porkchop plots

Matija Milenovic, Co-Founder & CEO of porkchop

Anytime I tell people that I am co-founding a company called porkchop, I get at least one of the following:

“So, why the name?”, “The name makes me hungry, haha!”, “I’ve heard of you guys, the name has stuck in my head!”, “Maybe use a vegetarian name?”

In a nutshell: The name porkchop actually comes from NASA — back in the day, they would plan various types of interplanetary missions. In particular, they wanted to see how much fuel they’d need to complete the trip, depending on which particular day they depart Earth and on which day they want to arrive on Mars.

So as any good engineer would do, they made a graph of this, plotting the departure date on the horizontal (x-axis) and the arrival date on the vertical (y-axis). They ended up with what they decided to call “porkchop plots” (there’s a whole Wikipedia page about it here: https://en.wikipedia.org/wiki/Porkchop_plot)

Credits: http://nbodyphysics.com/blog/gravity-engine-doc-1-3-2-2-2/demonstrations-2/porkchop-plots-visualize-transfer-delta-v/

The long story: I love telling this story every single time, but if you’re still reading by now, I have a feeling that you might want to know some porkchop plot trivia. This is a bit of an oversimplification, but understanding the concept is more important.

I stretched the truth when I said that the plot tells you how much fuel you need to bring. Porkchop plots actually show what characteristic energy a spacecraft needs to have in order to arrive on a certain date, given a departure date. Characteristic energy is a measure of the excess specific energy (energy per kg of the spacecraft) required to achieve escape velocity from a planet’s gravity well. It’s the square of the velocity with which the spacecraft is escaping Earth.

The reason engineers care about characteristic energy is because having too little of it means not being able to escape Earth, but having too much means the spacecraft will unnecessarily require more propellant to accelerate to the desired velocity — or worse still, miss its target completely.

Let’s humour Elon and look into an example of an Earth-Mars transfer (described in more detail here). The trajectory looks something like this:

Credits: https://marspedia.org/Earth-Mars_Transfer_Trajectory

When the spacecraft leaves Earth, it follows the black transfer trajectory. If its characteristic energy is too low, the black trajectory won’t be able to reach Mars’ orbit around the sun, and will miss it. If the characteristic energy is too high, the spacecraft will zip past Mars too quickly and also miss it. To plan accordingly, engineers create porkchop plots, which show how to get the transfer just right:

Credits: https://marspedia.org/Earth-Mars_Transfer_Trajectory

As I mentioned before, characteristic energy is the square of the spacecraft’s excess velocity, meaning we can also look at porkchop plots from a velocity perspective too (as in the case of the graph above).

Further, porkchop plots show you how the timing of the manoeuvre play a role. If Earth and Mars are not properly aligned, which is the case most of the time, it’s still possible to do the transfer, but it will just require an insane amount of energy to achieve. Conversely, you can minimise the energy (or velocity) needed by looking at the minima on the graph (shown in pink). In the above plot, that would be to launch around early May 2018 and arrive in mid-December 2018, hopefully in time for Christmas.

In summary:

  • Since rockets need fuel to accelerate to certain velocities, porkchop plots are also an indirect measure of how much fuel to bring along for the ride. The last thing you want is for your tank to be empty and just miss your target in a >$1B mission.

Porkchop plots also have applications beyond transfer trajectories, and can be used to minimise other manoeuvres, including rendezvous of two spacecraft together. They also make a great name for companies seeking to establish an interplanetary economy.

Edit: Spelling



porkchop is a Stockholm-based startup with the goal of establishing an interplanetary economy.

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porkchop is a Stockholm-based startup with the goal of establishing an interplanetary economy.