how to get an object into (and out of) orbit

8/12/2019 How To Get An Object Into (And Out Of) Orbit

1/4

How to Get an Object Into

(and Out of) OrbitBrian Williamson


2/4

Whether you're orbiting a satellite, a shuttle of humans, or an intercontinental ballistic missile,

there are fundamental principles that need to be followed in order to achieve the desired result; to have an

object moving so quickly around Earth that it can't fall towards it enough to hit it. Some "basic" things

that you'll need are a rocket, or rockets; your object or a vessel for your object; and a parachute. Here I

will detail the four (albeit simplified) steps to achieving orbit: preparation, launch, orbit, and landing.

To begin, you're going to need a rocket, but not just any rocket. Specifically you're going to need

a rocket with enough thrust to achieve escape velocity, and escape the atmosphere. Mathematically, this

means a rocket that can accelerate you by 9.3-10 km/s. The most efficient way to achieve this is by having

multiple rockets staged. Firing the rockets in sequential stages allows you to drop off the dead weight of

empty fuel tanks and unused rockets. Proper staging is especially crucial for longer range missions, where

every bit of fuel counts. Spent stages are separated with devices called decouplers. Sometimes explosive

decoupling is necessary to avoid separated parts colliding with necessary parts of the rocket, potentially

ending in catastrophe. Your rockets should be staged so that the more powerful rockets that require more

air are launched first, to overcome the stronger gravity closer to the Earth and, more importantly, the air

resistance. Smaller, multidirectional rockets, called Reaction Control System (RCS) rockets, should be

attached to the body of the craft for more precise movement in the vacuum of space, where even a little

thrust can make a big difference. Once you have all of these things, know that the easy part is over.

Getting it off of the ground is where we will now focus.

Don't worry, you'll be blasting off soon enough. Or rather, you should be worrying. This is where

you find out if your equipment will hold up to the massive stresses of leaving the atmosphere and

traversing the cold vacuum of space. Your rocket should be facing skyward, all systems ready for launch.

Once you are ready, turn your rockets to full blast and you're on your way. You'll want to maintain going

straight upward for several kilometres, to reduce the amount of time you spend in the atmosphere. To

conserve fuel you'll want to go as fast as you can, without exceeding terminal velocity. Terminal velocity,


3/4

in the classical sense, is defined as the speed that a falling object achieves when the frictional drag force

from fluid (air) resistance is equal to the downward gravitational pull, and the object no longer

accelerates. Terminal velocity will vary from rocket to rocket, depending on the efficiency of the

aerodynamics, and increase as you move upward into thinner atmosphere. As you are launching upwards

against gravity, terminal velocity tells us something different. It tells us the speed we should maintain

where if we were to go any slower, we would spend too much time in the atmosphere; any quicker, and

we are fighting much stronger drag forces relative to shorter amount of time, and therefore wasting fuel.

After a few kilometres of launching straight up, you'll want to begin turning your engines and gaining

some lateral speed. After all, the largest percentage of your acceleration will need to be lateral, lest you

fall straight back down to Earth, vaporising in the atmosphere; or find yourself drifting endlessly outward

into the blackness of space. Begin slowly levelling off your craft so that it is mostly level around the 170+

km mark of altitude, which is where you'll be accelerating to ensure you reach orbital velocity (~7.8 km/

s). This is the speed required to ensure you don't come tumbling back to Earth. What you're doing now is

making sure that the periapsis of your orbit is far enough away from Earth that you won't hit it. The

periapsis is the point of your orbit that comes closest to the body you are orbiting, in this case, Earth. The

apoapsis is opposite the periapsis, and therefore the highest altitude from Earth. After accelerating for

long enough, you should have yourself an orbit, though maybe not the most circular of orbits. In the next

step, I'll talk about how to circularise the orbit and make any necessary corrections.

Now that you've gotten yourself into orbit, I'll show you how to maintain that orbit. You'll want to

be orbiting above ~200km to keep the orbit as stable as possible. Any lower than this, and you'll be

subject to atmospheric drag that could significantly slow down your craft over time. To make this easier,

and your orbit a bit prettier, you'll want to circularise your orbit. Right now, it's probably more of an

ellipse than a circle. You'll want to equalise this, so that your apoapsis and periapsis are close to the same

altitude. To do this, there are a couple of terms you should become familiar with; prograde and retrograde.

If you want to raise the altitude of your apoapsis or your periapsis, you will need to accelerate prograde to

your orbit, at the point 180 opposite the point you wish to raise. If you wish to increases the altitude of

your apoapsis, you'll want to do the prograde burn at your periapsis, and vice-versa. Prograde simply

means that you are oriented with the direction of your orbit, so that firing the rockets will speed you up.


4/4

Retrograde then means the exact opposite, to accelerate opposite the direction you are orbiting, and

therefore to slow you down. Now, as you may have figured out, to lower the altitude of any point of your

orbit, you will want to perform a retrograde burn 180 from the point of orbit you wish to lower. Small

thrusts in little-to-no atmosphere, and therefore little-to-no drag, make tremendous differences in speed

and destination. A little bit of fuel can go a long way in the vacuum of space. These are the basics of

orbital manoeuvring, and allow you to do more advanced things such as orbital transfers by pushing your

orbit out enough so that you're pulled into the orbit of another body.

Finally, I'll be telling you how to land your craft back on sweet Earth; after you've done a lap or

two, of course. This is called deorbit and reentry. You may have already figured out what you need to do

here. You're going to need to decelerate your craft so that it can fall back to Earth. You must be careful not

to hit the atmosphere too quickly, though; lest you be cooked and vaporised. You'll want to do as much

deceleration above the atmosphere as you can. In order to do this, you will need to orient your craft to

perform a retrograde burn. If you remember correctly from before, this means you'll want to be firing

your rockets backwards to counter your speed. Doing this, you can remove a lot of your lateral motion,

and fall straighter back to Earth. Doing this will lower your periapsis back to the surface. Once you have

enough lateral speed gone, you will begin falling into the atmosphere, which will slow you down much

further. By the time you reach altitudes inside 20,000 ft., you should be deploying your parachute. This

will decelerate you the necessary amount to safely land back on the ground, and end your mission.

Now you have the necessary fundamentals to get yourself, or something, into orbit. These are

grossly simplified steps compared to the immense undertaking of performing this, but I hope this goes to

show that it isn't quite as complex as it may seem. There are tens of thousands of objects orbiting our

planet, and I hope yours becomes one of them.

how to get an object into (and out of) orbit

Documents