Intel's Ronler Acres Plant

Silicon Forest
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Saturday, July 28, 2018

Big Fancy Rocket

Making Life Multiplanetary

I do wish Elon had given the BFR a different name. I use the F-word frequently, but I try to refrain from using it in public, and my wife gives me grief if I use it, or any other blue words in her presence. Expletives have their place, but if every other word out your mouth is a curse, you're not doing it right.

BFR v Airbus 380 size comparison

The BFR is a big, new rocket project at SpaceX. It's going to be about the same size as an Airbus 380. It sounds like they intend to make it reusable, like their Falcon rockets, that is the boosters will return to their launch pad and safely land under their own power. This is quite a trick involving navigation, control and fuel management.

Falcon Heavy & Starman
The landing occurs near the end at 1:33.

There a couple of concepts at play here. One is that if it takes 90% of your initial mass to propel your rocket to top speed, it will take 90% of the remaining mass to decelerate it to zero. But since you only have 10% of your mass left after reaching your target speed, you only need 9% of your original mass to decelerate.

Since we are talking about boosters, we are disregarding the payload and a bunch of other stuff. The point is that after the booster has done its work by getting the payload on its way, it has used most of its fuel, and after dropping the payload, it is much like an empty beer can, it has relatively little mass and so is going to need very little fuel to return to the launch pad.

However, it is going to need some fuel, because it has a bunch of contortions to go through. First, when it drops off the payload it is going at near orbital speeds, on the order of miles per second, so it has to slow down. It has to slow down to zero before it can begin flying back to base. As its speed drops, gravity takes a bigger role. When its speed is zero it is like a big rock (or a big empty beer can) falling out of the sky. Now it needs to accelerate back towards the launch pad, which might be 100 miles away, but don't go too fast, because halfway back it will need to turn around and start decelerating for the landing. Speed up, turn around, slow down, speed up, turn around, slow down, and do this all while maintaining a heading that will keep you off of the ground and slow enough that you don't burn up in the atmosphere.

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