Peter Beck from Rocket Lab in New Zealand talks about the importance of minimising the mass of a rocket to make it as efficient as possible. He explains the ‘spiral of doom’ in which any extra mass requires a lot more fuel and fuel tanks. Keeping mass as low as possible is the way to keep rocket size and cost as low as possible.
For Rocket Lab, efficiency is everything. Our rocket wasn’t very big, but the reality is everybody else who understands what we are doing, it was like how on Earth did you make that thing so small? So really, you know, the size of our rocket was an indication of its efficiency rather than it’s just a small rocket, and the only way we can achieve that is to use very low mass materials, and carbon composites provide that advantage for us.
One of the important things to explain about the design of a rocket and the whole efficiency equation, if you will, is for every gram of inert mass – when I say inert mass, it means like propellant tank, fins, you know – the ratio is around about 10 grams of fuel. So for every 1 gram of mass you put in there, you need 10 grams of fuel.
So say I want to add 10 grams of screws to the front of the rocket. All of a sudden, now I need to carry 100 grams of propellant to lift those 10 grams of screws. Now because I’ve put more propellant in there, I need a bigger tank. So I need to add another 10 grams of tank to carry that propellant. But now I’ve added more inert mass, I need another 100 grams of fuel but I need a bigger tank. And we refer to that within Rocket Lab as the spiral of doom.
Now you can make that spiral of doom work for you. If you take out 10 grams of mass, now you need 100 grams less propellant, so the rocket gets smaller. Now because you’ve taken out 100 grams of propellant, your tank is smaller, and the rocket is lighter so you’ve taken out effectively more mass so you can take out more fuel, and the rocket gets smaller and smaller and smaller. It’s an ever decreasing graph, and there is a sweet spot obviously, so that is why you know we spend the time we do and focus on things we do around mass. It’s because it’s just such an important driver.