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Back to Basics - Recovery Systems Print
Written by Joe Pfeiffer   
Sunday, 11 July 2010 11:00

What goes up must come down (assuming a closed universe) - what distinguishes model rockets from missiles or fireworks is that we have the goal of bringing our rockets down without damage. The part of the rocket devoted to accomplishing this is called the" recovery system."

In most rockets, the recovery system isn't anything more than what I just said - and it's made to be as simple and reliable as possible. In others, making an elaborate system is the whole point of the rocket. In this installment, I'll be describing several of the different recovery systems in use today.

Common Features

The one thing that all of these systems have in common is the motor ejection charge. Last time, I discussed the parts in a model rocket motor, including the delay and ejection charges.

Remember that the delay charge acts like a timer, waiting for a few seconds between the end of the propellant charge and the firing of the ejection charge; this gives the rocket time to reach the top of its flight before the recovery system is activated.

The ejection charge fires after the delay. Most of the force from the ejection charge is directed up the rocket body instead of out the nozzle. The recovery system is activated either by the pressure from the ejection charge, or (sometimes) by the heat of the charge.

Recovery Systems based on Drag

The simpler forms of recovery systems use drag to slow the rocket down. How much drag is needed, and so the type of recovery system, depends on how large the rocket is. There are three basic drag-type recovery systems: tumble recovery, streamer recovery, and parachute recovery.

The simplest type of recovery system is tumble recovery. In a tumble recovery rocket, the motor is free to move back a ways in the body tube. When the ejection charge fires, the motor moves back, and the rocket becomes unstable (it's kind of like what would happen to an arrow if the arrowhead were in back instead of in front). Instead of flying straight down into the ground, the rocket tumbles instead, so it lands much more slowly. As you can imagine, this is only useable for a very light rocket - for most rockets, even though it would be coming down much more slowly than if it weren't tumbling, it would still hit hard enough to destroy the rocket.

The second simplest recovery system is streamer recovery. With this system, the ejection charge blows the nose cone off of the front of the rocket (the nose cone is attached to the body of the rocket with a long piece of elastic called a shock cord). There's a streamer attached to the nose cone, which unfurls and makes the rocket come down slowly. This system is appropriate for a model that's heavier than a tumble recovery rocket, but still pretty light.

A parachute recovery system is only a little bit more complicated than a streamer. It works just the same as a streamer system, only except for ejecting a streamer it ejects a parachute. Parachute recovery is suitable for an arbitrarily heavy rocket; it was used for all of the manned spacecraft until the space shuttle, and is still used for the shuttle's Solid Rocket Boosters.

The figure shows the basic mechanism of a streamer or parachute system. The ejection charge ejects the nose cone, allowing the streamer or parachute to come out of the body tube.

One other thing to mention here is the recovery wadding. Parachutes and streamers are not normally fireproof, so the heat of the ejection charge would destroy them. Because of this, fireproof paper called “recovery wadding" is put in the body tube below the streamer or parachute.

Illustration of ejection system

Lift-based systems

All three of the systems I just talked about are based on drag: drag acts to slow things down, so increasing the drag slows them down more. The next three systems are all based on lift: using the motion of the rocket through the air to provide lift, so it won't come down as quickly. There are three basic lift-based systems: rocket gliders, boost gliders, and helicopters.

A boost glider is a glider which is connected to a rocket. The rocket (called a power pod) is used to launch the glider; the ejection charge separates the two. The power pod comes down using one of the drag-based recovery systems, while the glider glides down. Sometimes it's a real challenge to keep the power pod's recovery system from getting tangled in the glider! The space shuttle is actually a boost glider: the orbiter glides down, the two Solid Rocket Boosters parachute down, and the main fuel tank isn't recovered.

A bigger challenge than a boost glider is a rocket glider: it goes up like a rocket, but the whole thing glides down. To do this, it has to change from behaving like a rocket (in its" boost configuration") to behaving like a glider (in its " glide configuration"). There are several ways to do this; the motor can slide back (but not enough to make a tumble recovery out of it!), or the wings can slide forward, or the angles of the wings can be changed. This is probably the hardest kind of recovery system to get to work.

Helicopter recovery is very similar to a rocket glider. With this scheme, the ejection charge makes helicopter blades unfold from the rocket which provide the lift to bring it down slowly.

In More Detail: Shock Cord and Parachute Sizing

There are some rules of thumb that can be applied to determine the length of the shock cord, and the size you should use for a parachute or a streamer.

First, the length of the shock cord. The shock cord should be at least three times the length of the body tube. One way to see if your shock cord was long enough after a flight is to examine the body tube: if the front has been folded in, it probably got hit by the nose cone because the shock cord was too short. Estes kits are famous for being supplied with shock cords that are too short, and some people refer to damage at the front as the "Estes dent."

A good rule of thumb for parachute diameter is that the diameter (in meters) should be the square root of the mass (in kilograms). A rule of thumb I like even better might be called the "Tim Taylor More Power Rule" - the parachute should be big enough that it's a little bit of a challenge to pack the chute. The idea here is that (as long as it doesn't get stuck in the body tube) no rocket ever landed too hard because its chute was too big! —Joe Pfeiffer

 

Last Updated on Sunday, 11 July 2010 11:14
 

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