Space, Spaceflight

Why was the 1st Demo-2 launch scrubbed?

Wednesday (May 27th) was meant to mark a momentous occasion in the history of spaceflight, a stepping stone to a more sustainable, accessible future: the first crewed launch of a commercial vehicle, and the first orbital launch from the US in 9 years!

We should have seen Doug Hurley and Bob Behnken launch to the space station in SpaceX’s Crew Dragon aboard a Falcon 9, but unfortunately the Florida weather had other ideas! The weather wasn’t looking good as we got closer to T=0, but patches of good weather arose here and there, so they held on as long as was safe, until finally at T-16min54sec, the ‘launch scrubbed’ announcement was heard over the comment channels 😦

A scrubbed launch isn’t the end of the world, as NASA always has back up windows (for this demo 2 launch, the next launch attempt is set for tomorrow at 8:22pm BST/3:22pm ET), and according to NASA administrator Jim Bridenstine in today’s press conference, “a good scrub is better than a bad launch any day”. Although loss of crew would still be unlikely during a bad launch, it uses a lot more resources than simply loading and off loading.

So what kept it from launching?

Everything – the rocket, the capsule, communication and ground systems- were ‘go for launch’, but the weather had been incredibly variable that day, and it just happened that the launch ‘window’ fell while dark storm clouds loomed above. I say ‘window’ because it’s actually an instantaneous window, with the rocket systems programmed down to the second. Why? 

Well there are 2 main reasons:

  1. The Rocket: the fuelling process starts at T-35 minutes, and involves super-cooled rocket grade kerosene (RP1) and liquid oxygen being pumped in. Loading it at such a low temperature, means it’s denser, so more can be packed in, and it’s expansion when heated adds an extra bit of thrust! However, as soon as it enters the rocket, it starts to warm up and boil off, reducing the amount of fuel. This could mean that the rocket may not reach orbit, or that the reusable booster might not make it safely back to Earth.
  2. The Space Station: travelling at 17,500 mph, the ISS orbits ever 90 minutes… but because the Earth is also spinning, it doesn’t actually return to the same spot above ground, so you can’t just delay an hour or so like when launching satellites

So even though the weather would have cleared up enough for launch just 10 minutes later, we had to wait. If you’ve ever been to Florida (or live in the UK which is just as variable but less extreme), you’ll understand- in the afternoon’s especially, you can go from 35 degree perfect sun to torrential rain and thunder in minutes! It was a gutting moment at T-16 minutes and 54 seconds, but as Administrator Bridenstine reminded us, the priority is always the safety of the astronauts, and any cost of a scrub is minimal when compared to the investment made to reach this point, and is insignificant compared to the value of the astronaut’s safety.

Other than the weather, human factors all have to be considered too: sleep cycles and the stress of a scrub. Sleep is super important to maintain focus, so the crew needs to be well rested, and awake, for crucial parts of the mission, like booster and capsule separation (and obviously rendezvous with the station), just in case something goes wrong. And even though the astronauts and mission control will have prepared to the point that it’s pretty much second nature, launches and repeated scrubs are tiring and stressful, so back to back launches aren’t the best for their physical and mental health. 

All this can be prepared for and controlled, except the weather, and with this new launch system comes extra regulation on the weather, because not only does it have to be in ‘green’ conditions at the launch site and trajectory, but also any possible in-case-of-abort landing zones. The weather conditions required for the launch site are detailed in the image below, but the main points are: 

  • Wind speed (at ground and high altitude), which could knock the Falcon off course, or prevent the boosters from returning safely.
  • Storms and natural lightning 
  • Thick clouds reaching up to freezing temperatures, where ice could form on the rocket, making it heavier or warping the heat shield. 
  • Rain, mainly for the same reason as it could freeze onto the rocket.

Wind direction also matters, especially at the launch site: a west to east wind can give you an extra boost to get you to space, but an east to west wind runs the risk of landing on land in a populated area in case of an early in flight or pad abort.

Do not launch if the sustained wind at the 162-foot level of the launch pad exceeds 30 knots. Do not launch through upper-level conditions containing wind shear that could lead to control problems for the launch vehicle. Do not launch for 30 minutes after lightning is observed within 10 nautical miles of the launch pad or the flight path, unless specified conditions can be met. Do not launch within 10 nautical miles of an attached thunderstorm anvil cloud, unless temperature and time-associated distance criteria can be met. Do not launch within 10 nautical miles of a detached thunderstorm anvil cloud. Do not launch within 3 nautical miles of a thunderstorm debris cloud, unless specific time- associated distance criteria can be met. Do not launch within 5 nautical miles of disturbed weather clouds that extend into freezing temperatures and contain moderate or greater precipitation, unless specific time-associated distance criteria can be met. Do not launch for 15 minutes if field mill instrument readings within five nautical miles of the launch pad exceed +/- 1,500 volts per meter, or +/- 1,000 volts per meter if specified criteria can be met. Do not launch through a cloud layer greater than 4,500 feet thick that extends into freezing temperatures, unless other specific criteria can be met. Do not launch within 10 nautical miles of cumulus clouds with tops that extend into freezing temperatures, unless specific height- associated distance criteria can be met. Do not launch within 10 nautical miles of the edge of a thunderstorm that is producing lightning within 30 minutes after the last lightning is observed. Do not launch through cumulus clouds formed as the result of or directly attached to a smoke plume, unless time-associated criteria can be met.

On Wednesday, storm clouds crowded the area, with both cumulonimbus (storm clouds) and anvil clouds (the tallest storm clouds). This meant that precipitation levels were too high, and there was actually a tornado warning in place! Thunderstorms are also accompanied by lightning, and although none was observed close to the launchpad, too much electricity was in the atmosphere (high voltage was measured by weather balloons sent up in succession in the hours and minutes prior to launch). Launching in high voltage conditions is incredibly dangerous, as the rocket ‘s exhaust plume act as conductors, and can trigger lightning even when there would be none otherwise. This lightning flows through the rocket and could fry the electronics or worse. 

For the areas marked as potential abort splashdowns, wind speed and wave characteristics are also checked. 

Scrubs may be annoying, but they are entirely necessary, and a completely normal part of spaceflight- Doug Hurley’s first launch on the shuttle was scrubbed 5 times before it launched! 

For now, Doug and Bob have an extra day to be with their families (who have also been in quarantine, to prevent the possibility of COVID19), relax, enjoy the last few moments of gravity/terra firma, and launch model rockets on the beach! (One of the many new traditions part of this new era of crewed spaceflight.)

Finally, I want to remind everyone that this mission is crewed not manned, and that since being made aware of this fact I have changed my previous blog posts– language matters, and needs to be inclusive to reflect that space is for everyone, with no limitations!

Fingers crossed for good weather tomorrow, and make sure to check out my Instagram stories (I have a highlight called Launch America if you want to check out previous content), and twitter for live updates, including when/if you will be able to see the crew dragon capsule fly overhead! Ad Astra!


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