Great article. I've not heard about any of the other traditional rocket builders pursuing reusable rockets recently. If SpaceX is successful in their goal of reusability, they will be able to undercut everybody else. But, they will have no incentive to reduce prices significantly until they have competition.
Yeah, no one else has made any announcements wrt re-usability. It seems odd, but I guess that's why they call it "dino-space".
Ariane 6 seems to be in response to the SpaceX challenge, at least; they aim to halve the cost per kg through cheaper and common upper/lower stage components. Which is good, but still not competitive with F9, much less a reusable version. At one point they almost made it all-solids in the first two stages, plus boosters, which would have totally doomed any future attempts at re-usability. The final design uses a F9-style common component liquid core, so they could maybe do the same thing as SpaceX later. They are still stuck with solid boosters, though, in a pork-provision move for Italy.
I've got to imagine that ULA is quietly looking into it. But it's quite possible they're just hoping SpaceX will eventually implode of its own internal contradictions and they can go back to their nice little government-launch monopoly. Or maybe they've decided they can't do it without absurd expense, based on their internal development costs. Or maybe they just hope to coast on their existing contracts for a while, and deal with it later once SpaceX has taken all the lumps.
NASA, via SLS, sure isn't looking at re-usability anymore. Lower cost to orbit via research should be their main task, if you ask me, but Congress would rather have them fly giant rockets a couple times a decade, apparently.
The Russians, well, are not much into changing things up. (They've been running Soyuz and Proton, their current launch families, since the sixties!) And their recent aerospace development programs have been less than impressive. But they have a real industry to protect, so one would hope they have a strategy.
I think that everyone's waiting for SpaceX to make it work, and then they'll get to copying. My long-term guess is that this'll kill several of the incumbents, a few will make the transition eventually, but most of SpaceX's competition will come from new entrants. In the meantime, SpaceX will be free to reap the profit from the margin between what it costs them and what their old-fashioned competitors can do. And/or capture the whole darn market. And/or grow it with lower prices. Maybe all of the above.
You could always say they are competing with themselves. The lower the price, the more customers they can get, which means more launches, which lowers the cost further.
SpaceX is very, very far ahead of the field, although it's not quite obvious at the moment, though it will be soon.
You see, there are two ways to build reusable rockets. One way is to build a purely reusable vehicle from the ground up, but this is extraordinarily expensive because of all the testing involved before it can be used commercial. The other is to build an expendable/reusable hybrid. A rocket that can be used in an expendable configuration but has all of the needed elements to allow for reusability down the road. This is vastly easier because it means you can bootstrap development on the back of commercial launch business, and do testing that is largely subsidized by paying customers. And this is what SpaceX has been doing.
However, there's a problem. You have to head into reusability from the get go, you can't just modify any old rocket for reusability, you need to design from a place of suitability for reuse from square one. And while that's not an enormous difficulty it is very much non-trivial, and imposes a substantial overhead on R&D and manufacturing costs. This is because most of the optimizations for building a purely expendable rocket drive the design away from the optimizations you'd want for a reusable rocket. In an expendable design it's common for there to be a mix of propellants and a diversity of engine designs between the first and upper stages, it's also common for the upper stage to be more expensive than the first stage, because that stage is more critical for payload performance and launch precision. Also, it's common to minimize the number of engines on the first stage. Even the Saturn V had only 5 first stage engines. And generally there'd be no reason to add first stage engine restart capability on an expendable vehicle.
All of those things results in an expendable vehicle that is completely unsuited to reuse. It leads to a first stage that is very difficult to modify for reuse, and an overall rocket design that does not reward the easiest type of reuse (of the first stage) because most of the cost is in the upper stage anyway. SpaceX went to a great deal of effort to ensure that they built a rocket that could serve the expendable launch market immediately (and pull in hundreds of millions of dollars in revenue) while also being suitable for reuse with comparably moderate modifications. The first stage has 9 engines, so throttling down to a thrust level suitable for precision landings of a mostly empty stage is possible by using only one engine. The first stage use restartable engines (using onboard TEA/TEB igniters). And the first stage is about 3/4 of the hardware cost of the vehicle, since the 2nd stage uses only one engine and is also LOX/Kerosene fueled.
That foresight has allowed SpaceX to be in the launch business for several years, proving the capability of the company and gaining much needed experience in building and flying rockets while also poising the company to bring to market a reusable launcher. In comparison, every other established launch provider in the world could only do reusability by designing a new launcher from scratch.
