Manned space flight options?

My understanding of air launch is that it is usually better to have a larger payload subsonic aircraft than a smaller payload supersonic aircraft. The major advantage of air launch is better orbital insertion and not requiring a stage to get the rocket off the ground. The speed boost isn't as significant.
Yeah, unless you can get up to Mach 3 or above, it's almost always better to get bigger payload than get supersonic.
 
phx1138 wrote:
That much freight is way more than I need, as noted. So swap for seats & call it, what, 20 men? Maybe a bit of "luggage"?

E of Pi wrote:
I'm not sure that 10,000 lbs for the t/Space concept isn't (again) the orbital vehicle's total mass, including structure. The vehicle they depict seems to be about the size of SpaceX's Dragon, and was stated to carry 4. Also, the CXV airlaunch platform (a "custom Scaled carrier aircraft" were actually based on early Rutan concepts for Stratolaunch's Roc. Stratolaunch, when publicly released a year or so later, only managed 6,100 kg of total launch mass--which would include both payload and the vehicle itself.

The link is to the old concept which required either the "Custom" carrier or serious modification to an existing (747) aircraft. Their new concept (see: http://www.nasa.gov/offices/c3po/partners/tspace/) had a new configuration that fit under a less modified 747.

It's explained in this paper here: http://mae.engr.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7835.pdf, along with showing the older configuration on a "long-gear" 747 concept. The original design was 13.5ft in diameter, and had a launch weight or about 290,000lbs and was a "single-barrel*" TSTO. The new concept was only 7.25ft in diameter but was much wider (a bit over 51ft) as it was a "three barrel*" TSTO with a launch mass of around 207,000lbs. Now the launch mass is the whole vehicle weight at launch time recall, but from what I understood the basic payload was the same in both versions, which I mis-typed as it's supposed to be 4 crew OR around 10,000lbs of "payload" (with no capsule) and around the same up-mass of cargo as the Dragon.

The capsule may in fact be a bit "smaller" than the one cited and closer to the size of the HMX HX Transfer Vehicle proposal for launch on surplus Titan-II missiles as proposed by Gary Hudson. (Sorry no links I can find but mentioned/shown here: http://www.hobbyspace.com/AAdmin/archive/Interviews/Systems/GaryHudson.html, similar itself to the Phoenix CEV concept for the Falcon-V here: http://rascal.nianet.org/wp-content/uploads/2015/07/2006-RASC-AL-UMD-Tech-Paper.pdf)

*- The difference is the "one-barrel" is a traditional two stage rocket design, while the "three-barrel" has two outer boosters with a central "core" stage that is the second stage.

Something to keep in mind is that both designs were built around the VAPAK self-pressurization system so the LV is of the pressure-fed variety which was to keep the design and operation simple and cheap. Arguments can be made for using a similar but more efficient pump-fed design though my personal preference is to keep the propellant combo (LOX and Cryogenic Propane) in order to keep the bulk of the LV smaller.

phx1138 wrote:
All of which had me thinking the top-carry/launch was the better option to begin with. (Hell, I've seen pix of the loading of the X-1 into an NB-29: they park the 'fort over a pit.)

If that's what you got out of my explanation then I obviously failed to highlight the disadvantages :) See the above cited paper for an experts opinion, (on the other hand there are a lot of other experts, the DARPA Air Launch report comes to mind, who continue to assume that top-launch is always better anyway :) ) on the tradeoffs.

As for the loading, they did that with the early atom bombs too and for the same reason; Not enough clearance under the aircraft to roll the item under the airframe. If the X-15 wasn't carried on a wing pylon but in a centerline mount they would have had to figure out a way to raise the B-52 or put the X-15 in a pit to load it :)

The Mach-6 BETA-II had the Orbiter being towed with its own landing gear under the carrier vehicle, which had landing gear that could lift it higher off the runway. Once mated using on-board hoists the carrier would lower it's gear to a "normal" position. Note that the AirLaunch/T-Space concept now simply over-fills the landing gear olos which is within acceptable maintenance and operations procedures to allow the same thing.

