7 ms·

I'm puzzled by what added benefit being 10km up in the air and going at 300-400kmph gives over regular rocket launches since speed and altitude are a very small

by sfifs 8y ago

I'm puzzled by what added benefit being 10km up in the air and going at 300-400kmph gives over regular rocket launches since speed and altitude are a very small fraction of what's required to orbit. The article says it will be less affected by weather windows but (1) would a normal LEO commercial launch be so highly time bound to find this of value? (2) if weather was bad for a rocket launch on surface, would you fly a heavily laden giant plane into it? Leads me to wonder if the real customers of this is the military who do value very short turnaround launches.

alex_anglin 8y ago
It's probably a more enjoyable launch experience for passengers.
mikeash 8y ago
The amount of rocket you need to reach a given speed is exponential with respect to that speed: https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation Shaving off a small amount of speed can have a large payoff. This is why most launch sites are near(ish) the equator.
- simonh 8y ago
  That's not actually a very big effect compared to mid latitudes. The main reason to launch near the equator is because a lot of payloads go into equatorial orbits and plane changes are expensive. From the equator you can go into any orbit equally easily (because you just wait for the earth to rotate until you’re on the desired orbital plane), but the only way to go into an equatorial orbit without a plane change is from the equator.
  - mikeash 8y ago
    That’s a big effect, certainly, but the payload boost from the additional speed is still significant. You can’t really get into any orbit equally easy when launching from the equator. The higher the inclination, the more delta v you need. For example, the Shuttle lost several thousand pounds of payload capacity when launching to the ISS compared to what it would have been able to carry to the Space Station Freedom, which would have been at an inclination matching KSC’s latitude. It’s just that the penalty is vastly worse for launching into a low-inclination orbit from high latitudes (thus why we didn’t tell the Russians to suck it and deal with an inclination better suited to KSC).
    - simonh 8y ago
      That’s a good point, the higher the inclination the more of the equatorial rotations Speed you need to cancel out. I’d still argue it gives you more flexibility overall compared to a higher latitude launch site.
      - mikeash 8y ago
        It's definitely a very small effect. Equatorial launch sites are almost always preferable in terms of the launch itself, and the fact that most launch sites aren't smack on the equator is due to logistical or political issues. Vaguely related, Baikonur is an interesting example of political issues. It's at 45.6 degrees latitude, but the minimum inclination it can reach is 51.6 degrees, because anything less would involve dropping boosters on China, and China is not enthusiastic about the idea.
wl 8y ago
Rockets waste a lot of fuel punching through the atmosphere at lower altitudes.
khuey 8y ago
You get to skip the densest part of the atmosphere, and you get to use an air-breathing jet engine (instead of a rocket engine that carries its own oxidizer) to get up there.
knowaveragejoe 8y ago
I thought speed was one of the most crucial elements of achieving orbit. Granted the extra 3-400kmph you mention is basically negligible for that.
dgudkov 8y ago
From Wikipedia [1]: "40,000 feet (12,000 m) is only about 4% of a low earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaces a costly first-stage booster." [1] https://en.wikipedia.org/wiki/Pegasus_(rocket) https://en.wikipedia.org/wiki/Pegasus_(rocket)
- simonh 8y ago
  It could replace an extremely weak, low capability first stage. In comparison the F9 first stage throws the second stage up to an altitude of 65km at 2km/s, or 26% of orbital velocity. So this air launch approach only achieves about one fifth as much as an F9 first stage.
  - ianai 8y ago
    That sounds like an 80/20 rule application.
jpindar 8y ago
Planes can fly above (some) bad weather.
ilaksh 8y ago
One of the biggest challenges for acheiving orbit is the dense atmosphere near the ground. Takes much much more fuel to go the same speed than it does when higher. It is dramatically less dense and then goes away as you get higher.
Gravityloss 8y ago
A rocket engine nozzle produces thrust only for the part where the pressure is greater than atmospheric pressure. (Makes sense, right?) Launching at lower ambient pressure (greater altitude) makes the engine work better. It gives more thrust for the same fuel flow (higher ISP). You can also make the nozzle bigger, getting even more thrust and ISP. This is a significant advantage AFAIK. Also, AFAIK, if your rocket engine is low-tech, you will likely have low chamber pressure, meaning the nozzle is small. The beneficial effect is proportionally larger then.
- ThenAsNow 8y ago
