385 Feet of Crazy: The Most Audacious Flying Machine

Point in case, note the size of the Pegasus orbital rocket [2]. Or the orbital anti-satellite missile, sized like a typical cruise missile, all thanks to air launch [3] from a typical fighter jet.

Now that SpaceX is able to recover the first stages from some launches, the cost of first stage is a bit less of a factor, but still there are other considerations, like ability to launch from any decent runway vs having to maintain a dedicated facility, unusable for any other commercial purposes, far away from humans due to incredible amount of noise rocket engines create.

[1] https://en.wikipedia.org/wiki/Air_launch_to_orbit

[2] https://en.wikipedia.org/wiki/Pegasus_(rocket)

[3] https://en.wikipedia.org/wiki/ASM-135_ASAT

Think of the airplane part of this as an efficient, highly reusable first stage with poor performance.

Also, by launching from an aircraft over an ocean one can avoid many of the ground realities of rocket operations. Land-based sites much deal with shipping notices and noise issues. Stratospheric can fire these things in different directions, to different orbits, without worry. They might also be able to launch multiple rockets on a single flight, a totally new service that might hit an unexplored market.

Lastly, politics. Stratolaunch could travel the world to launch rockets from basically any big airport. So if the Saudis want to launch something without the hassle of US/Russian security checks, they can have Stratospheric show up to handle the launch locally. There are probably enough countries wanting to launch something locally for prestige alone.

Or... someone just wants to have the biggest airplane in the world (paging Dr. Freud) and this is the way to make that happen.

When you watch a SpaceX launch they actually have to really dial down their acceleration to get through Max-Q, the highest dynamic pressure, due to air resistance. Air resistance is a function of speed SQUARED and the density of air. By starting the rocket that much higher, I believe they are essentially removing that inefficiency- by the time they're moving fast enough for it to matter, there isn't enough air for it to matter.

There's also the whole tyranny of rocketry equation. The first foot of elevation gained is the hardest and the first mile per hour of speed is the hardest. Starting even a little bit higher and faster is a big win over taking off from sea level.

And as others have said, they've got incredible flexibility here. Whatever orbit you want to be in, the launch will take place in that orbit. A lot of traditional rocket launches have 'instantaneous' launch windows where missing by 1 second is a no-go because the ground is only in the right spot at one moment.

Most important of all: let them try! If this whole thing is a bust, it's private enterprise and you and I lose nothing over it while getting to see an enormous jet. And if it does work, we all win from the increase in availability of satellites and access to space.

To use the example of the Falcon 9 again, consider the second stage with a dry mass of 4 tonnes, 107.5 tonnes of propellant, and an exhaust velocity of 3.41 km/s. With a 5 tonne payload the stage can achieve a delta-V of: ln((107.5 + 4 + 5)/(4 + 5)) * 3.41 km/s = 8.7 km/s. Now, if we substitute the first stage sea level exhaust velocity of just 2.76 km/s the overall delta-V is 7.1 km/s, and to get to the same delta-V (8.7 km/s) now requires a mass ratio of 23.4:1, resulting in an effective payload of just 0.8 tonnes, less than 1/6th as much as the original payload!

In actuality the overall performance differences won't be quite as severe but this does point out the potential for a rocket launched from higher altitude to have equivalent performance to a much larger rocket launched from sea level.

Another major advantage is operational flexibility. You can launch from a wider variety of launch locations, which might make it easier to launch in a more timely manner, or could make it easier to launch certain kinds of payloads (which don't need to be shipped around the world, the launcher can come to you). And potentially it makes it possible to launch from more equatorial locations, gaining a slight orbital mechanics advantage.

However, all of this comes at some pretty steep costs. With this design it's harder to make incremental changes to the vehicle. One of the classic mechanisms for improving launcher performance is stretching stages, but that's out of the question here. You can't make the rocket bigger without making the carrier aircraft bigger as well.

Additionally, and most damning, the tradeoff here is a bad one. On the one hand you have a sea level launched rocket which needs to expend more propellant to reach orbit. However, propellant is dirt cheap, it's the cheapest part of a launch pretty much. Especially for a LOX/Kerosene rocket the propellant isn't even going to crack 7 digits in terms of cost, even for a big rocket. With high altitude launch some dirt cheap propellant is saved at the cost of an inordinately expensive carrier aircraft, and a very complex and dangerous flight profile (you need to have a piloted aircraft takeoff with a fully fueled rocket). Moreover, you've now cut off all of the most straightforward avenues for incremental improvements of the launch system: you can't make any part of it bigger without starting from square one with a new iteration.

And in a world where reusable rockets are becoming ever more commonplace this makes even less sense.

Single engine out isn't probably much of a concern as there are two redundant engines per side.

I'm just speculating though.

https://news.ycombinator.com/item?id=14456602

If you start at 40,000 feet, you can accelerate largely sideways and reduce gravity losses without running into the atmosphere too hard and needing a stronger structure.

It is. Just that it's the first such pair of aircraft that stay parked next to each other in all regimes, not just the one.