Top loading requires something like the Shuttle/747 mate/de-mate stands to accomplish.

RanulfC wrote:
No what he's describing IS insanely risky The aircraft is stalling, fast, (unless as in the cited air-launch concepts above you do something even MORE crazy like putting a Space Shuttle engine in the tail of the 747 and light it up to keep the 747 flying while you release the LV) meanwhile once you release you carrier aircraft is trying to "push-over" (with a top-mounted LV) or pull into a loop (with a bottom mounted LV) and did you remember to seriously reinforce the whole airframe to actually pull off this maneuver in the first place? No? Well not to worry, firing the SSME probably tore your wings off anyway, if not the negative (push-over) or positive (loop) Gs will probably do that anyway...

phx1132 wrote:
That does depend, to some measure, on the engine the L/RV is using, doesn't it? I'd guess an X-15 (or something close) wouldn't produce the exhaust blast of an SSME.)

That's not even the engine of the LV we're talking about but the rocket engine in the tail of the carrier aircraft to push it up to an acceptable AoA for launch :) Remember your "sweet-spot" is an angle to the local horizon of between 35-and-70 degrees which is what the carrier aircraft has to pull up to be in a proper launch position.

Or you put wings on the LV and have IT perform all the needed maneuvering, (which is how Pegasus does it), or use something like T-Lad.
But the cited proposal actually had the carrier aircraft doing the maneuver (it's called a "Gamma maneuver" for some reason, maybe because "Omega" maneuver would have been off-putting for the flight crew? :) ) and the LV had to light off two of the 10 RL10s powering it to ensure separation.

phx1138 wrote:
I'm not opposing a dedicated "carrier", even one capable of Mach 6 & costly. I'm after something to replace throwaway (or nominally reusable) lifters like the Saturns or SRBs for crew-only missions: essentially, split the STS task in two, & make the L/RV truly "flyback" (within limits; IDK I'd demand jets for cross-range, when you'd want to return to a maintenance &/or launch base anyhow). I'm happy if it's a Citation-size crew load, rather than a DC-3 or L049.

E of Pi wrote:
If you're building a custom Mach 6 flyback carrier, you might as well just make it about 50% larger and get a payload that's actually usable for commercial cargo missions as well as just passengers.

From my readings around 10,000lbs is considered a "viable" up-mass for orbital use, but that's not clearly defined as to what all that entails as the definition varies. For example, Cargo-Dragon, Cyngnus, and CST-100 all are supposed to deliver about the same mass to ISS/orbit but they are shown to only be delivering around 5,000lb to 7,000lbs (2,268kg to 3,175kg) of the launch vehicles "payload" capacity which varies between 21,610lbs (9,800kg) to 63,450lb (28,790kg) and that last is a Delta-IV I might add. On the other hand you'll note that the Beta and proposals like the Black Horse specify 10,000lbs of "payload" in addition to at least two crew members. (That would "work-out" to something like 20 people including the crew but without things like seats, extra life support, etc. So offhand I'd figure no more than 15 and most likely 10 including crew) However your other constraint is physical space as most concepts have a "bay" or housing that's not designed for people but a general "cargo" configuration. The generally circular configuration is going to have a lot of space eaten up by seating arrangements that allow functional use of the space either in orbit or on the ground. (If you've ever seen the pictures of the mockup for the Big Gemini capsule, take a close look at where the astronaut is sitting in the aft-upper-left side seat. As the vehicle is on the launch pad and his back is "down" getting into the seat is pretty easy. Now look at the angle it's at in the mockup and then notice that the capsule is still not "landed" yet and he's got about another 10-15 degrees of "face-down" angle to deal with AND he's like 6 ft off the "floor" in the horizontal position. How the heck does he get out of his seat without lots of help or killing himself? :) )