  > A rocket engine nozzle produces thrust only for the part where the pressure is greater than atmospheric pressure. A rocket engine produces thrust when its chamber pressure exceeds ambient pressure, which is trivially true for any rocket engine. The flow coming out of the nozzle can have lower pressure at the exit plane than ambient and still make plenty of thrust. This is called "overexpanded" flow, and is the regime that produces shock diamonds in rocket and afterburning jet engine plumes at low altitudes. Reduced to quasi-1D form, thrust is given by: T = mdot*(delta V) + (p_e - p_a)*A_e Where mdot is mass flow coming from the nozzle, delta V is change of velocity of that efflux with respect to the vehicle reference frame (in a rocket, it is just exit velocity, in an airbreathing system, you have to take into account the incoming momentum), p_e is static pressure at the exit plane, p_a is ambient pressure, and A_e is the area of the exit plane. In an overexpanded scenario, p_e < p_a , but the majority of thrust is coming from the momentum change term, which is the one containing mdot. Nozzle performance is optimal when pressure at the exit plane matches the ambient pressure. This is when the exhaust velocity is maximized and the pressure thrust term vanishes. It can be shown (undergraduate aeropropulsion course homework) that you get more thrust from ideal expansion than from pressure thrust (underexpanded regime). The point you are trying to make is that an air-launched system allows the rocket to start with a higher area ratio than if it were ground-launched. In the ground-launched scenario, a large nozzle area ratio will suffer overexpansion losses and how large you can go will also be limited by structural implications from the internal nozzle separation that will occur with gross overexpansion. While this is all true, the reality is that the additional rocket performance you can get from a larger nozzle area ratio is not that compelling in light of building, operating, and maintaining an entire air-launch aircraft platform. You're better off putting that money into incrementally larger propellant tanks due to the lower mission-averaged Isp associated with the ground-launched vehicle.
jcims 8y ago
Pegasus launches occur at closer to 900km/h. It takes a Falcon 9 about a minute and roughly 150,000kg of propellant to get to 10km altitude, at which point it's going to be travelling closer to 1200km/h. If you can move that part of the mission to a carrier, you can substantially reduce the size and fuel capacity of the stage or maybe even entirely skip a staging operation (edit: not skip, carrier becomes first stage, thanks DenisM!). Back to the Falcon 9 at mid-first stage, from 10km to stage separation you're running on motors that had to be built to operate at sea level. This means that they are 'underexpanded' for operating at 10km altitude and are therefore not as efficient as they could be. If you know your engines will always start off at 10km, you can build bigger expansion nozzles and improve the overall efficiency.
- DenisM 8y ago
  You’re not skipping staging, your carrier is your (reuseable) first stage.
  - SubiculumCode 8y ago
    And, I presume, much much more efficient
    - umeshunni 8y ago
      Why is it much more efficient? Doesn't it still have to carry the same mass up to the same altitude and velocity?
      - newnewpdro 8y ago
        If you accelerate your car 0-60MPH in 3 seconds vs. 30 seconds, which uses less energy?
        sfifs 8y ago
        Should the energy change be roughly the same = 0.5mv^2 The power would be different (energy is used up much faster in the 3 second case)
ThenAsNow 8y ago
The real answer, which is somewhat saddening given the immense amount of effort put into Stratolaunch, is that air-launch, when all is accounted for, has negligible benefits from a performance perspective when you are deploying expendable stages. AIAA paper 2004-872 "Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics" gives a decent and readable treatment of this scenario. The advantages air-launch inarguably has are letting you pick the location and azimuth at which the high-performance launch vehicle stage begins its run as opposed to being constrained by your launch site and populations under the launch vehicle trajectory. If those are your most important metrics, air-launch may "win". If you are releasing a reusable stage from the air-launch platform, that could also be a unique value proposition as you could do a downrange landing of the stage (reserve no propellants for boost-back) to any desired site. SpaceX accomplishes this with a landing barge for their system. By virtually any other measures, air launch from a subsonic platform does not make engineering sense over a ground-launched system. DARPA's ALASA program explored air-launch from an F-15, and was struggling to get ~100 lbs. to orbit. Even if you use a "big" supersonic platform like a B-1, by the time you take into account the volume associated with a launch vehicle, you quickly realize that at absolute best, you are talking a low supersonic staging Mach number. It would require a new high-thrust aircraft platform if you wanted a supersonic