kevin-rf - 18/1/2008 6:08 AMTwin 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.
CFE - 17/1/2008 3:01 PMQuotekkattula2 - 16/1/2008 9:21 PMTraditionally 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?
kkattula2 - 16/1/2008 9:21 PMTraditionally 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.
joema - 18/1/2008 7:24 AMQuotekkattula2 - 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.
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.
vanilla - 18/1/2008 12:10 AMI 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".
libs0n - 18/1/2008 12:06 AMQuotevanilla - 18/1/2008 12:10 AMI 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).
CFE - 19/1/2008 2:43 AMQuotelibs0n - 18/1/2008 12:06 AMQuotevanilla - 18/1/2008 12:10 AMI 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.
libs0n - 19/1/2008 3:40 AMThe 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.
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...
joema - 19/1/2008 10:32 AMWhether you use captive carry & air drop or towing, the performance advantages of subsonic aerial release are small.
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.
Sid454 - 19/1/2008 2:38 PMI wonder if the airbus A380 also be could converted into a launch vehicle platform?
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...
joema - 20/1/2008 6:42 AMQuotelibs0n - 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:
joema - 19/1/2008 11:32 AMThis 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.