Well, I’ve always asked myself why can’t we use some kind of train or catapult to help the rockets to save fuel or to go further. Can anyone explain me why?
In: Engineering
They need to get up to 28,000km/hr to get into orbit.
Starting at 0km/hr or 40km/hr or even 400km/hr is practically insignificant (especially when the Earth’s own rotation adds more than that) and only adds complexity and failure points.
When your plan is to accelerate hundreds of tons of rocket fuel in a literal missile to 28,000km/hr, minimizing failure – especially on the ground with people around – is important.
Wasn’t Gagarin launched into space with a catapult?
The amount of acceleration need to get to orbital velocity requires that the acceleration be accomplished in a reasonably smooth fashion.
There have experiments with artillery launches to orbit but the short violent acceleration just destroys the projectile. A catapult launch to orbit would face the same problem. There’s more.
We are all familiar will dead satellites, or manned spacecraft, burning up on re entry into the atmosphere. The only way to prevent the payload from burning up in the atmosphere *on the way up* would be to build your catapult taller than the atmosphere. Not practical at all.
There are actual ideas along those lines, but they are still theoretical because they would need to be extremely huge beyond anything we can easily build today.
The important thing is that it is not so much about getting into space (which is only a 100 km up) but getting into orbit which means going several dozen times the speed of sound on earth.
Rockets are difficult because you need to carry fuel to carry more fuel around, so that your typical rocket that goes into space is mostly fuel and very little actual payload.
This had let many to think of how we can do this easier, better and cheaper.
The most famous idea for an alternate orbital insertion system is the space elevator, basically climbing a rope up into space high enough that you reach the point where orbital velocity which you need to stay up is the same speed that you are already going thanks to the rotation of the planet.
Everyone agrees that such a space elevator would be wonderful and cheap and easy and generally much better than rockets with just the minor problem that we don’t have any materials strong enough to build one.
Other famous ideas include shooting up your spaceship in a giant gun. This is an idea that actually predates modern rocketry and another thing that would work in theory.
One problem is that chemical explosion based guns accelerate their projectile really really hard, so that would not work for anything fragile like humans. Magnet based rail guns could work with a much lower acceleration though.
The bigger problem is that any space gun would need to be enormous. It is another thing that would work in theory but that we don’t have the right tech for yet to actually build it. The last guy to actually spend serious money on building anything like that was Saddam Hussein and he was more concerned with reaching some place more down to earth rather than the stars. (Things didn’t work out well for him.)
Another thing that might be come close to a catapult would be a space bola or rotating space tether. It is basically a cut down version of a space elevator that rotates in orbit and has its ends reach down into the atmosphere where a fast flying plane could attack a payload to be catapulted up into higher orbit by the spinning death strand. It is close technology wise to what we might actually be able to build with current tech, but would still be a major undertaking.
In any case, anything that shoots or catapults stuff into space could save some rocket fuel but you would always need some rockets to make an actual orbit out of it.
The point is that people have thought along the lines you suggested, come up with ways that would work, but ended up not being able to built the things they came up with because we don’t have the materials or the money for that yet.
The mechanism would have to be absolutely massive and at best it would only save a little bit of fuel. Some sort of mechanism the size of the tallest skyscraper would probably only be able to accelerate the thing up to a hundred miles an hour or so without subjecting the rocket to too much inertial stress, and rockets get to that sort of speed after only a few seconds.
You’re adding more systems than you need, which means you’re adding more individual pieces that can fail.
Further, rockets are kind of constrained in terms of speeds at lower altitudes because the faster you go, the more stress you have on the structure of the rocket caused by passing through the atmosphere at speed. Thus, most rockets will shoot straight up, get out of the thick part of the atmosphere, and then nose-over and throttle up their engines to get up to the sheer speeds needed for orbital velocity.
It’s really just not worth it.
There are plans out there for things like this, but they are extreme, even for modern engineering.
In order to understand the problem, it might help you to understand what a normal rocket has to do in order to get into orbit around the earth. A basic rocket launch has two stages. The first stage is to launch up and out of the Earth’s atmosphere. It uses up a ton of fuel, and puts a sustained 5-10 times as much stress on the rocket compared to sitting on the ground. (The rocket, and its payload experience 5-10 g’s of force.)
The second stage of the launch involves turning sideways as you exit the atmosphere, and accelerating to crazy high speeds, so that you are moving so fast you “miss” the earth as you fall. If you want to stay in space, you have to get going this fast outside of the atmosphere. This is called orbiting the earth.
Now, a catapult or launch rail system can only do step one. The payload launched up out of the atmosphere has to complete step two on its own. That means delicate rocket engines, fuel tanks, and control systems have to survive the catapult launch.
This is the big problem. Unlike a normal launch, where the rockets main engine can accelerate you out of the atmosphere at 5-10 g’s for a few minutes, a catapult has to do it all at once. That means subjecting the payload to 20-30 g’s of force immediately. This would likely kill a human passenger, and probably damage anything aboard the rocket, like those engines and fuel tanks.
None of this is to say its impossible, only that to do it with modern tech would be way harder than just launching a rocket, and probably way way more expensive.
How fast can a train go while carrying a rocket? Idk but I bet it’s hella slow. Almost nothing compared to how much speed it needs to take off.
But, the idea is used.. kinda. That’s where multi-stage rockets come in. They basically use a rocket to launch another rocket. The space shuttle uses this idea. The tanks used for liftoff are let go and fall back to earth, which means it doesn’t have to carry around the extra weight after launch.
Same reason that we don’t build launch sites on mountaintops. The energy it takes to achieve *altitude* is a rounding error compared to the energy it takes to achieve the necessary *speed*.
Consider the projectile that comes out of a 16-inch battleship gun: make it go *ten times* as fast, and you can put it into orbit. Try that with a catapult.
Getting into orbit requires 3000 G.kms. If you want to accelerate at 3Gs then your catapult needs to be 1000km long. Even if you can tolerate 10Gs that’s still a 300km catapult.
I understand that you’re talking about catapulting a rocket, so the catapult doesn’t do all the work. Even doing just 10% of the work results in needing a huge catapult, like 100km long. And the penalty for adding the rocket is that your catapult now has to handle 10 times more weight.
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