air-drop. And regardless of staging speed, if the flight path angle ("gamma") at release is too low, the deltaV advantage significantly diminishes (I recall the cited paper above shows this). Consider the following issues: * The Strato carrier aircraft is (unless even more megadollars are spent) is one-of-a-kind. If anything happens to it, no paying customer will be able to launch. * Any stage stack that is launched from the carrier aircraft has to be rated for manned safety. This is its own expensive undertaking that autonomous ground-launched systems do not have to contend with. This is in addition to any public safety issues associated with population overflight (to be fair, air-launch may be able to diminish those concerns, but compliance with the former is much more onerous than the latter). * The carrier aircraft presents very significant structural and volumetric constraints to the launch vehicle. The rumored lift capability is in the neighborhood of 500 klbs. For something like a hydrogen-fueled launch stack, you can do some rough sizing and realize that geometry will constrain you faster than mass. Don't forget to include flexible body effects, as that will be very significant with this airframe. There is definitely an engineering accomplishment in the flight of this aircraft. However, unless Strato does something that they can uniquely do, or they lose money hand-over-fist on each launch (Pegasus is very expensive already, and I think SpaceNews reported that Strato shut down their own internal launch vehicle development efforts), they cannot be commercially-competitive. That launch point and azimuth control has to be critical, or air-drop just doesn't make sense. As much as Burt Rutan has to be admired for his ability to think creatively, Stratolaunch was an insufficiently vetted architecture sold to an enthusiast billionaire who was probably delighted with the result of their SpaceShip One collaboration. And yet you see that in SS1, Scaled basically designed a one-off experimental vehicle architecture that fared very poorly when it was scaled into what was intended to be a sustainably-operated commercial system. Strato is another example of this kind of mindset.
curtis 8y ago
There's three advantages you get from air launch. One is that your first stage rocket engines can use nozzles that are optimized for vacuum or close to vacuum resulting in better performance through the entire first stage burn. (The RS-68 used on the Delta-IV, for example, has a sea-level ISP of 365s and a vacuum ISP of 420s, which is a pretty substantial difference.[1]) The second is that a launch aircraft at 30,000 or 40,000 feet is providing not just forward velocity but has also allowed the first stage to avoid some gravity loss (to accelerate at 1G at launch you need 2Gs of thrust). It's also allowed the first stage to side-step a major portion of the aerodynamic resistance. Finally, an air-launched system is not as strongly constrained to specific launch windows. When targeting a specific orbit, you don't have to wait for a specific launch window at the primary launch site (the airfield), you can simply wait until one is relatively nearby and fly to it. This might be a big operational advantage even when it's not a big performance advantage since you can launch your aircraft when you have decent weather over the airfield and the fly to a more ideal spot to launch the rocket. If the radius of the loaded launch airplane is wide enough, this could be a very big advantage. Even if you add all of these effects up, it's still a pretty small fraction of the 9,000 m/s of delta-v or so that you need to reach orbit. But keep in mind that rockets only deliver a few percent of their launch mass to orbit. A relatively small reduction in the delta-v needed to reach orbit might result in big gains in payload into orbit. In extreme cases, maybe it could double it. The biggest disadvantage of air launch is that you are seriously constrained in the maximum size of the rocket. The Stratolaunch aircraft can carry about 230 metric tons of payload, whereas a Falcon Heavy weighs in at something like 1,400 metric tons[2]. [1] http://www.astronautix.com/r/rs-68.html http://www.astronautix.com/r/rs-68.html [2] https://en.wikipedia.org/wiki/Falcon_Heavy https://en.wikipedia.org/wiki/Falcon_Heavy
- ethagknight 8y ago
  Comparing stratolauncher payload to Falcon Heavy’s gross weight at takeoff is not really a helpful comparison. FH has a 60 metric tons to LEO max payload, roughly 5% of its total weight. I’m sure someone has done the calcs on the theoretical potential of the Stratolauncher, but it looks like a Pegasus can get 1000 lbs to LEO at a total weight of 40,000 lbs, so same 5% ratio. Seems to be in the same ballpark, and Stratolauncher (would?) use a fraction of the resources.
newnewpdro 8y ago
I think it's just a matter of needing less fuel, which saves weight, further lowering fuel requirements. So it just makes launches cost less. Using the rocket to get off the launchpad is wasteful. It's basically the wrong tool for the first stage but the simplicity of the approach is nice.