And just to stir things up even more, why build a 'dedicated carrier aircraft" at all even if you want to use air-breathing propulsion?
May I introduce the "Spacejet" LV concept:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780003229.pdf
http://www.secretprojects.co.uk/forum/index.php?topic=11026.0
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810020560.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810024617.pdf

Your building "aircraft" to house your jets engines but the lift and flight control comes mostly from your LV. The design only addressed an orbital vehicle with "Shuttle" capacity payload and a staging speed of around Mach-3.5 but the overall idea is scalable. (I've done some really rough BOTE stuff on using a Black-Horse sized Orbiter with smaller versions of this concept that stages at similar velocity and it shows a real solid payload of a bit over 10,000lbs, though how you actually fit that in something that's supposed to be the size of an F-15 I'm not at all sure :) ) And you can in fact get the turbojets to operate up to around Mach-6 with some additional systems installed (Mass Injection, Pre-Compressor Cooling using water and lox doubles your jet compressor face Mach number capability and thrust for example) and if you build a Beta-like "over-under" or combined turbo-ramjet system...

As long as you're not concerned with cost, (and that's really the rub because costing-aeronautical/aerospace systems is a pretty well defined art and going supersonic is expensive, while hitting hypersonic really starts to cost money) and as long as you don't demand excessive air-breathing time, (lets fact it do you really NEED to go faster than Mach-6 with all the structure and costs that go with it? Most times the answer is a definite "no") or exotic propulsion (SCramjets for a really good example) the whole concept is probably feasible. If you've got the money.

On the other hand a more "reasonable' approach was a concept called "CRoSSBoW" which can be seen here:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070002822.pdf

Which supposes (again) to reduce your carrier aircraft to the smallest (and therefore cheapest) possible platform. Here it's a subsonic platform but all flight control is from the LV with only enough to autonomously return to the launch site. The LV provides the thrust (and that way you check out the rocket engine before release) for the gamma-maneuver and then releases the carrier to fly back to base while it continues on to orbit.

The study is actually more of a general study of Air Launch options and trades than a full-up concept and quite an educational read overall.

Randy
 
RanulfC said:
The link is to the old concept which required either the "Custom" carrier or serious modification to an existing (747) aircraft. Their new concept (see: http://www.nasa.gov/offices/c3po/partners/tspace/) had a new configuration that fit under a less modified 747.

It's explained in this paper here: http://mae.engr.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7835.pdf, along with showing the older configuration on a "long-gear" 747 concept. The original design was 13.5ft in diameter, and had a launch weight or about 290,000lbs and was a "single-barrel*" TSTO. The new concept was only 7.25ft in diameter but was much wider (a bit over 51ft) as it was a "three barrel*" TSTO with a launch mass of around 207,000lbs. Now the launch mass is the whole vehicle weight at launch time recall, but from what I understood the basic payload was the same in both versions, which I mis-typed as it's supposed to be 4 crew OR around 10,000lbs of "payload" (with no capsule) and around the same up-mass of cargo as the Dragon.

The capsule may in fact be a bit "smaller" than the one cited and closer to the size of the HMX HX Transfer Vehicle proposal for launch on surplus Titan-II missiles as proposed by Gary Hudson. (Sorry no links I can find but mentioned/shown here: http://www.hobbyspace.com/AAdmin/archive/Interviews/Systems/GaryHudson.html, similar itself to the Phoenix CEV concept for the Falcon-V here: http://rascal.nianet.org/wp-content/uploads/2015/07/2006-RASC-AL-UMD-Tech-Paper.pdf)

*- The difference is the "one-barrel" is a traditional two stage rocket design, while the "three-barrel" has two outer boosters with a central "core" stage that is the second stage.

