Rendezvous Part 3 – Getting to Orbit

In my book Shuttle, Houston: My Life in the Center Seat of Mission Control, I bring readers deep into the middle of how Mission Control worked during the Space Shuttle era. I give readers a glimpse of both the technology and the humanity required to put humans in space on a routine basis. There are many parts to this thirty-year story, and only a small portion of the tales fit into the book – so here is an excerpt from an unused chapter on the process of rendezvousing the shuttle with other objects in orbit. 

Now, of course, no matter how well you model the thrust, gravity, drag – and all the rest of the variables – you are never going to get performance exactly as you predict. There will always be some things you just can’t account for. Heck, at the moment the main engines cut off, you are accelerating at 3 G’s. That’s 100 foot per second of velocity change every second. The half a million pounds of thrust from each main engine doesn’t stop instantaneously – it takes a few seconds to tail off, and all the while, the acceleration is dropping. You have to know how the engine thrust drops off in order to predict how the acceleration will decrease – all so that you know when to start the engine shutdown so that you reach your exact target velocity. It is a tricky bunch of calculations, for sure.  Given that an error of just one foot per second means that your altitude halfway around the globe is going to be off by half a mile, you need to be as close as you can – but there ae still going to be errors – and that is where the art of rendezvous really begins – at MECO (Main Engine Cut-off). Determining the starting point for the rendezvous is all about figuring out exactly what orbit you ended up in at MECO.

Let’s assume that you made it into the same orbital plane as your target. That’s mostly ground trajectory calculations and getting off the ground on time. The next thing you have to understand is that you are always going to be chasing your target. Basic orbital mechanics says that if you are lower than your target, you will be catching up to it. Higher, and you are losing ground. In order to go higher, you have to ADD velocity by doing an engine burn that accelerates you. But the closer you get to your target’s altitude, the slower you catch up with it. Remember that doing burns takes fuel – and you only have a limited amount.

Imagine, if you will, that you are driving a car, heading to an intersection with a stop sign. You slow down gradually, getting a feel with light braking for how much more braking you will need to get stopped at the line. You do this automatically, of course, having learned how to stop on a line the very first time you drove a car (I hope). You might start with a big deceleration, and then slowly let off to come to a smooth stop. If you are aggressive, you might start slowing and then slam on the brakes at the end. It’s not very elegant, but some do. Well, closing on a target in a rendezvous is similar – you slow your closure rate by raising your orbit. You do this by adding velocity a little at a time. Remember that if you overburn, you could overshoot, and that means you’ll have to burn backwards (retrograde), and that is wasteful. And you don’t have fuel to waste! You need a certain amount to come home!

So how much velocity change are we talking? Well, in low earth orbit, where the shuttle lived, we used the rough rule of thumb that one foot per second of velocity change (Dv) would raise or lower your orbit by 1/2 nautical miles half way around the planet. If you start in a circular orbit, doing a one foot per second posigrade burn, will raise your orbit ½ mile on the other side of the planet – but you’ll come right back down to where you were when you get back to your burn point. You have turned your circular orbit into an ellipse. If you want to raise the orbit by half a mile all the way around, you have to do TWO burns – the first one foot per second, and a second when you get to the high point (apogee) half way around. The second burn is also one foot per second. There – you’ve raised your orbit by half a mile.

That rule of thumb works pretty well no matter what you want to do. To bring a space shuttle home, you have to drop the perigee (low point) of the orbit to about 80 nautical miles in order to get captured by the atmosphere. So, let’s say that you start at 200 miles (circular). You need to lose 120 nautical miles at the far side of the planet to be at 80 miles, where you’ll be captured and start your entry. One-hundred twenty nautical miles takes 240 fps – so that is the size of your deorbit burn. Just burn retrograde so that you slow down (and lose altitude) rather than burn posigrade, going higher, and stranding yourself without enough fuel to come back down! Most Shuttle missions got to orbit with about XXX fps of Dv capability to be used for rendezvous, maneuvering, and deorbit burns. It was the trench’s job to figure out how to use it. The PROP officer owned the engines and the fuel, of course, but FDO knew where the Shuttle needed to go.


We’ll bring you the next portion of “Rendezvous” next week right here! If you enjoyed this look inside the Shuttle program, you can find many more details and stories in the book – look for it wherever you buy your paper books, or add it to your favorite E-reader or audio book account.

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Christmas is coming!