a much larger mothership.

Offline khallow

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RE: Air-Launched SSTOs
« Reply #78 on: 01/20/2008 04:35 pm »
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joema - 20/1/2008  6:42 AM

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libs0n - 19/1/2008  11:21 AM
...I am aware that air launching does not greatly reduce the delta-v change a launch vehicle must execute....I concur that the greater expense of air launch for the benefits it provides, especially when we start talking about the construction of a new giant aircraft, may make it a marginal endeavour when compared with land launch or sea launch...
You are correct -- subsonic air launch has little delta-v benefits. A crude comparison of potential energy at orbital altitude vs kinetic energy at orbital velocity illustrates this:

The potential energy of a 10,000 kg vehicle raised vertically to 160 km orbital altitude is:

Potential and kinetic energy are not delta v! Here's my crude take. Traditionally, it takes 9.3-10k m/s of delta v to get to orbit. You also have 1-2k m/s of gravity and air resistance losses (I almost need a cheat sheet for this sort of discussion). I don't recall the details but you get negligiable benefits from higher altitude (around 30 m/s), some benefit from lower air resistance (100-200 m/s maybe), and considerable improvement from higher efficiency engines, optimized to work in vacuum. I just remember the last effect was considerably larger than air resistance, maybe 400 m/s (using comparisons of the ISP of a sea level optimized RS-68 engine operating in vacuum compared to vacuum optimized RS-68 operating in vacuum). And on top of that you get roughly 300 m/s from subsonic velocity of the carrier vehicle.

I can see considerably more delta v than 3.5% taken care of in this way. Maybe 6-8%. That means considerably better mass ratios for a launch vehicle.
Karl Hallowell

Offline vt_hokie

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RE: Air-Launched SSTOs
« Reply #79 on: 01/20/2008 04:40 pm »
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joema - 19/1/2008  11:32 AM

This is illustrated by the X-15 itself. It had many times the energy performance of Rutan's SpaceShipOne, yet wasn't remotely capable of achieving orbit. However the X-15 almost maxed out the B-52 payload capacity.

Therefore a manned orbital air launch vehicle would require a mothership much bigger than a B-52.

t/Space (http://en.wikipedia.org/wiki/T/space) is thinking of having Rutan build a gigantic mothership to air launch a man carrying orbital vehicle - January 2006 Air & Space Magazine.

The required mothership size would be titanic -- gross weight of one million lbs, payload 150 tons (3x a 747-400 freighter), wingspan 320 feet (1.5x a 747). It would the largest aircraft ever constructed.

Since development cost increases with gross vehicle weight, a sufficiently large mothership to air launch a manned orbital vehicle would be very expensive. However despite that cost, it would impart little performance benefit, if subsonically launched.

A high supersonic launch would impart some perf. benefit, but a 1,000,000 lb, Mach 3+ airbreathing launch vehicle is even more expensive. Picture something like an XB-70, but 4x the gross weight.

So, what are your thoughts on the likelihood of this existing?  Is it even remotely feasible?

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