As I prepared for this post tonight, I realized that I wasn’t really modifying the rocket equation at all–I have been using the rocket equation and a summation of mass terms to find the payload fraction, which I consider an especially useful value to know.
Furthermore, if you read my previous post, you probably figured out pretty quickly that all of the dry mass in the rocket doesn’t correlate to the propellant mass. That’s a pretty good guess for very large rockets with large delta-V’s, but as you get smaller, the assumption really starts to fall apart.
One of the big masses in a rocket outside of the propellant tanks are the engines themselves, and they really don’t correlate to propellant mass at all. They correlate to the gross mass, because the engines are typically sized to give you some particular value of thrust-to-weight when you light them up. So I went ahead and extended the previous derivation of payload fraction to include this important feature.
So let’s define some terms. Lambda is the relationship between the vehicle dry mass that correlates to propellant, and phi is the relationship between vehicle dry mass that correlates to gross mass.
We plug these definitions into an equation for gross mass, and I simplified things by just going ahead and combining the propellant-sensitive mass into (1+lambda)*propellant_mass. Then I went ahead and replaced propellant mass with the expression from the rocket equation that related propellant mass to gross mass, and now I had the equation entirely in terms of payload mass and gross mass, which is exactly what I was after.
Then it was a matter of doing the algebra to simplify things down until I had an expression for payload fraction like I did before.
Take a look at the final equation. You’ll note a few things. The first one is that if phi is zero, then the expression is just the same as the one I derived before. The second one is how lambda and phi impact the payload fraction differently. You can see that the effect of lambda is reduced somewhat by the mass ratio having one subtracted from it, but that phi has no such reduction.
This is because in the case where you had a vanishing small impulse, MR would be very close to one. And one minus one would be zero and the impact of lambda would be almost eliminated. But because phi is multiplied by one, its effect is still present even if the impulse was very small. This is because any rocket whose engines were sized to deliver a particular thrust-to-weight at ignition would still pay a mass penalty for those engines, even if it just briefly turned them on and then turned them off.
In an upcoming post I will show how to calculate lambda and phi from other vehicle design parameters, like mixture ratio, tank mass per unit volume, propellant densities, thrust-to-weight ratios of engines and vehicles, and so forth.
Latest posts by Kirk Sorensen (see all)
- Baroness Worthington at the US Space and Rocket Center - June 26, 2012
- Sorensen TEDxYYC Thorium Talk - April 23, 2011
- Save U-233, explore space video - January 28, 2011