Rocket Science for Danny

Michael M. Butler (
Sat, 08 Aug 1998 16:25:27 -0700

>At 14:29 8/8/98 EDT, you wrote:

>>In a message dated 98-08-08 05:59:00 EDT, you write:

>>Im sorry I dont understand what you're saying. If you go 6 miles above sea
>>level, and drop a ball, its not going to burn up on its way down. Why not
>>slow the ballistic object down before entering the atmosphere? Thats what I
>>asked initially.

If you read the paragraphs that follow the one you quoted, you will see my response to that question. I'll try to explain the same facts a different way.

To do what you suggest necessitates treating reentry to Earth surface as if Earth were an airless planet. This means that every gram of spacecraft has to have all its orbital velocity removed by a rocket engine and propellant carried up to orbit.


Well, heck, how hard can that be?

Here's a somewhat simplified example to help guide your intuition:

Let's say you wanted to use a rocket engine to stop a car moving 5 mph (miles per HOUR). Suppose your computations yielded a result that suggested you only had to burn _one pound_ of propellant to accomplish this. Great.

How much propellant would it take to stop the same car going _5 miles a SECOND_?

Based only on kinetic energy being proportional to velocity squared, it would take (3600)^2 times as much propellant. That works out to 12,960,000 pounds of propellant used in a rocket with the same specific impulse as the first example.

Now how much did it cost you to orbit those thirteen million pounds of propellant you are using to slow the car back down?

Oh, only $10,000 a pound.
Or 129.6 billion dollars.
For each car you want to deorbit.

So you see it costs a lot to do that. Too much, even if my back of the envelope figures are wrong by a factor of a thousand, and I doubt they are.

Alternate scenarios:
1) Come up with much more efficient propulsion, or don't use regular rockets, or both, and the numbers change. Nuclear propulsion, Rotons, beanstalks, laser launch... they're all considered far out today, so they're hard to fund.

2) Come up with a low enough ballistic coefficient, and reentry can be induced so gradually that heating is minimized. Like a biiig dandelion seed. But you'd have to manufacture such structures in space, as we have no practical way of orbiting them today.


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