by sfifs 8y ago

alex_anglin 8y ago

It's probably a more enjoyable launch experience for passengers.

mikeash 8y ago

The amount of rocket you need to reach a given speed is exponential with respect to that speed: https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation Shaving off a small amount of speed can have a large payoff. This is why most launch sites are near(ish) the equator.

simonh 8y ago
That's not actually a very big effect compared to mid latitudes. The main reason to launch near the equator is because a lot of payloads go into equatorial orbits and plane changes are expensive. From the equator you can go into any orbit equally easily (because you just wait for the earth to rotate until you’re on the desired orbital plane), but the only way to go into an equatorial orbit without a plane change is from the equator.
- mikeash 8y ago
  That’s a big effect, certainly, but the payload boost from the additional speed is still significant. You can’t really get into any orbit equally easy when launching from the equator. The higher the inclination, the more delta v you need. For example, the Shuttle lost several thousand pounds of payload capacity when launching to the ISS compared to what it would have been able to carry to the Space Station Freedom, which would have been at an inclination matching KSC’s latitude. It’s just that the penalty is vastly worse for launching into a low-inclination orbit from high latitudes (thus why we didn’t tell the Russians to suck it and deal with an inclination better suited to KSC).
  - simonh 8y ago
    That’s a good point, the higher the inclination the more of the equatorial rotations Speed you need to cancel out. I’d still argue it gives you more flexibility overall compared to a higher latitude launch site.
    - mikeash 8y ago
      It's definitely a very small effect. Equatorial launch sites are almost always preferable in terms of the launch itself, and the fact that most launch sites aren't smack on the equator is due to logistical or political issues. Vaguely related, Baikonur is an interesting example of political issues. It's at 45.6 degrees latitude, but the minimum inclination it can reach is 51.6 degrees, because anything less would involve dropping boosters on China, and China is not enthusiastic about the idea.

wl 8y ago

Rockets waste a lot of fuel punching through the atmosphere at lower altitudes.

khuey 8y ago

You get to skip the densest part of the atmosphere, and you get to use an air-breathing jet engine (instead of a rocket engine that carries its own oxidizer) to get up there.

knowaveragejoe 8y ago

I thought speed was one of the most crucial elements of achieving orbit. Granted the extra 3-400kmph you mention is basically negligible for that.

dgudkov 8y ago

From Wikipedia [1]: "40,000 feet (12,000 m) is only about 4% of a low earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaces a costly first-stage booster." [1] https://en.wikipedia.org/wiki/Pegasus_(rocket) https://en.wikipedia.org/wiki/Pegasus_(rocket)

simonh 8y ago
It could replace an extremely weak, low capability first stage. In comparison the F9 first stage throws the second stage up to an altitude of 65km at 2km/s, or 26% of orbital velocity. So this air launch approach only achieves about one fifth as much as an F9 first stage.
- ianai 8y ago
  That sounds like an 80/20 rule application.

jpindar 8y ago

Planes can fly above (some) bad weather.

ilaksh 8y ago

One of the biggest challenges for acheiving orbit is the dense atmosphere near the ground. Takes much much more fuel to go the same speed than it does when higher. It is dramatically less dense and then goes away as you get higher.

Gravityloss 8y ago

A rocket engine nozzle produces thrust only for the part where the pressure is greater than atmospheric pressure. (Makes sense, right?) Launching at lower ambient pressure (greater altitude) makes the engine work better. It gives more thrust for the same fuel flow (higher ISP). You can also make the nozzle bigger, getting even more thrust and ISP. This is a significant advantage AFAIK. Also, AFAIK, if your rocket engine is low-tech, you will likely have low chamber pressure, meaning the nozzle is small. The beneficial effect is proportionally larger then.