Something to keep in mind is that both designs were built around the VAPAK self-pressurization system so the LV is of the pressure-fed variety which was to keep the design and operation simple and cheap. Arguments can be made for using a similar but more efficient pump-fed design though my personal preference is to keep the propellant combo (LOX and Cryogenic Propane) in order to keep the bulk of the LV smaller.
That's interesting stuff. Thx for that, & for the links.:cool::)
RanulfC said:
If that's what you got out of my explanation then I obviously failed to highlight the disadvantages :)
That's not all I got,:p but I'd already guessed it was a marginal (if not outright bad) idea.;)
RanulfC said:
the above cited paper for an experts opinion, (on the other hand there are a lot of other experts, the DARPA Air Launch report comes to mind, who continue to assume that top-launch is always better anyway :) ) on the tradeoffs.
I'd guess there's always a tradeoff when there's something to be carried. How much of the design of jet engine pylons is tradition? An overwing pylon would work pretty well, too, no? So...
RanulfC said:
As for the loading, they did that with the early atom bombs too and for the same reason; Not enough clearance under the aircraft to roll the item under the airframe. If the X-15 wasn't carried on a wing pylon but in a centerline mount they would have had to figure out a way to raise the B-52 or put the X-15 in a pit to load it :)
I have a vague recollection of the Bomb being loaded that way. I'm not surprised.:) That's the thing: if the RV has to be loaded that way, how close to scraping the runway will it be on takeoff?:eek: And I knew that was an issue as far back as the X-1.;)

Top-carry means issues at launch, which is why I proposed JATO to clear the carrier before lighting the main engine; IMO, some sacrifice on launch beats the risk of catastrophe on takeoff.:eek:
RanulfC said:
That's not even the engine of the LV we're talking about but the rocket engine in the tail of the carrier aircraft to push it up to an acceptable AoA for launch :) Remember your "sweet-spot" is an angle to the local horizon of between 35-and-70 degrees which is what the carrier aircraft has to pull up to be in a proper launch position.
I'm just beginning to realize how little I understand of this issue...:eek:
RanulfC said:
From my readings around 10,000lbs is considered a "viable" up-mass for orbital use.... I'd figure no more than 15 and most likely 10 including crew) However your other constraint is physical space as most concepts have a "bay" or housing that's not designed for people but a general "cargo" configuration. The generally circular configuration is going to have a lot of space eaten up by seating arrangements that allow functional use of the space either in orbit or on the ground.
That's a good reason, IMO, to use a dedicated crew L/RV, rather than a "combi"; if it needs to bring back (some) cargo, it can, but volume, & airframe mass, don't get wasted for its primary mission. I'm also seeing this, in case I wasn't clear, as a "point to point" vehicle, so the crew would stay in their seats (more/less) until rdv with the orbital station (which I picture as a "super Mir", with more modules, rather than ISS).
RanulfC said:
(If you've ever seen the pictures of the mockup for the Big Gemini capsule, take a close look at where the astronaut is sitting in the aft-upper-left side seat. As the vehicle is on the launch pad and his back is "down" getting into the seat is pretty easy. Now look at the angle it's at in the mockup and then notice that the capsule is still not "landed" yet and he's got about another 10-15 degrees of "face-down" angle to deal with AND he's like 6 ft off the "floor" in the horizontal position. How the heck does he get out of his seat without lots of help or killing himself? :) )
I wouldn't sign off on it for this task.:rolleyes:;)
RanulfC said:
And just to stir things up even more, why build a 'dedicated carrier aircraft" at all even if you want to use air-breathing propulsion?
May I introduce the "Spacejet" LV concept:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780003229.pdf
http://www.secretprojects.co.uk/forum/index.php?topic=11026.0
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810020560.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810024617.pdf

Your building "aircraft" to house your jets engines but the lift and flight control comes mostly from your LV. The design only addressed an orbital vehicle with "Shuttle" capacity payload and a staging speed of around Mach-3.5 but the overall idea is scalable. (I've done some really rough BOTE stuff on using a Black-Horse sized Orbiter with smaller versions of this concept that stages at similar velocity and it shows a real solid payload of a bit over 10,000lbs, though how you actually fit that in something that's supposed to be the size of an F-15 I'm not at all sure :) ) And you can in fact get the turbojets to operate up to around Mach-6 with some additional systems installed (Mass Injection, Pre-Compressor Cooling using water and lox doubles your jet compressor face Mach number capability and thrust for example) and if you build a Beta-like "over-under" or combined turbo-ramjet system...