Werner von Braun originally wanted to use reusable boosters about the size of Space-X's Falcon, launch huge numbers of them, and build a big space station. See his 1952 book, "The Mars Project". (That was written before we found out that Mars barely has an atmosphere.)
The article suggests that SpaceX does not use specialized, radiation hardened onboard computers, but rather they use triple redundant off-the-shelf computers.
Why wouldn't they use radiation hardened computers, though? Are they that much heavier and more expensive than the more commonly available components? Out of a $40 million spacecraft, the difference of a few thousand dollars or even a hundred thousand dollars seems rather trivial, in exchange for a more reliable avionics system.
> It's really not the expense that drives it. We're committed to having the best possible parts in all of our designs. So if it cost a lot and we needed it, we'd go get it. We were already required to have all this redundancy in the computers to meet all the different safety requirements. Then we started looking at what parts do we want to use and what is appropriate for this design. And what really is more important to us than the cost of the parts is the capability of the parts – how much power do they use, how much memory do they hold, how much do they process, and how physically big are they. That's the first thing.
> The second thing is what tools they come with. We run the Linux operating system, we program everything in C++, and that enables us to tap into a huge pool of very talented people and find the absolute best people in the computer and software industry to work with us. If you go into the radiation hardened parts, they are very limited in terms of what languages you can work in, what support packages there are for them, who knows how to program in them. It really limits your ability to work with the parts. And the other thing it really does is they all take a little longer time to get and they're a little harder to come by.
At least for Falcon, I imagine radiation-hardened components probably aren't much of a gain. A rocket's operational lifetime is on the order of an hour. Triple- or 5-redundant computers are really effective for shorter missions like this because loss of one computer (due to radiation damage) doesn't have a long-term impact like it would on a deep space mission. At least that's my guess.
Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch?
Also, there is something about using the same engine on every stage to gather more data per launch, but iirc different engine profiles are used for different velocities and air densities long the launch. I wonder what's the efficiency loss of using a single engine profile for the whole launch.
Probably not. You only need a few seconds of propellant for landing: a mostly-empty stage weighs much less than a full one, and even one engine is enough to slow it down real fast. I've done some really rough guess-numbers before, landing on 5-10% of a full propellant load needed for landing.
It's probably not even as bad as that, since they certainly fly with some performance reserve already, at least several percent, and most payloads are probably not max-weight. The landing propellant in many cases may be entirely free, mass-budget-wise. They do add some additional mass for the legs and fins, in addition to any extra propellant they need, so it's not free, but the first stage is not as performance-sensitive as higher stages (see: rocket equation), so they can get away with it. I don't see 'em getting second stages back any time soon, though, and I think they've given up on that, at least for F9.
WRT engines, the efficiency based on air density is mostly a function of the nozzle, and the upper-stage Merlins have a nozzle extension to deal with this. Higher-efficiency fuels (i.e LH2) are historically preferred for upper stages, since they are more performance-sensitive, so SpaceX is losing some performance there. But LH2 is super-cryogenic and a beast to work with, and an entirely different engine. SpaceX's thing is that they are willing to sacrifice maximum efficiency for reasonable cost, so they decided to make that tradeoff. It seems like a good plan so far.
Elon Musk said the fuel costs $300,000. The first stage and 9 Merlin engines cost ~$40,000,000.
The Merlin 1-D Vacuum used on the upper stage does have a wide vacuum optimized nozzle, but the other parts are almost the same, to save costs during manufacturing and testing.
The article is ok, but says nothing new to people who have followed SpaceX for years. There is a lot of info online about the company and its products.
> Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch?
It does reduce payload capacity (30-40%, IIRC) but the expected upside is drastic cost reductions. If I can launch several payloads for the price of one as a result, it becomes well worth it, and if I've got a large payload I can either use the upcoming Falcon Heavy or we'll start doing more in-orbit assembly.
Thanks to the wonders of the rocket equation mass that is only present on the first stage is much cheaper than mass that has to be boosted through the second stage as well. And air resistance is a big deal so it helps a lot that it's now working for you rather than against you.
It's funny seeing people complain about throwing away fuel when we're already throwing away rocket engines now. Spoiler: the engines are more expensive.
I am sure they did the math but at first I thought that landing legs would be unnecessary if you have a system that 'catches' the rocket on the ground. Since the rocket can hover, it would be possible to grab the rocket with a structure/claw.
The weight of the legs is probably quite low so this might have been the easiest solution.