ThenAsNow 8y ago
> A rocket engine nozzle produces thrust only for the part where the pressure is greater than atmospheric pressure. A rocket engine produces thrust when its chamber pressure exceeds ambient pressure, which is trivially true for any rocket engine. The flow coming out of the nozzle can have lower pressure at the exit plane than ambient and still make plenty of thrust. This is called "overexpanded" flow, and is the regime that produces shock diamonds in rocket and afterburning jet engine plumes at low altitudes. Reduced to quasi-1D form, thrust is given by: T = mdot*(delta V) + (p_e - p_a)*A_e Where mdot is mass flow coming from the nozzle, delta V is change of velocity of that efflux with respect to the vehicle reference frame (in a rocket, it is just exit velocity, in an airbreathing system, you have to take into account the incoming momentum), p_e is static pressure at the exit plane, p_a is ambient pressure, and A_e is the area of the exit plane. In an overexpanded scenario, p_e < p_a , but the majority of thrust is coming from the momentum change term, which is the one containing mdot. Nozzle performance is optimal when pressure at the exit plane matches the ambient pressure. This is when the exhaust velocity is maximized and the pressure thrust term vanishes. It can be shown (undergraduate aeropropulsion course homework) that you get more thrust from ideal expansion than from pressure thrust (underexpanded regime). The point you are trying to make is that an air-launched system allows the rocket to start with a higher area ratio than if it were ground-launched. In the ground-launched scenario, a large nozzle area ratio will suffer overexpansion losses and how large you can go will also be limited by structural implications from the internal nozzle separation that will occur with gross overexpansion. While this is all true, the reality is that the additional rocket performance you can get from a larger nozzle area ratio is not that compelling in light of building, operating, and maintaining an entire air-launch aircraft platform. You're better off putting that money into incrementally larger propellant tanks due to the lower mission-averaged Isp associated with the ground-launched vehicle.

jcims 8y ago

Pegasus launches occur at closer to 900km/h. It takes a Falcon 9 about a minute and roughly 150,000kg of propellant to get to 10km altitude, at which point it's going to be travelling closer to 1200km/h. If you can move that part of the mission to a carrier, you can substantially reduce the size and fuel capacity of the stage or maybe even entirely skip a staging operation (edit: not skip, carrier becomes first stage, thanks DenisM!). Back to the Falcon 9 at mid-first stage, from 10km to stage separation you're running on motors that had to be built to operate at sea level. This means that they are 'underexpanded' for operating at 10km altitude and are therefore not as efficient as they could be. If you know your engines will always start off at 10km, you can build bigger expansion nozzles and improve the overall efficiency.

DenisM 8y ago
You’re not skipping staging, your carrier is your (reuseable) first stage.
- SubiculumCode 8y ago
  And, I presume, much much more efficient
  - umeshunni 8y ago
    Why is it much more efficient? Doesn't it still have to carry the same mass up to the same altitude and velocity?
    - newnewpdro 8y ago
      If you accelerate your car 0-60MPH in 3 seconds vs. 30 seconds, which uses less energy?
      - sfifs 8y ago
        Should the energy change be roughly the same = 0.5mv^2 The power would be different (energy is used up much faster in the 3 second case)