As long as you're not concerned with cost, (and that's really the rub because costing-aeronautical/aerospace systems is a pretty well defined art and going supersonic is expensive, while hitting hypersonic really starts to cost money) and as long as you don't demand excessive air-breathing time, (lets fact it do you really NEED to go faster than Mach-6 with all the structure and costs that go with it? Most times the answer is a definite "no") or exotic propulsion (SCramjets for a really good example) the whole concept is probably feasible. If you've got the money.

On the other hand a more "reasonable' approach was a concept called "CRoSSBoW" which can be seen here:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070002822.pdf

Which supposes (again) to reduce your carrier aircraft to the smallest (and therefore cheapest) possible platform. Here it's a subsonic platform but all flight control is from the LV with only enough to autonomously return to the launch site. The LV provides the thrust (and that way you check out the rocket engine before release) for the gamma-maneuver and then releases the carrier to fly back to base while it continues on to orbit.

The study is actually more of a general study of Air Launch options and trades than a full-up concept and quite an educational read overall.
Wow.:eek::eek::cool::cool::cool: I'd never have dreamed. Given a flyback RV, that makes enormous sense.:cool: Lifting the L/RV is really the only issue; it could very well be a kind of Thunderbird 2. (I am ashamed I never thought of it...especially as big a fan of "Thunderbirds" as I was.:eek::eek:)
 
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So a modified B-70 might have enough speed to allow some interesting things?
Speed, yeah. Carrying mass...not so much. A single hydrogen stage could put about 1,300 kg into orbit using the XB-70's 70,000 lb carrying weight. Two hydrogen stages could maybe push that to 3,000 kg--still only about half what a 747 could carry. Modify it to exhcange the B-70's fuel load for a heavier rocket, push that from 70,000 lbs to more like three times that (there's room in the design for lifting 300,000 lbs of fuel, which we won't need all of)...maybe you get closer to parity with the 747. Call it optimistically 9,000 kg to LEO, using a two-stage hydrogen vehicle, though hydrogen has so many handling issues. That could be a satellite to LEO or a vehicle and its payload.

OTOH, the 747 will likely be far cheaper to operate. It's certainly easier to get a hold on one for anyone but the USAF. You really need to go bigger than the B-70 and preferably also faster for real benefits. Mach 3 and 25 km is just kind of piddling on rocket scales. A proper first stage gets going about twice as fast and several times as high, and as SpaceX proved, it is possible to recover a stage from those conditions.
 

Delta Force

Banned
Speed, yeah. Carrying mass...not so much. A single hydrogen stage could put about 1,300 kg into orbit using the XB-70's 70,000 lb carrying weight. Two hydrogen stages could maybe push that to 3,000 kg--still only about half what a 747 could carry. Modify it to exhcange the B-70's fuel load for a heavier rocket, push that from 70,000 lbs to more like three times that (there's room in the design for lifting 300,000 lbs of fuel, which we won't need all of)...maybe you get closer to parity with the 747. Call it optimistically 9,000 kg to LEO, using a two-stage hydrogen vehicle, though hydrogen has so many handling issues. That could be a satellite to LEO or a vehicle and its payload.

OTOH, the 747 will likely be far cheaper to operate. It's certainly easier to get a hold on one for anyone but the USAF. You really need to go bigger than the B-70 and preferably also faster for real benefits. Mach 3 and 25 km is just kind of piddling on rocket scales. A proper first stage gets going about twice as fast and several times as high, and as SpaceX proved, it is possible to recover a stage from those conditions.

What if an air-augmented rocket akin to the Gnom ballistic missile were used?
 
