Rocketry Frequently Asked Questions

How can one read the Estes date of manufacture on their motors?

What is model rocketry? What is high power rocketry?

Model rocketry is a hobby where people build real flying rockets out of lightweight materials such as cardboard, plastic, and balsa wood. They weigh 1.5 kg (3.3 pounds) or less and they use pre-manufactured solid propellant rocket motors which use black powder or composites as propellant. Model rocketry is a very safe hobby as long as you fly by the NAR (or CAR if you live in Canada) safety code. Model rockets use motors which generate less than 320 N-s (Newton seconds) total impulse. and no single motor can exceed 160N-s total impulse meaning that you would have to use two of these motors to reach the 320 N-s limit.

High power rocketry is model rocketry on a much larger scale. High power rockets are made of stronger materials and they use motors ranging in power from 320 N-s (H class) to 40 960 N-s (O class). Also, any rocket that uses a combination of lower power motors that meets or exceeds 320 N-s is also considered to be a high power rocket.

What are all those weird letters and numbers on my rocket motor?

All rocket motors have a code stamped on the side of it which provides the user with important information. The code consists of a letter, a number, a dash, another number, and in some cases another letter. For example: G40-7W

This first letter indicates the total impulse or power of the motor. Each letter has twice the power of the previous letter. In other words, a full power G motor is twice the power of a full power F motor and so on. The power ranges of each motor class are listed in the table in the Glossary.

This number indicates the average thrust of the motor. The higher the number, the higher the thrust. Some people (including a hobby shop employee I talked to once) believe that this number is the burn time of the motor. This is NOT the case. For any given motor class such as the G motor in this example, a lower number will indicate a longer burn time than an equivalent motor with a higher number. All things being equal, a G80 will deliver twice as much thrust as a G40, but will burn for only half the time. Both motors have the same total impulse however.

This number indicates the delay in seconds between propellant burnout and the ejection charge that deploys the recovery system. This delay allows the rocket to coast after the propellant burns out so that it can reach its maximum altitude before activating the recovery device.

Some manufacturers, Aerotech in particular add a letter at the end of the code to indicate what kind of propellant the motor is using. The letter codes are listed below:

W	White Lightning	This propellant gives off a bright white flame and thick grey smoke much like the propellant in the space shuttle's solid rocket boosters. If I remember correctly, they are the same propellants. There is a variant of this propellant called "Fast White Lightning" that produces the same visual effect as regular White Lightning propellant, but it burns quickly like Blue Thunder propellant.
T	Blue Thunder	This propellant gives off a bright blue flame and little smoke. Typically, this motor has a short burn time and it produces very high thrust.
J	Blackjack	This propellant spews out thick black smoke... no trouble tracking this rocket! Typically, this propellant has a long burn time and low thrust.

Some people say "rocket engines" and some people say "rocket motors"... What's the diff?

A motor is a device that produces motion. An engine is a device that produces motion through the use of a combination of moving parts. An engine is a motor, but a motor is not necessarily an engine. Solid propellant rocket motors of the type used in model & high power rocketry have no moving parts and are therefore motors.
But then how come Estes stamps "Model Rocket Engine" on their motors?
Well "engine" sounds more sophisticated and powerful than "motor", doesn't it? On the newer Estes motors, they're still stamped as "Model Rocket Engines", but they're also stamped as "Model Rocket Motors" in really small letters since the California State Fire Marshall requires them to.

What's a CATO? Why does this happen?

CATO stands for Catastrophe At Take Off and it refers to a general failure of the motor which results in an explosion. This is caused by the inability of the motor's casing to contain the high pressures of the hot gases which either forces the casing to rupture, to blow out the nozzle, or to blow out the front end of the motor, whichever is the weakest point. CATO's can be caused by some of the following...

A crack in the propellant grain can allow more propellant to burn at once than the casing was designed to handle since more propellant surface area is exposed. This is common in black powder motors that have been dropped.

A small chunk of propellant can fall into the nozzle and block it which can cause a pressure buildup and an explosion.

There may be a defect in the casing, nozzle, propellant, binding of the propellant to the casing, or any of a combination of these.

I'm not sure if my homemade rocket will fly straight. How do I test its stability? What's the swing test?

There are a number of ways to test the stability of a rocket. The easiest way is to use the swing test. To perform the swing test, do the following:

1) Insert the most powerful motor that you expect to be using in this rocket since it's likely to be the heaviest. This will test the rocket's stability with the maximum possible weight at the rear of the rocket. If it's stable with a heavy motor, it'll be stable with any of the lighter ones (provided of course that the lighter motors have enough thrust to move the rocket fast enough to keep it stable).

3) Hold the rocket by the string and move it along the airfame until the rocket balances horizontally. This is your rocket's center of gravity (balance point).

4) In an open area, swing the rocket around you from the end of the string. If it flies straight, your rocket will be stable in normal flight. If it flies in a wavy pattern or if it loops all over the place, you need to do one or more of the following:

       a)    Add more weight to the front of the rocket.
       b)    Increase the size of the fins at the rear of your rocket.
       c)    If you have any fins that are far forward of the center of gravity, consider moving them farther back along the airframe or removing them altogether.

