Why do rocket launch structures not push the rocket up (in addition to rocket thrust).


As I understand it the initial motion of the rocket launch is the most energy intensive. Why is there not some propulsion method (electromagnetic or old skool motors) that assist with pushing the spacecraft up. This would also reduce the fuel load?

Basically, why do we not slingshot the craft up and let the onboard fuel take it the rest of the way once it’s got some momentum, even if it’s the first 100 meters?

In: 1

Rockets are very not strong. They are like a soda can (or beer can if you prefer). Anything that could toss them would run the risk of buckling then, causing a rapid unexpected disassembly.

What you’re thinking of is a [non-rocket launch](https://en.m.wikipedia.org/wiki/Non-rocket_spacelaunch), and there’s a lot of different methods proposed. Some of them are more practical than others, but theoretically offer a lot of advantages over rockets.

As to why we still use rockets? It comes down to the fact that we know *a lot* about them. After all, we’ve been developing them for almost a century and have a great deal of experience with their design and operation.

As a result, it’s preferable to engineer a traditional rocket propulsion system than it is a non-rocket system that requires vast amounts of engineering and money to create.

> As I understand it the initial motion of the rocket launch is the most energy intensive.

You understand incorrectly!

In fact, getting a rocket to space is not very hard. If you’re a good engineer with some resources, you can build a rocket that will reach space pretty easily in a backyard shed. And you can launch it from miles up by just floating it up on a weather balloon, too.

What’s hard is *staying* in space, because you have to go sideways *really really really* fast. The amount of energy necessary to get to space is only a fraction of the amount of energy you need to speed up enough to stay in orbit. Rocket engineers measure the ‘energy’ needed in something called [delta-v](https://en.wikipedia.org/wiki/Delta-v_budget)*, and getting to orbit requires about six times more delta-v than just getting to space.

* (It’s actually not a fixed amount of energy, but it captures the idea of “how hard” it is to get from A to B reasonably well.)

They can and do…that’s the entire reason for air-launched rockets like Virgin Orbit or Pegasus or Stratolaunch. In that case, the “rocket launch structure” is an airplane and you get a free hundred mph and a few miles of altitude for “free” from the rocket’s point of view.

For larger terrestrial rockets, the short answer is that launch towers aren’t much taller than the rocket itself and, at launch, rockets are *really* heavy and their structures are (relatively) not very strong…they can’t handle much more force…if you pushed them with something on the launch tower you’d have to reduce the engine thrust to match so that the total load on the rocket wasn’t too high. So you’d only gain the altitude of the launch tower (a few hundred feet) and you wouldn’t be going meaningfully faster when you cleared the tower. It would help, but not enough to be worth it for the extra complexity.

You’d have to have the engines up and running before launch just so you know that they *will* be running when you clear the tower.

There is another more sutble version though…if you move the entire rocket launch structure somewhere that the entire structure is going fast…like the equator…then you get a free boost. This is why most launch facilities are as far south as their respective countries can get them and why it’s preferred to launch in the direction of earth’s rotation. The gain from doing this is *far* higher than anything you could get from pushing the rocket up the tower.

Getting into orbit requires 3000 G.kms. You’re talking about 100 metres; that’s a tenth of a km, so a tower that big could throw an object to orbital velocity, no rocket required, using an acceleration of 30 000 Gs. Notice how this is a stupidly big number that will destroy just about any payload; it’s like firing a bullet from a gun.

OK, you say, I only wanted the tower to help a little. But probably the largest practical boost the tower could give would be around 3 Gs. Anything bigger than that is going to require stupidly strong attachment points and reinforcement of the rocket, making it heavier. But 3 Gs only helps out by one part in 10 000 (see above). It’s not even close to being worthwhile.

The energy requirement would be insane and not reduce fuel all that much

The vast majority of the energy isn’t used for going up, its for going sideways because the rocket needs to get up to 8000 m/s to keep its payload in orbit.

If you wanted to give a Falcon 9 a boost over the first 100 meters so you double its acceleration during that time you would need a launch stand that’s 30% taller than the rocket (F9 is only 70 meters tall) and it would need to push upwards with 770 tons of force. That’s feasible but generally we’d just call that another stage of the rocket as you’d need the power from 9 Merlin engines to pull that off and give you the 760 MJ of energy.

While this would result in some energy savings it adds yet another point of failure, added cost and complexity to a launch, and helps the payload by maybe 1-2%. This is pretty similar to launching off a mountain, it could help but its wayyy less convenient.