ThenAsNow 8y ago

The real answer, which is somewhat saddening given the immense amount of effort put into Stratolaunch, is that air-launch, when all is accounted for, has negligible benefits from a performance perspective when you are deploying expendable stages. AIAA paper 2004-872 "Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics" gives a decent and readable treatment of this scenario. The advantages air-launch inarguably has are letting you pick the location and azimuth at which the high-performance launch vehicle stage begins its run as opposed to being constrained by your launch site and populations under the launch vehicle trajectory. If those are your most important metrics, air-launch may "win". If you are releasing a reusable stage from the air-launch platform, that could also be a unique value proposition as you could do a downrange landing of the stage (reserve no propellants for boost-back) to any desired site. SpaceX accomplishes this with a landing barge for their system. By virtually any other measures, air launch from a subsonic platform does not make engineering sense over a ground-launched system. DARPA's ALASA program explored air-launch from an F-15, and was struggling to get ~100 lbs. to orbit. Even if you use a "big" supersonic platform like a B-1, by the time you take into account the volume associated with a launch vehicle, you quickly realize that at absolute best, you are talking a low supersonic staging Mach number. It would require a new high-thrust aircraft platform if you wanted a supersonic air-drop. And regardless of staging speed, if the flight path angle ("gamma") at release is too low, the deltaV advantage significantly diminishes (I recall the cited paper above shows this). Consider the following issues: * The Strato carrier aircraft is (unless even more megadollars are spent) is one-of-a-kind. If anything happens to it, no paying customer will be able to launch. * Any stage stack that is launched from the carrier aircraft has to be rated for manned safety. This is its own expensive undertaking that autonomous ground-launched systems do not have to contend with. This is in addition to any public safety issues associated with population overflight (to be fair, air-launch may be able to diminish those concerns, but compliance with the former is much more onerous than the latter). * The carrier aircraft presents very significant structural and volumetric constraints to the launch vehicle. The rumored lift capability is in the neighborhood of 500 klbs. For something like a hydrogen-fueled launch stack, you can do some rough sizing and realize that geometry will constrain you faster than mass. Don't forget to include flexible body effects, as that will be very significant with this airframe. There is definitely an engineering accomplishment in the flight of this aircraft. However, unless Strato does something that they can uniquely do, or they lose money hand-over-fist on each launch (Pegasus is very expensive already, and I think SpaceNews reported that Strato shut down their own internal launch vehicle development efforts), they cannot be commercially-competitive. That launch point and azimuth control has to be critical, or air-drop just doesn't make sense. As much as Burt Rutan has to be admired for his ability to think creatively, Stratolaunch was an insufficiently vetted architecture sold to an enthusiast billionaire who was probably delighted with the result of their SpaceShip One collaboration. And yet you see that in SS1, Scaled basically designed a one-off experimental vehicle architecture that fared very poorly when it was scaled into what was intended to be a sustainably-operated commercial system. Strato is another example of this kind of mindset.

curtis 8y ago

There's three advantages you get from air launch. One is that your first stage rocket engines can use nozzles that are optimized for vacuum or close to vacuum resulting in better performance through the entire first stage burn. (The RS-68 used on the Delta-IV, for example, has a sea-level ISP of 365s and a vacuum ISP of 420s, which is a pretty substantial difference.[1]) The second is that a launch aircraft at 30,000 or 40,000 feet is providing not just forward velocity but has also allowed the first stage to avoid some gravity loss (to accelerate at 1G at launch you need 2Gs of thrust). It's also allowed the first stage to side-step a major portion of the aerodynamic resistance. Finally, an air-launched system is not as strongly constrained to specific launch windows. When targeting a specific orbit, you don't have to wait for a specific launch window at the primary launch site (the airfield), you can simply wait until one is relatively nearby and fly to it. This might be a big operational advantage even when it's not a big performance advantage since you can launch your aircraft when you have decent weather over the airfield and the fly to a more ideal spot to launch the rocket. If the radius of the loaded launch airplane is wide enough, this could be a very big advantage. Even if you add all of these effects up, it's still a pretty small fraction of the 9,000 m/s of delta-v or so that you need to reach orbit. But keep in mind that rockets only deliver a few percent of their launch mass to orbit. A relatively small reduction in the delta-v needed to reach orbit might result in big gains in payload into orbit. In extreme cases, maybe it could double it. The biggest disadvantage of air launch is that you are seriously constrained in the maximum size of the rocket. The Stratolaunch aircraft can carry about 230 metric tons of payload, whereas a Falcon Heavy weighs in at something like 1,400 metric tons[2]. [1] http://www.astronautix.com/r/rs-68.html http://www.astronautix.com/r/rs-68.html [2] https://en.wikipedia.org/wiki/Falcon_Heavy https://en.wikipedia.org/wiki/Falcon_Heavy

ethagknight 8y ago
Comparing stratolauncher payload to Falcon Heavy’s gross weight at takeoff is not really a helpful comparison. FH has a 60 metric tons to LEO max payload, roughly 5% of its total weight. I’m sure someone has done the calcs on the theoretical potential of the Stratolauncher, but it looks like a Pegasus can get 1000 lbs to LEO at a total weight of 40,000 lbs, so same 5% ratio. Seems to be in the same ballpark, and Stratolauncher (would?) use a fraction of the resources.

newnewpdro 8y ago

I think it's just a matter of needing less fuel, which saves weight, further lowering fuel requirements. So it just makes launches cost less. Using the rocket to get off the launchpad is wasteful. It's basically the wrong tool for the first stage but the simplicity of the approach is nice.