What if an air-augmented rocket akin to the Gnom ballistic missile were used?
By 50,000 ft, you're down to air that's only 15% as dense as at sea level. There's barely air to augment with, and there's certainly not much as soon as you start climbing--which as Ranulf notes is task number one for air launch. A lot of the benefit of air launch in subsonic forms comes from starting above so much of the atmosphere you can essentially just fit a vacuum nozzle. I'd have to run some more specific numbers, but my gut is that you'd be lucky if the air augmentation system increased payload enough to make up for its weight. The issue is that a supersonic drop platform is doing the part of a normal first stage's job where air augmentation might help the most.
 
By 50,000 ft, you're down to air that's only 15% as dense as at sea level. There's barely air to augment with, and there's certainly not much as soon as you start climbing--which as Ranulf notes is task number one for air launch. A lot of the benefit of air launch in subsonic forms comes from starting above so much of the atmosphere you can essentially just fit a vacuum nozzle. I'd have to run some more specific numbers, but my gut is that you'd be lucky if the air augmentation system increased payload enough to make up for its weight. The issue is that a supersonic drop platform is doing the part of a normal first stage's job where air augmentation might help the most.

Air augmentation of a rocket was another thing that was widely studied using various methods. For straight-up simple, augmentation (like the Gnom) it's best used from sea-level and only works to about Mach-2 or so. Mostly because you're using very simple ducts, intakes, and exhausts. It was on par with using SRBs to boost take-off performance though it was less mass. The downside that got it set aside was simply that SRBs can burn longer and therefore give a more effective boost which made it very less economic for an expendable rocket stage.

On the other hand augmentation of an air-breathing engine to produce more thrust and operate at higher altitudes/speeds was a staple of early jet engines due to their lack of thrust-to-weight capability. Everyone settled on water-injection early on because it was the easiest to work with operationally but there were a huge number of "injectents" and injection points studied. But it all went by the wayside as jet thrust and efficiency increased.

Now having said that we know quite well that water injection works quite well to increase mass flow, what was hinted at in earlier work was that its not just the 'water' but the fact it cools and condenses the incoming air as well. Research in pursuit of the concept of LACE (Liquid Air Cycle Engines) where incoming air was turned into liquid air or oxygen and fed into a rocket engine turned up an interesting fact that wasn't appreciated at the time.

Long before the air becomes a liquid it gets very, very dense. Chamber tests simulations found that this densification effect worked quite well from sea-level to altitudes up to 100,000ft with the higher speed at altitude the better. (Makes sense, there's less air so the faster you go the more you 'ram' into the intake as you go, though IIRC the speeds they were discussing around there were Mach-10 or higher)

Thing was the focus of the effort was to get Liquid Air/Oxygen not dense air so other than effects it had on intake air "choking" and such it wasn't really considered.

Then comes the 1990s and a project called RASCAL, which proposed to use "off-the-shelf" F100 fighter engines and a process called "Mass Injection, Pre-Compressor Cooling" (MIPCC) using water and LOX injected into the engines to allow them to operate at high thrust and speed to altitudes of around 70-75,000ft. Testing found that the system could allow the engines to double their thrust while also doubling their capable Mach number. (Maximum mach number is derived from the materials temperature limits of the compressor face so in this case they could not reach Mach-4 instead of just Mach-2 and some change)

Funny thing was that while most people thought the LOX was augmenting the air at high altitude, which it was to an extent, it's main purpose was to super-cool the incoming air and significantly increase it's density over just using water alone. (It had a secondary role of stabilizing the combustion chamber flame at high altitude) Bu the main take-away from this was that you suddenly had a way to increase the thrust of a jet engine at high altitude with very little added mass and complexity.

Well SOME jets I should say, as the process works best on turbojets (which almost no one flies these days) or low-bypass turbofan (such as modern military engines) as the way a high-bypass turbofan works doesn't lend itself to using the system.

Not to leave them out though, during the various "Shuttle-II" studies (Spacejet among them) the idea of using "commercial" jet engines was revisited numerous times and one method of significantly increasing the thrust of a high-bypass turbofan was to include a system that introduced "duct-burning" (burning a fuel in the airflow of the bypass duct) which doesn't seem to work all the well with standard kerosene but works quite well (in tests, not sure if anyone has ever actually flown one of these) with hydrogen.

