[oconnor663]

https://web.archive.org/web/20120503175355/https://www.nasa....

> The percent propellant has huge implications on the ease of fabrication and robustness in achieving the engineering design (and cost). If a vehicle is less than 10% propellant, it is typically made from billets of steel. Changes to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say. I can easily overload my ¾ ton pickup by a factor of two. It might be moving slowly but it is hauling the load.

> Once the vehicles become airborne, the engineering becomes more serious. Light weight structures made of aluminum, magnesium, titanium, epoxy-graphite composites are the norm. To alter the structure takes significant engineering; one does not simply weld on another chunk to your airframe if you want to live (or drill a hole through some convenient section). These vehicles cannot operate far from their designed limits; overloading an airplane by a factor of two results in disaster. Even though these vehicles are 30 to 40% propellant (60 to 70% structure and payload), there is room for engineering to comfortably operate thus there is a robust, safe, and cost effective aviation industry.

> Rockets at 85% propellant and 15% structure and payload are on the extreme edge of our engineering ability to even fabricate (and to pay for!). They require constant engineering to keep flying. The seemingly smallest modifications require monumental analysis and testing of prototypes in vacuum chambers, shaker tables, and sometimes test launches in desert regions. Typical margins in structural design are 40%. Often, testing and analysis are only taken to 10% above the designed limit. For a Space Shuttle launch, 3 g’s are the designed limit of acceleration. The stack has been certified (meaning tested to the point that we know it will keep working) to 3.3 g’s. This operation has a 10% envelope for error. Imagine driving your car at 60 mph and then drifting to 66 mph, only to have your car self-destruct. This is life riding rockets, compliments of the rocket equation.

[kitd]

Interesting post. I'd never thought of it that way. Not consciously anyway.

Might that make an air-launched system more reliable? Even if it's less efficient, the TCO would be lower using a winged system for the initial phases of launch.

[oconnor663]

Several air launch systems have been tried, with limited but non-zero commercial success. The altitude and speed you get from the plane is very small compared to the total work the rocket needs to do, so the benefit in terms of "the rocket can be smaller" is minimal. The main benefit in practice has been launching from ~wherever you like, since regular fixed launch sites usually have strict limits about the direction you have to fly to avoid people. But the economics of reusability are pushing rockets to get bigger, and no air launch system can fly anything nearly as big as a Falcon 9, much less a Starship + Super Heavy. Other scattered problems:

- Hanging under-wing is a totally different set of forces than standing vertically, especially for a big rocket with thin walls. You're more like a bridge than a tower, or rather like a bridge one moment and then a tower the next. You need reinforcement for that, which makes the vehicle heavier.

- Modern reusable rockets do quick "load and go" filling to keep their propellant as cold and dense as possible. You can't do that if you need to fuel on the ground and then hang off an airplane for ~an hour while it climbs.

[kolinko]

It wouldn’t help much, sadly. Getting to orbit is about speed, not height — you need 27000 kph to get to orbit, and having an air launched platform would shave off 1k kph off it at most, perhaps 5k with some insane hypersonic engineering.

[m4rtink]

Main advantage of air launch is that you can better match your target inclination or perhaps even orbit timing - just drop the rocket at the right time in the right direction over the ocean. With a fixed launch site you always need to adjust for some difference of your point of origin versus the orbit you want to achieve.

[panick21_]

Sure but the main disadvantage that you need another highly complex vehicle. Best case its a existing commercial platform like Virgin Orbit, but then you are limited by the size. Or you design another whole vehicle that more complex then the rocket itself.

And even then you need to have all the infrastructure flying and integrated rather as nice static pad.

You are also putting human in bigger danger unless you want to have another autonomous highly complex vehicle.

The benefits are simply better addressed by having 2-3 launch pads.

[kjkjadksj]

How big a trebuchet would be needed to chuck a cubesat directly into LEO?

[chainingsolid]

cubesat + small rocket. Orbital mechanics require a burn at a later time in the orbit, to avoid coming back to say hello again to Mr. Trebuchet (Ignoring Earth's rotation)

[00N8]

100 or 200 km tall at point of release ought to do it.

[NetMageSCW]

The tip would still have to be doing 7 km/s.

[NetMageSCW]

Lookup SpinLaunch.

[bruckie]

It helps a bit more than you imply, though: if you can launch from a higher altitude, you have less atmosphere to plow through. That lets you use more of your propellant to speed up instead of to push air out of the way.

[notahacker]

You've just got the problem of building a fixed wing aircraft which can carry your rocket full of explosive propellant, successfully release it pointing in the right direction and then get the hell out of the way....

[Rover222]

You're extremely limited by the amount of mass you can even launch from a mothership aircraft.

There's no future in this idea outside of small sat, and probably not even there.

[NetMageSCW]

That isn’t very helpful considering rocket launches only spend a few seconds in atmosphere mostly going vertically to get above the majority as quickly as possible .

[Cthulhu_]

There are some companies working on that and / or there have been some experiments with it, but there's two factors there; the one is of course how much weight an aircraft can carry. The other one is the altitude and / or angle; a big plane goes to about 10 kilometers (maybe more, idk), but that's a 'flat' flight, ideally you launch while angled upwards and that's a bit more involved.

But that's how a lot of the X projects were / are done.

[NetMageSCW]

There’s actually been commercial launch services using air launch - Virgin Orbit, which went bankrupt, and Northrop Grumman’s (acquired as part of Orbital ATK) Pegasus, which hasn’t launched since 2021 and has one launch planned in 2026.

[awongh]

That’s what Virgin Galactic was.

[NetMageSCW]

No, Virgin Orbit. Virgin Galactic is still in business and does sub-orbital tourist flights.

[rramadass]

Nice and to the point.

Thanks for the link.

[Laremere]

To add to this excellent explanation: Rockets have a fundamental problem. They need to go absurdly fast. If you have a rocket that can reach speed X, to go faster than X you need to reach X but also have fuel left over. However to get that fuel to speed X, you need even more fuel. This is the tyranny of the rocket equation.

Roughly put, the rocket equation is: change in speed = (engine efficiency) * log(mass of the rocket with fuel / mass of the rocket without fuel). So there's limited parameters to play with:

- The speed you need to reach is fixed.

- You can change the weight of the payload. Payload (eg, satellite) designers try to make things as light as possible, rocket designers try to give as much capacity as possible, and everyone prays they can meet in the middle.

- You want as little propellant as possible for cost and practicality, but mostly the other parameters fix how much you need. If the other parameters aren't good enough, you can easily get results like needing a rocket the size of Central Park. [1]

- You can make the engine more efficient. This means running it hotter with higher pressure, pushing the limits of material science. [2]

- You can make the non-payload static parts of the rocket lighter. This means removing structural integrity. It also means making the lightest parts to complete hard tasks like being a valve for cryogenically cooled, literally the smallest element, hydrogen.

Both the engine and non-payload static mass are essentially asking the question "How far can I push this without it breaking". Get your answer to that question even slightly wrong on any of the thousands parts in a rocket, and suddenly all of the fuel that you're using to go in one direction fast decide that you should instead go in every direction fast.

[1] https://what-if.xkcd.com/24/

[2] Or not using chemical propulsion. However things like ion engines don't have enough thrust to get through the atmosphere and into orbit, and things like nuclear propulsion spew fallout everywhere.

[ianburrell]

Nuclear rockets aren't suitable to get orbit, they are too heavy. Also, nuclear rockets can separate the reactor and the propellant, called close cycle. I think the solid core reactors that are feasible send propellant through reactor but all of the fuel is encased.