It's not about getting high, its about going fast

My hero and god, Randall Munroe makes this point infinitely more entertaining than I ever could, but essentially getting into space is not about how high you can shoot yourself up, its about how fast you can encourage yourself to go sideways. You can spend all day firing things up in the air with whatever legal (or illegal) means you can acquire. You might even get above the 150km-above-sea-level mark and be in 'space' (or not depending on which circles you travel). But if you don't build up enough sideways speed, you're always going to fall right back down.

Sounding rockets and even balloons can get you to the edge of space

To get to space you need to first get in orbit, and to get in orbit you need to travel in a circle. Once your spacecraft starts moving in circle fast enough, the force of gravity keeps you going in a circle instead of forcing you back down to the ground.

A diagram showing orbital velocity, via aerospaceweb.com

How much speed? Well at 600km above sea level, about where the Hubble Space Telescope sits, you need to be going at 7.56 km per second, or about 27 000 km per hour. And so to accelerate the 11 tonne hubble space telescope to that kind of speed, we pretty much have to rely on rockets.

Rockets. Goddamn Rockets. 

Rockets manage to be awesome and awful all at the same time.

Rockets are cool because all controlled explosions are cool and all in all they are pretty amazing feats of engineering. Rockets suck because they're huge, they weigh more than they should and shake a crap ton.

Let's deal with size and weight, To launch a heavy spacecraft or any lump of mass, you need to burn fuel. Fuel adds mass - mass that you will need to launch with more fuel. More fuel means more mass which needs more fuel. Hopefully you see where I'm going with this.

Eventually after a little bit of algebra you come up with the Tsiolkovsky rocket equation. The equation maps delta v (a fancy term rocket scientists use to indicate how much faster you want to go) against the ratio of payload mass (that we need to work in space) and fuel mass. The graph ends up looking like something below

The rocket equation, mapping mass ratio against delta V / exhaust velocity

There are two scary things about that above graph.

Firstly - look at that curve! The more delta v you need the steeper the graph gets (trust me you need more delta V to get further into space). That means that the amount of fuel you'll need gets insane unless you're launching something really really tiny.

Secondly - the x axis maps mass ratio, not absolute mass. So if you need to send up a payload twice as large, you'll need twice as much fuel. Imagine having to double the size of your rockets to send up a 10 tonne space telescope vs what is already huge to launch a 5 tonne communications satellite.

An example

No beginner's blog would be complete without having some horrendously inaccurate back of the envelope calculations. So why not dredge up one of my old university assignments and simulate ourselves an asteroid capture.

Let's say that we want to rendezvous with an asteroid that has roughly the same orbit as Earth but is some distance away from us. Capturing asteroids is all the rage lately, so lets say we want to hit it with NASA's Asteroid Redirect Vehicle - which will weigh aproximately 16 tonnes if you include all the fuel it's carrying.

Now any rocket carrying this vehicle will need to

• Launch into Earth Orbit
• Escape from Earth's Orbit
• Perform a manoeuvre to meet with the asteroid

All of these steps need (you guessed it) delta v. And as we have learned above, delta V needs lots of fuel. Now, accounting for ideal launch conditions, getting the Earth's spin and orbit around the sun to help us gain some speed, we eventually figure out that we need roughy 8km/s of delta V. And when you plug all the maths into the rocket equation and use the specs from the Atlas V user's guide (which is super easy to find btw), you end up with a little over 500 000kg of fuel.

Atlas V rocket launch

Five hundred thousand kilograms of fuel to launch something that is only 16 thousand kilograms. Fully 97 percent of the total mass of this mission is just fuel to burn to get it where it needs to be. That's not even accounting for the mass of the rocket parts like empty tanks or flight electronics. Is that not insane!?

To drive the point home, to get to space, you need to go fast. To go fast you need rockets. To go faster you need an unholy amount of fuel. Getting to space is hard, and gravity wells really suck. Aim small, slim down and conserve your weight.

Cool Links

• The Atlas V rocket by United Launch Alliance - go shopping online for a heavy rocket launch!
• NASA's Asteroid Redirect Mission Concept - they actually put up plans to grab an asteroid

PS: Some pedantic details

Ok so the rocket equation graph above doesn't strictly show the relationship between mass ratio and delta v. It maps mass ratio and delta v/v_e. v_e is what is known as exhaust velocity and essentially describes how efficiently your rocket turns mass of propellant into thrust. This varies - liquid and solid propellants having relatively 'bad' v_e and newer electric propulsion doing much better (I'll get into that in a later blog post). That detail aside - the graph is still pretty crazy if you consider v_e to be a constant. Getting to the point where your delta v/v_e ratio is around 3-4 is not unheard of for longer missions. So what I said above isn't completely wrong...

As for my actual rocket calculations.... they're pretty long winded and I took some liberties with estimation of exhaust velocities and getting a 'prefect' flight'. Send me a message if you want to see the full working out