Recovery systems - Discussion


A recovery system needs to make sure that the rocket has slowed down to the terminal velocity of the recovery system before it hits the ground. If the rocket is to stay in the air for the longest time possible, the chute needs to be fully deployed as high as possible.

There are several ways of determining when this will be (by no means an exhaustive list):

  1. Use an altimeter to monitor air pressure - triggering release when the pressure is lowest;
  2. Use a timer to measure the time elapsed from launch so as to trigger release after, say, four seconds (timers can be a clockwork device such as a Tomy timer, a balloon release, a damped plunger or anything else);
  3. Use a flap that will spring our when the air pressure due to speed falls at apogee thus triggering the release of the parachute; and,
  4. Have a nose that will separate on its own at apogee due to a drop in the drag force holding the nose onto the rocket and also the action of the rocket turning at the top of its flight.

1. Altimeter.

The Problem with monitoring absolute air pressure inside anything that is moving (such as when trying to measure height from the inside of a water rocket) is that the air moving past the sides of the rocket will reduce the pressure inside any chamber that is vented to the outside.

Rocket Speed
Height Equivalent
for Pressure
50 75
75 160
100 250
125 350

When stationary, air on both sides of a hole allow gas to diffuse through equally in each direction. However, when the body is moving, a hole in the side will allow gas from the inside to diffuse to the outside easier than the other way around therefore, the absolute pressure inside the nose cone of a rocket depends upon the velocity as well as the altitude.

If a rocket goes quite fast, the lowest air pressure measured may be at the end of the thrust phase where the speed is highest.

Many rockets travel between 75 and 100 mph (they have been clocked at over 100 mph) and so at this speed (possibly only a few tens of feet above the ground) the altimeter is going to think that it is a lot higher - at 75mph, just 10 feet of the ground, it is going to think that it is at 160 feet. Depending upon which strategy is chosen, either: as it slows down due to drag, the pressure increases and it thinks that it is on its way down; or, as the pressure reaches a set level and it thinks that it is at the right hight, it will release early. A compensation has to be made.

2 Timer

Timers are an excellent method of deploying the recovery system at a specific time. They can be a clockwork device such as those found in toys; a balloon type release (link on the recovery systems index page); a damped plunger that is actuated by the inertia of launch or anything else.

Their main disadvantage is that they are oblivious to just when is the best time. If you know that you want to deploy a parachute at four seconds, the timer can be made to do this and reliably so. Unfortunately, if something has gone wrong, the rocket may have already hit the ground by four seconds into the flight.

3 Sprung Flap

A sprung flap that is mounted on the side of the rocket is also a good method for detecting the lower air speed at apogee. Prelaunch, the flap stands out a little but its weight is enough to hold the nose cone in place. During the thrust phase, the flap is pushed flat against the side by the combined effects of acelleration and air pressure due to speed. After thrust phase, during free flight, the flap is held against the side of the rocket by the pressure caused by the speed of the wind which gradually reduces in magnitude until, at apogee, it is so low that the flap springs open and causes the release of the parachute. The main disadvantage of this method is when the rocket does not go sufficiently straight up for the wind speed to slow at apogee - the horizontal flight at apogee stops the flap from working.

4 NSA (Nose Separates at Apogee)

With the nose separates at apogee method, there are no complicated mechanisms to go wrong (just a simple one to go wrong). Prelaunch, the nose cone simply rests on the base of the unit which is fastened to the top of the rocket. During thrust phase, the acelleration of the rocket holds the nose cone in place. During free flight, the pressure on the front of the nose cone holds it in place. At apogee, if the rocket was one piece, it would rotate until it faced the Earth as it fell. As this is in two pieces, the sideways force on the nose as it rotates is enough to knock it off its base unit (this bit is very reliable and I have never had any problems with this part of the recovery).

With all of the methods, the parachute deploys on the way down (hopefully near the apogee) and the rocket lands safely on the grass.

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