Calculating Mission Fuel Requirements with a spreadsheet and the Rocket Equation

[Poppa Wheelie]

This tutorial demonstrates how to calculate your mission fuel requirements using a spreadsheet and the rocket equation.

I recently flew missions to Duna and Eve that each required orbital assembly in LKO. These ships were too large to fully assemble them in the VAB, and I realized that I didn’t have a method of determining how much fuel each mission would require. I’m sure there are plenty of other use cases where you may need something more than the VAB tools to determine your fuel requirements.

Too Long, Don’t want to Read? Do you just want a copy of the spreadsheet and the short version of how to use it?  Here you go:  the spreadsheet.

Short Version:

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The spreadsheet has 2 tabs:

• Empty 100 rows.  This is a “ready to go” spreadsheet.  See the “Short Instructions” below for how to use it.
• Tutorial Example Complete.  This the final result of the spreadsheet used in the tutorial.  You can use this as reference.

The Empty 100 rows tab of the spreadsheet has formulas filled in for 100 rows (rows 5 through 104).  If you need more, add them.

Short Instructions:

• Fill in Mission Phase and Mission Event columns
• Fill in dV Required column
• Add a Reserve row before each dropped fuel tank, and one at the end
• Put a Final Mass and Tankage row at the end, and fill it in (see row 29 of the tutorial example)
• Fill in Additional Mass Dropped column for all mass drop events, other than empty fuel tanks
• Determine mass (empty) and tankage for each tank drop event, and fill in Mass and Tankage columns

 

Longer version:  What are we doing with the Rocket Equation, how to create the spreadsheet from scratch and fill it with the Excel formulas that implement the rocket equation, and how to fill in your mission-specific details to get the results you want.

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What’s all this fuss about the Rocket Equation? “What” are we actually doing here, before we get to “how”

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You’ll see the Rocket Equation mentioned all over the forums. Here’s a good Wikipedia article on it.

One of the things the Rocket Equation does is to tell us how much fuel we need to achieve a specified dV, for a specified mass.

So we’ll have to work backwards, like this:

• Determine how much fuel is required to perform the very last burn for whatever vehicle completes the mission (including the mass of the empty tank that contained the fuel we’re calculating now)
• Determine how much fuel is required to perform the next-to-last burn, for the vehicle, the empty tank, and the mass of the fuel needed for the last burn
• Determine how much fuel is required to perform the second-to-last burn, for the vehicle, the empty tank, the mass of the fuel for the last burn, and the mass of the fuel for the next-to-last burn
• etc.

Each time we calculate fuel (again, working backwards), we’ll “assign” that fuel to the fuel tank, until we’ve assigned all the fuel the tank can hold – then it’s time for another tank to hold the fuel for “previous” burns.  I know, it’s kind of difficult to wrap your head around this “working backwards in time” thing, but hang in there. You’ll see it working in the tutorial example.

How to build your spreadsheet and populate the formulas

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The spreadsheet provided with the Short Version of this tutorial contains a tab called “Empty 100 rows”.  You are welcome to use this if you don’t want to build your own.

If you want to create your own, and get a better understanding of how it works, then follow these 3 steps:

Step 1. Create a spreadsheet, populate the headers, and fill in some constants

Step 2. Fill in the formulas on the first row

Step 3. Copy and paste the row of formulas.  Fill as many rows as you want.  If you find later that you need more, you can copy and paste more below the ones you’ve already done

How to fill the spreadsheet with your mission-specific details (with a follow-along example)

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For this tutorial, we’ll use an example mission to Duna and Ike.

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Step 1: Fill in Mission Phase and Mission Event

Step 2: Fill in dV Required

Step 3: Add Reserve rows

Step 4: Fill in Final Mass and Tankage

Step 5: Address Dropped Mass (not fuel tanks)

Step 6: Address Dropped Fuel Tanks

Here's the fully assembled ship, ready to go to Duna and Ike!