In some special cases, especially with long rockets, your rocket may loop around because it is overstable and it won't fly in a circle since it's trying to go straight. If you think that this may be the case, try lengthening the string or reducing the size of the fins at the rear, or lightening the front of the rocket.

Model rocket motors are the powerplant of the rocket which allow them to fly. They are basically very thick paper tubes which contain from back to front, a clay or ceramic nozzle, the rocket propellant, a delay charge, an ejection charge, and a clay or paper cap to keep it all inside the tube.

Propellant
Propellant is what actually causes the forward motion of the rocket. By electrically igniting it, the propellant burns and produces large volumes of very hot gas which escapes though the nozzle at the back at high speed. The propellant in model rocket motors is usually black powder, but it can also be a composite fuel. High power rocket motors use composite fuel almost exclusively to power the heavier rockets since they offer more thrust per amount of fuel although they often require higher temperatures to ignite and sometimes require special igniters to light them. NASA's Space Shuttle uses a four to one ratio of ammonium perchlorate oxidizer to aluminum powder fuel. Some of the composite motors use a nearly identical formulation. Once the fuel is finished burning, the delay charge begins.

Delay Charge
The delay charge is a combustible material which blows a lot of smoke out the nozzle to aid in tracking the rocket and also to let the rocket coast to it's maximum altitude before the ejection charge ignites. The delay charge produces no measurable thrust.

Ejection Charge
The ejection charge produces a short burst of hot gas to blow the recovery system out the front of the rocket. In rare cases, it's used to blow the motor casing out of the rocket or to move the motor casing to a different position in the rocket to change its center of gravity to either allow it to glide back to Earth or to make it aerodynamically unstable and allow it to tumble down.

Most model rocket motors use some form of black powder as their propellant. Propellant by the way, is the combination of fuel and oxidizer. Fuel is the material that burns and oxidizer is the material that supports the fuel's combustion. Most motors larger than "D" use what are called composite propellants. They are between 2 to 3 times more powerful than black powder motors with the same size casing. There are many fuels that are used by different companies, but most burn these fuels with ammonium perchlorate oxidizer. The space shuttle's solid rocket boosters and Aerotech's White Lightning motors burn powdered aluminum with ammonium perchlorate.

How can I read the manufacturing date code on Estes and Centuri motors?

On older Estes & Centuri motors, the date of manufacture is indicated by a number indicating the day, a letter code indicating the year, and another number indicating the month. The letter codes correspond to the years shown in the table below. From 2000 onwards, the practice of using a letter code to represent the year was abandoned.

Multi stage rockets are rockets which allow parts of the rocket to fall away once the propellant in those parts have burned out. There are two kinds of multi staging: series staging & parallel staging.

Series staged rockets contain motors that are stacked one on top of the other inside the rocket which are usually successively smaller the farther up the rocket they are. The motors in the first stage are ignited at the launch pad and once the fuel burns out, that section of the rocket falls away to reveal another motor for what's left of the rocket which then ignites and then continues upwards.

In the case of most model rockets, booster stage motors are used in all the stages except for the last (highest) one. Booster stage motors have no delay charge or ejection charge and carry designations such as B6-0, C6-0, etc. The zero at the end of the motor designation is the delay in seconds which in this case is zero since there's no delay charge. As the motor burns and there is only a tiny bit of fuel left at the end, the high pressure in the motors bursts through this bit of burning fuel and it flies upwards straight into the nozzle of the next motor just above it and ignites it. The lower stage falls away as the next stage ignites.

Model rockets usually have only two stages (booster & upper stage), but some have three and in some rare cases, some have more than three. This is because adding extra motors to the bottom of the rocket shifts the center of gravity (CG) far back which necessitates successively larger fins on each lower booster to keep the center of pressure (CP) behind the CG in order to keep the rocket aerodynamically stable. Also, larger fins in turn increases the rear weight more and also causes more aerodynamic drag. As you add more stages, sooner or later there is so much weight at the rear that some of the upper stage fins will be ahead of the CG which will actually move the CP forward. As you can see, there are limits to this technique.

Parallel staging occurs when the boosters fall from the sides of the rocket instead of falling off the bottom. This technique is used when a lot of power is needed at liftoff to lift heavy payloads. When the boosters finish burning, they fall from the sides of the rocket while the center part or "sustainer" continues on its way.

Parallel staging doesn't occur too frequently in model rocketry since most people aren't really sure how to go about having the boosters fall from the sustainer at the right time. The technique I plan to use hasn't been tested yet and once it's been tested and proven safe, I'll put it on this web site.

E	1974	M	1982	U	1990	C	1998
F	1975	N	1983	V	1991	D	1999
G	1976	O	1984	W	1992
H	1977	P	1985	X	1993
I	1978	Q	1986	Y	1994
J	1979	R	1987	Z	1995
K	1980	S	1988	A	1996
L	1981	T	1989	B	1997