A single Merlin engine, even at low throttle, has quite a bit more thrust than a (nearly) empty F9-1 stage has weight. Thus, it cannot hover; once it slows to 0, it quickly starts going up again.
Thus the landing profile is the "hover-slam": the engine is started at the exact point in time necessary to reach a "landable speed" (from 0-several m/s) at the ground, and is cut off when it gets to the target.
While modern avionics are quite good, it's still coming in pretty fast and from far away and will have some variance in location and speed, just because that's the real world. Probably they don't quite know how much yet, but it could easily miss the target by multiple meters and a few meters/s. And that's a heck of a condition to build some sort of giant grabbing apparatus for.
Perhaps once they're routinely landing on land, they may be able to determine they can hit the target well enough to build a grabbing contraption. Or maybe it's too hard. Or maybe they'd rather keep their infrastructure to a minimum: a concrete pad is a lot cheaper to build and easier to operate, and SpaceX wants to do a lot of flights.
Besides, the 4000lb of the legs is on a 40,000lb rocket.
This has been brought up multiple times in different places. The basic consensus is that a claw system would require more infrastructure on the ground as well as a more accurate guidance system.
The F9v1.1 cannot hover when landing as the TWR with an engine at 60% thrust (the minimum) is almost 2.
Has anyone looked at only using a solid rocket booster to get to orbit? Would that be cheaper, perhaps only needing a singe stage plus orbital insertion?
Even if unsafe could it still make sense for unmanned launches, supplies, cargo?
Sure. There are all-solid ICBMs, so there are flying examples. See Minotaur, a 5-stage solid booster made from converted ICBMs.
An earlier design for the Ariane 6 was 4 identical solid boosters, 3 on the 1st, and one for the 2nd stage--before they came to their senses.
Solids are convenient and reasonably simple and good at high thrust and smaller scale, but don't scale up as well as liquids. (You try casting a fuel grain the size of an office building!) So they're not well suited for launching anything particularly large into orbit. And unfortunately they have nowhere near the mass fraction to do SSTO. Liquids, though, are tantalizingly close.
All solid LV's haven't really been cheaper than liquids in the general case, the motors themselves tend to be quite expensive. Also, a single SRB is never sufficient, even with 0 payload a very large solid doesn't have the propellant-mass ratio to get even close to orbit. Something like Pegasus or (I think) Athena 1 were 3 stage designs. Being solid helps simplify the ground support infrastructure and launch pad, whose costs don't scale down well. With an all-solid, something like the Kodiak launch complex is feasible, an extremely simple assembly building + pad.
Can anyone point me to more information on this line from the article, regarding the space shuttle: "But the final design, modified to accommodate Air Force requirements, ended up being only a partially reusable launch system."?
There were several, but the two that stand out[1]:
1) The payload bay grew in size from 12 ft wide to 15 ft wide, in order to accommodate future unspecified military needs.
2) There were new cross-range requirements specified for reentry.[2] Basically, the military wanted to be able to launch satellites into polar orbits (useful for surveillance) and retain the capability to land after one orbit of the Earth. However, this meant that the Shuttle also needed to be able to -reach- the landing sites, which are normally not in the direct orbit path, but are divert 1000 miles away laterally (the cross part of cross-range).
In short: it needed to be capable of doing a once around polar orbit launch, which meant that when it came around back on the next orbit it would be several hundred miles away from the landing strip in Vandenberg. So it needed a huge cross range flight capability, which meant very large wings. Large wings meant more weight and a stronger airframe (also more weight), as well as a more complex and difficult thermal protection system. It raised the cost and complexity of the vehicle a great deal, though it wasn't the only reason why the Shuttle didn't meet its design goals.
Someone use drones already. Whoever is using drones to launch many air-breathing components that return on their own and launch stages that glide in after re-entry wins. The rest of this stuff has obvious gigantic weight disadvantages.
Current gas turbines are not optimized for pure thrust-to-weight or else why are military engines outperforming civilian in thrust-to-weight? Turbofan-to-turbo-jet for high Isp in atmospheric flight and then fly everything back on its own. Firefly's rocket has a max thrust of 90klb on the main while a big gas turbine pulls 100klb easy. While we're talking about Isp and re-usability, the entire first stage of a rocket could be gas-turbine powered and the engines just peel away and fly home after they're done. The engine needs gas. The gas takes space. Put the gas in a wing. Wing glides the engine home after launch. Easy as 1,2,3-a-bunch-of-engineering. Go!
