# NASA – The Tyranny of the Rocket Equation

This NASA document has been around for a while, but it remains relevant.  If you’re going to engage in space travel, you have some unyielding scientific and engineering realities to contend with.  This article is a bit dry, but it’s a pretty good introduction into the realities of spaceflight.

Tyranny is a human trait that we sometimes project onto Nature. This projection is a form of rationalization, perhaps a means to cope with matters that we cannot control. Such is the case when we invent machines to free us from the bounds of Earth, affecting our escape into space. If we want to expand into the solar system, this tyranny must somehow be deposed.

Rockets are momentum machines. They spew gas out of a nozzle at high velocity causing the nozzle and the rocket attached to it to move in the opposite direction. Isaac Newton correctly defined the mathematics for this exchange of momentum in 1687. Conservation of momentum applied to a rocket was first done by Russian visionary and scientist Konstantin Tsiolkovsky in 1903. All our rockets are governed by Tsiolkovsky’s rocket equation.

The rocket equation contains three variables. Given any two of these, the third becomes cast in stone. Hope, wishing, or tantrums cannot alter this result. Although a momentum balance, these variables can be cast as energies. They are the energy expenditure against gravity (often called delta V or the change in rocket velocity), the energy available in your rocket propellant (often called exhaust velocity or specific impulse), and the propellant mass fraction (how much propellant you need compared to the total rocket mass).

read the rest at NASA – The Tyranny of the Rocket Equation.

There is, of course, another tyranny this article doesn’t mention: the tyranny of economics.  In truth, you can brute force your way through the tyranny of the rocket equation if you have enough resources.  But to get those resources, there has to be a payoff.

We often hear talk about the spirit of exploration, evoking the age of discovery when Europe explored the rest of the world.  What is often not mentioned is that the age of discovery was motivated by economics.  Initially it was to find alternate trading routes to the spice islands (other than the difficult and dangerous overland routes through Asia), but later the incentives became extracting resources from far flung colonies.

I’ve said it before, but for the space age to really get started, it will need an economic incentive.  We need a space age version of the spice trade, only much bigger since space is much more expensive.  Until we find it, most of our exploration will continue to be by robots (of increasing intelligence and sophistication) with a few symbolic manned missions.  We may even put a colony on Mars, just to prove we can do it, but without an economic incentive, it’s unlikely to last.

So yes, the tyranny of the rocket equation must be contended with, but so must the tyranny of economics.

## 9 thoughts on “NASA – The Tyranny of the Rocket Equation”

1. Brett says:

If space launch capability isn’t inherently labor-intensive, then you could theoretically get to a point where launches are relatively cheaper for a richer society to afford, opening up space exploration to a larger group of people (including those who want to colonize parts of space just because they want to). That’s even if it’s a mature industry.

I’ve said it before, but for the space age to really get started, it will need an economic incentive. We need a space age version of the spice trade, only much bigger since space is much more expensive.

I think it’s the time frame and unknowns about investment that make it especially unattractive, since the values of some Platinum Group Metals are enormous – bring back several tons of them to sell on pre-purchased futures contracts for the metals and you might be able to easily make your money back. But most investors aren’t really up to the idea of putting so much capital into such an illiquid investment as an asteroid mining program with so much uncertainty about whether or not it would actually work.

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1. I definitely think mining offers a potential economic incentive. I agree that risk is a major obstacle right now. And here the government does have a role to play. For although the age of exploration was motivated by economics, the risky initial forays were funded by kings (governments) such as Henry the Navigator.

The problem is that space travel is so expensive right now, that the return on investment isn’t there yet. But as space travel becomes cheaper (hopefully) and raw materials on earth more scarce, that economic balance could shift.

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2. Brett says:

That really is a good essay by Don Pettit, though. It lays out the challenges and limitations of conventional rocketry quite clear.

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3. ” In truth, you can brute force your way through the tyranny of the rocket equation if you have enough resources.”

This is incorrect. Uh, that’s why it’s called “the tyranny.”

I’m sorry, it is just plain wrong. One hundred and eighty degrees. To think that you can is magical thinking of the most extreme kind.

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4. ” In truth, you can brute force your way through the tyranny of the rocket equation if you have enough resources. ”

A wagon-load of hay will feed a wagon’s horses from Bristol to London. Add more hay and you need more horses. Add more horses and the hay runs out faster. That’s the economics.

That’s why it’s called “the tyranny,” Michael.

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1. David,
I don’t think you understand my point with that phrase. But I believe it’s clear if you read the rest of the post.

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1. I think I understand your point perfectly well, Michael. You believe that you can always work your way around the rocket equation by adding “resources” and I think you’re wrong.

For any given technology there are limits set by the Law of Diminishing Returns. Adding resources works for a while and then it doesn’t.

Then you need to stop adding resources and switch to be better technology. Brute force only works for a while.

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