How rockets don’t work

WoodyI was only 10 years old but already had Space Fever thanks to Chesley Bonestell’s artwork in Collier’s and Life magazines.  I eagerly joined the the movie theater ticket line to see George Pal’s Destination Moon.  I loved the Woody Woodpecker cartoon (it’s 12 minutes into the YouTube video) that explained rockets to a public just getting used to jet planes.  But the explanation’s wrong.

Go ahead, follow the link and watch the cartoon.  I’ll wait here.

Pretty far-sighted for 1950, eh?  And it’s amazing how much they got right, including how the driving force for the Space Race was international politics.  But oh, the physics…

Yeah, they tacitly acknowledged Newton’s Third Law: For every action there is an equal and opposite reaction.  The cartoon implies that the action is the pellets coming out of the barrel and the reaction is Woody getting knocked back.  But that can’t be right: if it were true you wouldn’t get any kick when you fire a blank cartridge — but you do.  Let’s take a close look at just what actions are in play.

Maybe it’s the pellets plus the gases behind it pushing forward and the gun pushing backward?  Sort of, but where do the gases come from?  Right, the exploding charge next to your cheek in the receiver.  Those gases move equally in all directions.  Some of them push pellets down the barrel.  Some of them push on the back end of the receiver which pushes the gun stock which mashes your shoulder.  But there’s bunches of molecules that uselessly collide with the receiver’s walls.

Action and reaction balance out just fine but only when you consider the gases moving outward from the center of the BANG.  For instance, if left and right didn’t balance perfectly the piece would crash into your ear or swing around and flatten your nose or the back of your head.

Both shotguns and conventional rockets get their propulsive energy from chemical combustion.  The reason gun parts have to be strong is all those hot molecules dashing in every direction other than down and up the barrel.  A chemical rocket casing has to be strong for the same reason.

Chemical combustion is just not an efficient use of propellant mass.  Just look at this NASA image of a SpaceX Falcon 9 during a DSCOVR launch — huge side-flare from molecules that make no contribution to forward thrust:DSCOVR launch
Wouldn’t it be nice if we had a way to put all our propulsion energy into moving the vehicle forward?

There’s good news and not-so-good news.  People are working on a few other options, all of which depend on forces we know how to steer: electric and magnetic.  Unfortunately, each of them has drawbacks.

Unlike rockets, ion thrusters use an electric or magnetic field to accelerate ions (duh!) away from the vehicle.  It’s a much more efficient process because there’s little off-axis action/reaction — all the propellant heads out the nozzle (action) and all the push-back force (reaction) acts directly on the vehicle.

But… ions resist being crowded together so you can’t blast huge quantities out the nozzle like you’d need to for a launch from Earth.  Up in space, though, ion thrusters are perfect for satellite attitude adjustment and similar low-power tasks.  The Dawn mission to Vesta and Ceres used an ion thruster to boost the spacecraft continuously from Earth to target.  It’d be impractical to build a chemical-powered system to do that.

Rather than send out atoms one by one, a rail-gun drive could use high-power magnetic fields to accelerate lumps of iron down a track and away.  Iron goes one way, vessel goes the other.  Might work in the Asteroid Belt where lumps of iron are there by the billions, but on the other hand I’d rather not be a Belter tooling along in my mining tug only to be hit amidships by someone’s cast-off reaction mass.

And then there’s the Q-thruster and EmDrive.  I hope to eventually include enough physics background in this blog that we can discuss the controversies and prospects for new-physics drives based on space warps and such.  You can check out Dr Harold White’s video for some of that.  It’d be sooo cool if they work.

~~ Rich Olcott

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