You're advocating for air launch, not drones - the Falcon 9 / Dragon is already an unmanned drone craft.
It has been successfully tried - http://en.wikipedia.org/wiki/Pegasus_%28rocket%29 - but my understanding is payloads are pretty severely limited. You can see the size of the Pegasus launcher is much, much smaller than a Falcon 9 - it's certainly not going to work for SLS/Falcon Heavy/MCT-sized payloads.
edit: For ground-launch with jets as the first stage, I really don't think that's going to work.
A high thrust-to-weight air-breathing flyback first stage is certainly attractive. Load up a bunch of fighter jet engines on it, and go: no need for engine development or carrying a bunch of oxidizer.
Main obstacle there is that optimum staging altitude and speed are very high by aircraft standards. 100km and Mach 10. Not much air up there. So you couldn't replace an F9-1 like that. You could perhaps use it as a 0.5-stage to get an otherwise SSTO rocket over the hump, but it may not be worth it compared to a standard architecture: still requires a separate giant lower-stage airframe, and one with much different technology from the rest of the system.
[+] [-] stevep98|11 years ago|reply
[+] [-] jccooper|11 years ago|reply
Ariane 6 seems to be in response to the SpaceX challenge, at least; they aim to halve the cost per kg through cheaper and common upper/lower stage components. Which is good, but still not competitive with F9, much less a reusable version. At one point they almost made it all-solids in the first two stages, plus boosters, which would have totally doomed any future attempts at re-usability. The final design uses a F9-style common component liquid core, so they could maybe do the same thing as SpaceX later. They are still stuck with solid boosters, though, in a pork-provision move for Italy.
I've got to imagine that ULA is quietly looking into it. But it's quite possible they're just hoping SpaceX will eventually implode of its own internal contradictions and they can go back to their nice little government-launch monopoly. Or maybe they've decided they can't do it without absurd expense, based on their internal development costs. Or maybe they just hope to coast on their existing contracts for a while, and deal with it later once SpaceX has taken all the lumps.
NASA, via SLS, sure isn't looking at re-usability anymore. Lower cost to orbit via research should be their main task, if you ask me, but Congress would rather have them fly giant rockets a couple times a decade, apparently.
The Russians, well, are not much into changing things up. (They've been running Soyuz and Proton, their current launch families, since the sixties!) And their recent aerospace development programs have been less than impressive. But they have a real industry to protect, so one would hope they have a strategy.
I think that everyone's waiting for SpaceX to make it work, and then they'll get to copying. My long-term guess is that this'll kill several of the incumbents, a few will make the transition eventually, but most of SpaceX's competition will come from new entrants. In the meantime, SpaceX will be free to reap the profit from the margin between what it costs them and what their old-fashioned competitors can do. And/or capture the whole darn market. And/or grow it with lower prices. Maybe all of the above.
[+] [-] derekp7|11 years ago|reply
[+] [-] InclinedPlane|11 years ago|reply
You see, there are two ways to build reusable rockets. One way is to build a purely reusable vehicle from the ground up, but this is extraordinarily expensive because of all the testing involved before it can be used commercial. The other is to build an expendable/reusable hybrid. A rocket that can be used in an expendable configuration but has all of the needed elements to allow for reusability down the road. This is vastly easier because it means you can bootstrap development on the back of commercial launch business, and do testing that is largely subsidized by paying customers. And this is what SpaceX has been doing.
However, there's a problem. You have to head into reusability from the get go, you can't just modify any old rocket for reusability, you need to design from a place of suitability for reuse from square one. And while that's not an enormous difficulty it is very much non-trivial, and imposes a substantial overhead on R&D and manufacturing costs. This is because most of the optimizations for building a purely expendable rocket drive the design away from the optimizations you'd want for a reusable rocket. In an expendable design it's common for there to be a mix of propellants and a diversity of engine designs between the first and upper stages, it's also common for the upper stage to be more expensive than the first stage, because that stage is more critical for payload performance and launch precision. Also, it's common to minimize the number of engines on the first stage. Even the Saturn V had only 5 first stage engines. And generally there'd be no reason to add first stage engine restart capability on an expendable vehicle.
