n after watching the Apollo 11 launch. Why is it more efficient (if it is) to launch a spack shuttle vertically? Why haven't we invented planes that can slowly increase in altitude until they are in orbit? I feel like it's taking a pointless amount of energy to get it started when you can basically give it a running start on the back of some kind of huge plane. I guess you wouldn't know if there's a failure as quickly but what other reasons are there
In order to answer your questions we first have to review some things about space and what is required to reach it. A vehicle travelling to space needs to function both inside the atmosphere and out in the vacuum of space. In order to reach orbit, a spaceship must reach a speed of more than 7 kilometers per second. Relevant XKCD. The point is, space isn’t very high, but orbit is very fast.
The only way we can currently get enough power to lift something into space is by combustion. Combustion requires a fuel and oxidizer. In a vehicle designed to travel into orbit, you can use air-breathing engines for the first part of the trip, while the vehicle is still low in the atmosphere, but you have to bring oxidizer for the remainder of the trip because there is no air in space. Most airplanes carry only fuel, and take in air from the atmosphere. So that’s one reason you can’t just put an airplane into space.
You can’t just fly low in the atmosphere using air breathing engines to gain all the necessary speed either, because that atmosphere will cause a lot of aerodynamic stress on the vehicle, as well as severe heating. This is what happens when you travel at that speed through the atmosphere. Here is what a space craft looks like after it has travelled through the atmosphere at orbital velocity. The only way to survive that kind of speed through the atmosphere is with specialized shielding - an ablative heat shield in the case of most space capsules, and specialized thermal tiles in the case of the space shuttle. There’s no way to cover a rocket (or airplane carrying enough fuel to reach orbit) with that kind of shielding without making it way too heavy.
The fact that you have to carry oxidizer and reach such incredible speeds means that getting to orbit requires a lot of propellant (meaning actual fuel + oxidizer, usually liquid oxygen). So much fuel, in fact, that 90% of the total mass of a rocket may be made up of nothing but propellant. The Saturn V, which delivered the Apollo missions to orbit, was 85% propellant by mass, for example. The Space Shuttle was 80% propellant on the pad. The SpaceX Falcon 9 is something like 95% propellant by mass. That means that for every pound of vehicle which reaches orbit, a vehicle has to carry between 5 and 20 pounds of fuel. To carry all that fuel, an airplane would have to have huge wings, which would be very heavy and require 5 to 20 times that weight in fuel, requiring larger wings, etc etc. It’s called the tyranny of the rocket equation, and it means that the only way to get to orbit is to strip as much weight from the vehicle as possible. Wings, control surfaces, landing gear, etc, etc are all heavy. That’s why a flying fuel tank is the best way to get to orbit.
That being said, there are some horizontally launched airplanes designed to put a payload into orbit.
A company called Orbital Sciences has a space launch system called Pegasus, which is a rocket slung underneath an airplane. The airplane takes off, gains speed and altitude, and then drops the rocket, which ignites and then lifts a payload into orbit. Here is a video.
The whole thing with mass fuel means that that entire airplane/rocket bastard child could only put 1,000 pounds into orbit. That’s almost nothing.
A company called Stratolaunch aims to expand that type of launch system into one that can carry 6 tons into orbit, but we’re a ways off from that. Also, 6 tons isn’t enough to send crew to orbit.
A company called Reaction Engines has a very controversial proposal for a vehicle called Skylon, which is just what you envision - an airplane that flies into space. Specs aren’t anywhere close to final, but it is planned to be approximately the size of a jumbo jet, and will only be able to carry 15 tons to orbit. That’s just enough to carry crew. The only problem is it doesn’t exist, and relies on several highly advanced technologies which haven’t yet been developed. People familiar with it give it no better than 50/50 odds of being successful, at best. I for one am nearly certain it will never fly.
TL;DR space is hard.
