Calculations and Numbers

There are many different factors that go into a rocket's design and performance. This page will go into some of these factors and what they can affect.

Specific Impulse

Specific impulse is a measure of how efficiently an engine consumes propellant. More specifically it is the change in velocity delivered per unit of fuel consumed.

Delta V

Delta V, symbolized as "Δv" is the amount of velocity you can change, in any direction, measured in meters per second. For example, if a craft has 2,100 m/s of delta v, it can change its velocity in any direction a total of 2,100 m/s. The equation is

In this equation:

• "M start" is the mass of your vehicle with full fuel
• "M end" is the mass of your vehicle with no fuel
• "Isp" is specific impulse, it is a measure of how efficiently an engine uses propellant
• "9.81" is the gravity, it is 9.1 because this equation assumes you are calculating delta v on Earth, if it were on another body you would replace it with the gravity there

Thrust to weight ratio

The thrust to weight ratio, or TWR, its one of the most important numbers for a rocket. The rule of thumb is that the TWR must be greater than 1 in order for the rocket to move. It is a perfect example of Newton's second law, there must be unbalanced force (in this case, the force of the thrust must be able to overcome the force put on by gravity) to produce acceleration. The equation for TWR is thrust over mass times gravity, it it written like this:

Gravity

Gravity is a force that naturally pulls toward a mass. In this case it is what keeps things on the launch pad on the pad. This concept is important in thrust to weight ratio, which is an example of Newton's second law. In summary it means that in order to accelerate the thrust of the engines must be able to exert more upward force than the force of gravity pulling down.