All of those things results in an expendable vehicle that is completely unsuited to reuse. It leads to a first stage that is very difficult to modify for reuse, and an overall rocket design that does not reward the easiest type of reuse (of the first stage) because most of the cost is in the upper stage anyway. SpaceX went to a great deal of effort to ensure that they built a rocket that could serve the expendable launch market immediately (and pull in hundreds of millions of dollars in revenue) while also being suitable for reuse with comparably moderate modifications. The first stage has 9 engines, so throttling down to a thrust level suitable for precision landings of a mostly empty stage is possible by using only one engine. The first stage use restartable engines (using onboard TEA/TEB igniters). And the first stage is about 3/4 of the hardware cost of the vehicle, since the 2nd stage uses only one engine and is also LOX/Kerosene fueled.
That foresight has allowed SpaceX to be in the launch business for several years, proving the capability of the company and gaining much needed experience in building and flying rockets while also poising the company to bring to market a reusable launcher. In comparison, every other established launch provider in the world could only do reusability by designing a new launcher from scratch.
[+] [-] Animats|11 years ago|reply
[+] [-] blisterpeanuts|11 years ago|reply
Why wouldn't they use radiation hardened computers, though? Are they that much heavier and more expensive than the more commonly available components? Out of a $40 million spacecraft, the difference of a few thousand dollars or even a hundred thousand dollars seems rather trivial, in exchange for a more reliable avionics system.
[+] [-] speeq|11 years ago|reply
It's not about cost at all:
> It's really not the expense that drives it. We're committed to having the best possible parts in all of our designs. So if it cost a lot and we needed it, we'd go get it. We were already required to have all this redundancy in the computers to meet all the different safety requirements. Then we started looking at what parts do we want to use and what is appropriate for this design. And what really is more important to us than the cost of the parts is the capability of the parts – how much power do they use, how much memory do they hold, how much do they process, and how physically big are they. That's the first thing.
> The second thing is what tools they come with. We run the Linux operating system, we program everything in C++, and that enables us to tap into a huge pool of very talented people and find the absolute best people in the computer and software industry to work with us. If you go into the radiation hardened parts, they are very limited in terms of what languages you can work in, what support packages there are for them, who knows how to program in them. It really limits your ability to work with the parts. And the other thing it really does is they all take a little longer time to get and they're a little harder to come by.
[+] [-] rzimmerman|11 years ago|reply
[+] [-] slashnull|11 years ago|reply
Also, there is something about using the same engine on every stage to gather more data per launch, but iirc different engine profiles are used for different velocities and air densities long the launch. I wonder what's the efficiency loss of using a single engine profile for the whole launch.
[+] [-] jccooper|11 years ago|reply
It's probably not even as bad as that, since they certainly fly with some performance reserve already, at least several percent, and most payloads are probably not max-weight. The landing propellant in many cases may be entirely free, mass-budget-wise. They do add some additional mass for the legs and fins, in addition to any extra propellant they need, so it's not free, but the first stage is not as performance-sensitive as higher stages (see: rocket equation), so they can get away with it. I don't see 'em getting second stages back any time soon, though, and I think they've given up on that, at least for F9.
WRT engines, the efficiency based on air density is mostly a function of the nozzle, and the upper-stage Merlins have a nozzle extension to deal with this. Higher-efficiency fuels (i.e LH2) are historically preferred for upper stages, since they are more performance-sensitive, so SpaceX is losing some performance there. But LH2 is super-cryogenic and a beast to work with, and an entirely different engine. SpaceX's thing is that they are willing to sacrifice maximum efficiency for reasonable cost, so they decided to make that tradeoff. It seems like a good plan so far.
[+] [-] wolf550e|11 years ago|reply
The Merlin 1-D Vacuum used on the upper stage does have a wide vacuum optimized nozzle, but the other parts are almost the same, to save costs during manufacturing and testing.
The article is ok, but says nothing new to people who have followed SpaceX for years. There is a lot of info online about the company and its products.
[+] [-] mbell|11 years ago|reply
No, it reduces payload to orbit by ~30% or so. Fuel is insanely cheap in comparison to the cost of building the 1st stage.
[+] [-] ceejayoz|11 years ago|reply
It does reduce payload capacity (30-40%, IIRC) but the expected upside is drastic cost reductions. If I can launch several payloads for the price of one as a result, it becomes well worth it, and if I've got a large payload I can either use the upcoming Falcon Heavy or we'll start doing more in-orbit assembly.
[+] [-] Symmetry|11 years ago|reply
[+] [-] InclinedPlane|11 years ago|reply
[+] [-] kirk21|11 years ago|reply
The weight of the legs is probably quite low so this might have been the easiest solution.
[+] [-] jccooper|11 years ago|reply
Thus the landing profile is the "hover-slam": the engine is started at the exact point in time necessary to reach a "landable speed" (from 0-several m/s) at the ground, and is cut off when it gets to the target.
