Why SpaceX Had to Scrub Its Wednesday Launch

May 27th was supposed to be a historic day. For the first time in nearly a decade, American astronauts were supposed to launch into space from an American rocket on American soil. But at practically the last minute, the launch was scrubbed and rescheduled for Saturday. And it all came down to bad weather that cleared up ten minutes after the launch window. Unfortunately, ten minutes later is still “too late” and here’s why.

A Launch Scrubbed Due to Weather

If you were watching the SpaceX launch yesterday, you probably came out disappointed. At just under 17 minutes to launch, NASA aborted the mission. Before launching a rocket, the crew keeps a close eye on the weather.

At the time, the weather violated three conditions for launch—natural lightning, field mills, and the attached anvil. Field mills are a measure of the electrical charge in the atmosphere, and attached anvils indicate icy cloud tops associated with thunderstorms.

Had SpaceX and NASA gone ahead with the launch, there would have been a real chance the rocket could trigger lighting mid-flight. That would have been catastrophic. But ten minutes after the launch window, the weather cleared enough to launch. Ten minutes may not seem like long, but in rocket science, that’s an eternity, and far too late.

Rocket Science is Complicated

There’s a good reason why the old saying, “he’s smart, but he’s no rocket scientist” exists. When you want to send a rocket into orbit, you have to determine where exactly it’s going, how much the rocket weighs, and how much thurst you need to lift that weight to its destination.

But wait, there’s more: you also need to account for moving targets. Because the Earth rotates meaning even if you tried to shoot at an empty spot in space, it’s effectively moving while you’re shooting. And if you’re aiming at an object that moves independently from the Earth’s rotation, like the International Space Station that orbits the Earth, that adds another factor to your calculations.

Now that you have all that math squared away, you need to figure out fuel. You’ll need enough fuel to lift your weight (say 3.5 million pounds) to orbit, winning the fight against gravity that you fail every time you take a step. But adding fuel to a rocket adds weight. So you’ll need to add some more fuel to account for the weight of your fuel, which adds more weight, which makes everything complicated.

The Football Canon Equation

NASA and SpaceX’s ultimate goal is to launch a rocket in the most efficient way possible to use the least fuel necessary. That means a process of getting from point A to point B with as few course corrections as possible. Every time the rocket corrects its course, that requires more fuel. For that reason, launches to the International Space Station (ISS) require an “instantaneous launch window.”

An “instantaneous launch window” means you must launch at “T-minus Zero,” you can’t go any earlier or any later.

Think of it this way: Imagine you’re playing a game at a local fair. You have a football canon, and in the game, a football player figure with a hole in the middle runs sideways across a field. Your goal is to fire the football through the hole. But you can’t turn your canon left, right, up, or down. You can only fire at one particular spot.

How do you win the game? You can’t fire the football when the player figure is in the spot the football will hit. That’s too late; the player will move on before the football arrives. So you have to fire early. But fire too early or too late, and the football will miss.

Now imagine the football player is 254 miles away and moving 17,150 miles per hour. That’s the ISS, Wednesday’s intended target. As NASA administrator Jim Bridenstine put it, delaying the flight even 90 minutes means “the International Space Station won’t be anywhere near where we need it to be.”

But what about just ten minutes for the weather on Wednesday? Well, the other problem is fuel.

Cold Fuel for Efficiency

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If you were watching the launch attempt on Wednesday, you witnessed an unusual procedure compared to past American rocket launches. In the past, astronauts boarded the capsule after teams loaded fuel to the rocket.

But on Wednesday, the astronauts boarded the SpaceX Dragon Crew capsule before fuel loading. SpaceX waits until about a half-hour before launch to load fuel. It’s a somewhat controversial choice because moving highly explosive propellants comes with some danger of exploding.

SpaceX uses an extremely cold propellant to fuel its rockets, specifically a liquid oxygen propellant kept at -340 degrees Fahrenheit. That incredibly low temperature increases the SpaceX rocket’s fuel efficiency. Better fuel efficiency makes the math easier and lowers the cost of launch.

That choice isn’t without its problems, though. A few years ago, a SpaceX rocket exploded during testing. After a thorough investigation, SpaceX determined a series of events caused the cold liquid fuel to turn into a solid, which in turn caused the explosion. The rocket company put in additional measures to prevent a repeat.

Why an instantaneous launch window? Why do we have to wait until Saturday to attempt to launch again? Here's why: pic.twitter.com/ejHrgnCFsu

— NASA (@NASA) May 27, 2020

This leaves SpaceX with a different problem it can’t solve—as NASA explained during the launch broadcast, the fuel needs to stay cold. As the liquid fuel warms up, it becomes less efficient. If it gets too warm, it won’t have the performance to lift the rocket to orbit. So the rocket much launch within a half-hour of loading the fuel or not at all.

Ultimately that means the launch couldn’t wait ten minutes. Every minute means warmer fuel and less performance, and the calculations involved leave the slimmest of margins for fuel requirements. So instead, SpaceX will try the launch again on Saturday, March 30th, at 3:33 PM Eastern Time. Hopefully this time the weather cooperates.