This specific impulse calculator takes the figures you already have and applies the relationship behind specific impulse calculations for you, giving thrust-specific fuel consumption (tsfc), thrust to weight ratio (twr) and specific impulse (isp) from dry weight (w), gravitational acceleration (g0), mass flow rate (ṁ) and exhaust velocity (ve).
What Is the Specific Impulse Calculator?
This specific impulse calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns dry weight (w), gravitational acceleration (g0), mass flow rate (ṁ) and exhaust velocity (ve) into thrust-specific fuel consumption (tsfc), thrust to weight ratio (twr) and specific impulse (isp).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Specific Impulse Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with dry weight (w) set to 10, gravitational acceleration (g0) set to 5, mass flow rate (ṁ) set to 2, the calculator returns specific impulse (isp) of 20, and thrust (f) of 200, and specific thrust (fs) of 0.1. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out thrust-specific fuel consumption (tsfc)?
Enter dry weight (w), gravitational acceleration (g0), mass flow rate (ṁ) and exhaust velocity (ve). The calculator then works through the relationship and reports thrust-specific fuel consumption (tsfc), thrust to weight ratio (twr) and specific impulse (isp).
What does the specific impulse actually calculate?
It takes dry weight (w), gravitational acceleration (g0), mass flow rate (ṁ) and exhaust velocity (ve) and converts them into thrust-specific fuel consumption (tsfc), thrust to weight ratio (twr) and specific impulse (isp) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter dry weight (w)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.