While modern avionics are quite good, it's still coming in pretty fast and from far away and will have some variance in location and speed, just because that's the real world. Probably they don't quite know how much yet, but it could easily miss the target by multiple meters and a few meters/s. And that's a heck of a condition to build some sort of giant grabbing apparatus for.
Perhaps once they're routinely landing on land, they may be able to determine they can hit the target well enough to build a grabbing contraption. Or maybe it's too hard. Or maybe they'd rather keep their infrastructure to a minimum: a concrete pad is a lot cheaper to build and easier to operate, and SpaceX wants to do a lot of flights.
Besides, the 4000lb of the legs is on a 40,000lb rocket.
[+] [-] mrfusion|11 years ago|reply
Maybe they just wanted to keep complexity down? Or they simply can't land with sub meter resolution to garuntee they get within the claws reach?
Also I think they want to use the same system on Mars where there won't be any infrastructure.
[+] [-] zlsa|11 years ago|reply
The F9v1.1 cannot hover when landing as the TWR with an engine at 60% thrust (the minimum) is almost 2.
[+] [-] mrfusion|11 years ago|reply
Even if unsafe could it still make sense for unmanned launches, supplies, cargo?
[+] [-] jccooper|11 years ago|reply
An earlier design for the Ariane 6 was 4 identical solid boosters, 3 on the 1st, and one for the 2nd stage--before they came to their senses.
Solids are convenient and reasonably simple and good at high thrust and smaller scale, but don't scale up as well as liquids. (You try casting a fuel grain the size of an office building!) So they're not well suited for launching anything particularly large into orbit. And unfortunately they have nowhere near the mass fraction to do SSTO. Liquids, though, are tantalizingly close.
[+] [-] Solarsail|11 years ago|reply
[+] [-] tempestn|11 years ago|reply
What exactly were these "Air Force requirements"?
[+] [-] cyanoacry|11 years ago|reply
1) The payload bay grew in size from 12 ft wide to 15 ft wide, in order to accommodate future unspecified military needs.
2) There were new cross-range requirements specified for reentry.[2] Basically, the military wanted to be able to launch satellites into polar orbits (useful for surveillance) and retain the capability to land after one orbit of the Earth. However, this meant that the Shuttle also needed to be able to -reach- the landing sites, which are normally not in the direct orbit path, but are divert 1000 miles away laterally (the cross part of cross-range).
[1] http://en.wikipedia.org/wiki/Space_Shuttle_design_process#Ai...
[2] http://yarchive.net/space/shuttle/shuttle_crossrange.html
[+] [-] InclinedPlane|11 years ago|reply
[+] [-] knappador|11 years ago|reply
Current gas turbines are not optimized for pure thrust-to-weight or else why are military engines outperforming civilian in thrust-to-weight? Turbofan-to-turbo-jet for high Isp in atmospheric flight and then fly everything back on its own. Firefly's rocket has a max thrust of 90klb on the main while a big gas turbine pulls 100klb easy. While we're talking about Isp and re-usability, the entire first stage of a rocket could be gas-turbine powered and the engines just peel away and fly home after they're done. The engine needs gas. The gas takes space. Put the gas in a wing. Wing glides the engine home after launch. Easy as 1,2,3-a-bunch-of-engineering. Go!
[+] [-] ceejayoz|11 years ago|reply
It has been successfully tried - http://en.wikipedia.org/wiki/Pegasus_%28rocket%29 - but my understanding is payloads are pretty severely limited. You can see the size of the Pegasus launcher is much, much smaller than a Falcon 9 - it's certainly not going to work for SLS/Falcon Heavy/MCT-sized payloads.
edit: For ground-launch with jets as the first stage, I really don't think that's going to work.
http://en.wikipedia.org/wiki/Thrust-to-weight_ratio#Jet_and_... indicates the SR-71's engine had a thrust-to-weight ratio of 5.2. SpaceX's Merlin has a thrust-to-weight of 150. There might be room for improvement, but 30-fold? I doubt it.
[+] [-] jccooper|11 years ago|reply
Main obstacle there is that optimum staging altitude and speed are very high by aircraft standards. 100km and Mach 10. Not much air up there. So you couldn't replace an F9-1 like that. You could perhaps use it as a 0.5-stage to get an otherwise SSTO rocket over the hump, but it may not be worth it compared to a standard architecture: still requires a separate giant lower-stage airframe, and one with much different technology from the rest of the system.