Why do we have to fly rockets straight up and out of the atmosphere? Why can’t we fly a ship like a plane and just go higher and higher and higher, then rocket booster out past the exosphere. I know the air is thin way up there, so at the highest, tilt upwards and rocket boost
In: Engineering
There were two space programs running side by side in the 50’s and 60’s in the US, one used rockets, the other used planes exactly as you described. The rocket program got more early success and so got more funding and that was the one we stuck with.
Don’t have to but, fuel has weight and it takes a lot to reach that height. A large Craft carrying fuel to reach orbit needs to be as light as possible. Apollo fuel load was about one million pounds per stage. Going off memory here, could be wrong. Going straight up reduces forces on the Body of the rocket to mainly length-wise.
In a sense, that’s what we do. First we go straight up to get out of the thick atmosphere to reduce drag and fuel consumption. then as we get higher, we actually move towards the horizontal to build up speed and gain orbital velocity. If you took off from an air strip and flew up like a plane, you’d just be fighting the thick atmosphere and taking a lot longer to get to an altitude that lest you get your speed up enough to get into orbit.
Virgin Galactic tried to bridge that gap by making the orbital module launch at very high altitude from an aircraft designed to get it there, so kind of a hybrid concept.
Another idea was to put a launch platform under a bunch of ultra-high altitude balloons and launch from there. Whatever the case, the most expensive fuel consuming part of the flight is from ground level to very high altitude where you don’t have to fight the atmosphere to get your speed up.
ELI5: Air is thick, so you want to get above it as fast as possible. You can worry about getting orbital speed when you’re really high up.
You need wings and control surfaces to fly like a plane, and they become redundant (excess weight) when you leave the atmosphere. Orbital rockets start going straight up, but soon roll over to take an orbital path
The term for this is air launch to orbit. Its only slightly more efficient (space is surprisingly close to the ground), and leads to a large increase in mechanical complexity.
It does make a little bit of sense for very small payloads that don’t really need to make it to orbit.
Like rockets on a fundamental level are pretty simple things, and we still mess them up every now and then.
Getting into space isn’t about getting high up. It’s about going really really fast horizontally so the arc of your trajectory causes you to continually miss the ground as the earth curves away from you. The only reason why rockets go up at all is because they want to get out of the atmosphere (or at least to the very thin parts of it) so drag doesn’t slow them down.
So flying up into the atmosphere doesn’t really help achieve this objective all this much, other than slightly reducing the initial drag force. Planes need air to provide lift. The higher go, the less they can lift, so the smaller the payload they can lift. So an aircraft very quickly becomes ineffective at lifting any sizable rocket up into the air. In theory air-launch can be more efficient than vertical launch for some smaller rocket sizes, so they have been attempted. But the margins are actually quite small. And the maneuver from horizontal to vertical often requires added aerodynamic surfaces to perform, which means the added drag and weight of these features offset the theoretical efficiency gains made. They just aren’t worth the added engineering complexity.
As it turns out, the best vehicle for lifting a small rocket up high above the ground is a bigger rocket. This is basically what a two-stage rocket is.
Imagine I have a model plane with a rubber band attached to the ground. My goal is to lift the plane up until the rubber band snaps. Which is easier, pulling straight up, or pulling up while moving it around in circles?
Seems like the second way just wastes more effort since I’m moving in circles while fighting against the rubber band at the same time. My arm is getting tired. t’s easier just to pull it straight up until it snaps and not worry about going in circles.
Rockets don’t fly straight up, they launch vertically and soon after launch lean over to gain maximum horizontal velocity.
Getting to orbit is the easy part of a launch, it’s staying in orbit that’s hard.
To do that you need to go really really fast to the side.
For low earth orbit you need to be going at **7.8 km per SECOND**.
For comparison, the SR-71 is the fastest jet plane ever made, it took insane amounts of engineering to get the plane’s outsides to survive the kind of forces that its speed through the atmosphere was putting it through.
That speed was about **1** km/s
It was spending so much energy fighting against the atmosphere slowing it down, which was also heating up the aircraft and putting stress on the hull.
So rather than fighting the atmosphere, they boost past it and start going sideways after they’re out of the vast majority of it.
Furthermore, the SR-71 was operating at the edge of how high jet planes can possibly work at.
About 25 km up.
The minimum line for space is 100km up.
Low Earth Orbit is usually 200km up.
TL:DR even the best jets only work at a tiny fraction of what space ships need to work at.
Getting into orbit is much more about going sideways really fast than it is about just getting up above the atmosphere. Rocket trajectories are designed to minimise distance through the atmosphere so they can then accelerate in a mostly vacuum.
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