Recovery systems -
Discussion
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):
- Use an altimeter to monitor air pressure
- triggering release when the pressure is
lowest;
- 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);
- 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,
- 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
/mph |
Height
Equivalent
for Pressure
/feet |
| 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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