pythonphysics

# Calculating delta-v gives negative result

I'm trying to design my own model rocket completely by myself, but when I'm trying to calculate delta-v using the Tsiolkovsky equation, my code only gives negative answers.

I thought it may be down to my rocket not being powerful enough to have any delta-v so I used a real life example (Saturn V) and it gave an accurate result but still in negatives (first stage: -2000 delta-v).

This is my code:

``````import math
netMass = int(input('what is the  total mass of the rocket: '))
dryMass = int(input('what is the empty mass of the rocket: '))
Isp = int(input('what is the Isp of the engine: '))
fuelMass = netMass - dryMass
Δv = Isp*9.8*math.log(float(dryMass/netMass))
print(Δv)
``````

I also don't have Numpy at my disposal so can only use the math library.

Solution

• In the Tsiolkovsky rocket equation, the ratio which should be used is wet mass (or initial total mass) to dry mass (or final total mass). The code then should be

``````import math
netMass = int(input('what is the  total mass of the rocket: '))
dryMass = int(input('what is the empty mass of the rocket: '))
Isp = int(input('what is the Isp of the engine: '))
fuelMass = netMass - dryMass
Δv = Isp*9.8*math.log(float(netMass/dryMass))
print(Δv)
``````

Which will give you the correct (positive) value for an ideal rocket in a vacuum.