I should point out here that standard subsonic-combustion ramjets would benefit as much if not more from MIPCC type augmentation as well. And they have been designed and built to fly at high altitudes and speeds probably in excess of what you'd want for a booster stage.

Now getting into some more exotic, but proven technology you have other engine cycles that you can use such as Supercharged Ejector Ramjet, Rocket and Turbine Based Combined Cycle systems and again any of them benefit to varying degrees with augmentation.

Randy
 
Speed, yeah. Carrying mass...not so much. A single hydrogen stage could put about 1,300 kg into orbit using the XB-70's 70,000 lb carrying weight. Two hydrogen stages could maybe push that to 3,000 kg--still only about half what a 747 could carry. Modify it to exhcange the B-70's fuel load for a heavier rocket, push that from 70,000 lbs to more like three times that (there's room in the design for lifting 300,000 lbs of fuel, which we won't need all of)...maybe you get closer to parity with the 747. Call it optimistically 9,000 kg to LEO, using a two-stage hydrogen vehicle, though hydrogen has so many handling issues. That could be a satellite to LEO or a vehicle and its payload.

OTOH, the 747 will likely be far cheaper to operate. It's certainly easier to get a hold on one for anyone but the USAF. You really need to go bigger than the B-70 and preferably also faster for real benefits. Mach 3 and 25 km is just kind of piddling on rocket scales. A proper first stage gets going about twice as fast and several times as high, and as SpaceX proved, it is possible to recover a stage from those conditions.

First of all while LOx/LH2 is the king of propellants their bulk and operational conditions tend to make them far from "economical" as choice propellant. Especially for air-launch where you have to worry about bulk and drag. Methane is the most often suggested alternative as it's less bulky than LH2. Kerosene meanwhile is "preferred" by a lot of folks due to it's operational and infrastructure depth. I'm going to again point out my personal favorite combination: LOX and Cryogenic Propane. By cooling the propane to the same temperature as lox you get a very dense (about the same as kerosene) propellant with about 80% methalox's ISP. The only problem is finding a engine to run it in as the only one known is the ones AirLaunch designed or the RL10.

And on the subject of recovery, while SpaceX is doing amazing work the key point is that despite what Elon Musk wants their not going to be operating them with "aircraft-like" efficiency which is the key argument applied to using "aircraft" for boosters. Unfortunately that argument runs up against economic and operations questions the moment you go beyond anything currently operating as an aircraft. Supersonic aircraft are expensive. Big Supersonic aircraft are even more expensive. Big High-Supersonic or Hypersonic aircraft... Well we don't really know but evidence says we can't assume they will be cheaper at all :)

Can't find it at the moment but there is an interesting study out there on the maintenance and operations costs of a reusable (rocket powered in this case) booster based on the experience with the X-15. It does NOT leave a good impression on the possibility of such, however take with a grain of salt, (really, the X-15 was an experimental, high-speed, high-altitude, rocket powered vehicle and the complaint is it wasn't as "easy" as they think it should be?) and keeping in mind what the X-15 was and was not it does leave the impression that today we could build a reusable vehicle that while it may be more costly than a "normal" vehicle will still be economical in the context of it's use.

Couple that with concept that you don't actually have to build a "big" carrier if you exclude things like crew and such and it opens up some possibilities worth considering.

Randy
 
Musk may be thinking "scheduled", which the "wing-carrier" (rather than "carrier aircraft" might give you.
 
Any orbital variant of an X15 wouldn't have been launched from an airplane.

http://www.astronautix.com/craft/x15b.htm

actual planes involved various kludged together missile boosters.

Note, too, that it would have needed an aeroshell for protection on reentry, and some plans involved having the astronaut bail out and not even attempt to land the plane.

So it was a very, VERY strange plan. Far more expensive and harder to do than simple capsules.
 
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