Rocket Motors
There are three types of motor used in rocketry. The first, black powder or BP, uses ordinary gunpowder like the stuff used in fireworks. The second type is an ammonium perchlorate-based mixture, generally known as an AP motor. The third type is a hybrid motor, normally using nitrous oxide as the oxidiser and plastic as the fuel. These are described more fully below, but first, it's worth explaining how motor performance is classified.
Bottom to top: Pentamax K hybrid motor (54mm diameter), Sky Ripper G hybrid
(29mm diameter), D12 BP motor (24mm diameter), Pro38-G AP case (38mm diameter)
Here at the University, we have a range of motors available. As well as the black powder motors, which are all single-use, we have Cesaroni Pro38 one-grain (G impulse), two-grain (H) and five-grain (J) cases, plus a Congreve I case, all 38mm diameter. We also have a micro-hybrid (D impulse, working on nitrous oxide capsules), Sky Ripper G-69 and H-78, RATT I-80 (all 29mm) and a Pentamax J/K 54mm set, plus a Sky Ripper 38mm J on order, all hybrids. We have a RATT Works L/M combination hybrid motor on order for Level 3 flights. We are therefore able to fly on every impulse up to K (over 1280 Ns), and when the RATT arrives will have motors for L and M as well. Motors of H and I impulse require Level 1 certification, J, K and L require Level 2. M impulse and above requires Level 3.
Motor Performance
| Total impulse (newton-seconds) | Letter code |
|---|---|
1.26 - 2.50 |
A |
2.51 - 5.00 |
B |
5.01 - 10.00 |
C |
10.01 - 20.00 |
D |
20.01 - 40.00 |
E |
40.01 - 80.00 |
F |
80.01 - 160.00 |
G |
160.01 - 320.00 |
H |
320.01 - 640.00 |
I |
640.01 - 1,280.00 |
J |
1,280.01 - 2,560.00 |
K |
2,560.01 - 5,120.00 |
L |
5,120.01 - 10,240.00 |
M |
10,240.01 - 20,480.00 |
N |
20,480.01 - 40,960.00 |
O |
Classification of motors according to total impulse
Whatever the type, there are a number of different aspects to a motor's performance - thrust (maximum and average), total impulse and burn time. Thrust is, of course, important, not least because it determines how heavy a rocket the motor can lift. However, it is the total impulse that has most effect on the altitude the rocket will reach. Total impulse is simply the average thrust multiplied by the burn time. Different types of motor, as well as variants of a particular type, may have similar total impulse but different thrust profiles. For this reason, it is usual to classify motors by a letter code, representing its total impulse. Each letter represents a doubling of the impulse range, so an E motor will have around twice the total impulse of a D. Note, though, that this does not mean the thrust is doubled - the impulse can be doubled by doubling the burn time for the same thrust. In fact, an Estes D motor has more thrust than an E. The letter codes are shown in the adjacent table.
The thrust profile of a motor is also important - it affects the capacity of the motor to lift a rocket of a given weight, and also how much load is applied to the motor mount. If the motor produces more thrust than the mount is able to withstand, it will break free in flight, usually destroying the rocket. This is known as a 'shred'. Motors that produce a high thrust initially, then reducing thrust as the motor burns are called regressive motors, and those that show an increasing thrust with time are called progressive. A motor that produces essentially constant thrust over the whole burn is called neutral. For any particular impulse (measured in newton-seconds), almost any combination of thrust and burn duration are possible, provided the product of the two equals the total impulse; thus a 280Ns H motor could produce 280N average thrust for one second, 140N for two seconds, or 560N for half a second, or any other combination, depending on how the motor grain and nozzle are designed.
BP Motors
A black powder motor has the advantage that it is easy to handle, cheap and simple to operate. All that is needed is a small igniter (usually supplied with the motor) and a suitable launch controller. Most BP motors are made in the USA by Estes. No explosives certificate is required - anyone over 18 can buy them. Their limitation, though, is that they are only available in impulses up to E, and their performance in relation to their size and weight is poor.
AP Motors
Closer view of the D12, Pro38 and Sky Ripper G63 motors
Ammonium Perchlorate-based motors are available in a huge range of impulses, from F through to M (at least). They provide a compact, reliable means of getting large impulses for high-power rocketry, and are the most common type used after BP. They are simple to light, needing only a commercially available igniter and a low-voltage launch controller. AP motors do suffer from two major drawbacks, though - they are quite expensive (an M reload costs almost 300 pounds) and the larger ones require an explosives certificate, issued by the police, to buy and store them. The motor itself typically consists of a case (often aluminium alloy, as in the pictures on this page) and a 'load' or 'grain'. The grain is the AP mixture itself and needs to be replaced for every launch. The case is reusable. Most AP motors also use a replaceable nozzle, together with various O rings used for sealing the motor. Most also include a delay fuse and an ejection charge - the Pro38 motor shown has an adjustable delay time, and a special tool is available to remove part of the delay fuse to achieve the desired timing.
Hybrid Motors
Hybrid motors overcome some of the limitations of cost and storage of the AP motors, but introduce some drawbacks of their own. Because they contain no explosives, they do not require licensing to buy or use them, and they are much cheaper to run. However, they are much more complex to light, are much larger and heavier for a given impulse, and are more expensive to buy in the first place.
Hybrids are technically any type of motor that uses more than one type of propellant, but for amateur rocketry this will inevitably refer to a solid fuel with a liquid oxidiser - usually plastic fuel and nitrous oxide. Nitrous oxide (known variously as laughing gas, dentist gas or party gas) is not an oxidiser at room temperature, but disassociates into nitrogen and oxygen at 570 degrees Celsius. It is stored in pressurised bottles as a liquid, then the motor is filled immediately before flight. Once full, the trick is to raise the inside of the motor to this critical temperature, then start the flow of nitrous - chemistry and physics do the rest. It is this complexity that makes hybrids so expensive to set up and more complex to fly, but since the plastic fuel grains and nitrous oxide are both quite cheap (one-third the cost of an equivalent AP reload, or less), they are economical to operate once all the ground support equipment, or GSE, has been purchased.
There are several ways to achieve this high temperature. One common way is to introduce gaseous oxygen into the motor and use a high-voltage spark to start the fuel grain burning. The heat or thrust (depending on the motor) from this is used to start the flow of nitrous. The need to have oxygen bottles and an extra solenoid valve and more plumbing raises the cost of the GSE even higher, but it is a reliable method. The second way is to enhance the performance of a standard igniter by chipping off a small amount of AP from a solid motor fuel grain. This is popular in the USA, but has only recently been interpreted as legal in the UK. One drawback of this method is that pieces of AP can break off and block the nozzle, wih catastrophic results for the motor. A third way is to use a piece of plastic initiator cord (PIC) and a standard igniter or 'electric match', which is quite straightforward but not suited to all motors. A fourth way is to use Pyrodex pellets, again lit by a standard igniter.
One additional complexity is that hybrid motors do not provide a separation charge, so a different arrangement needs to be made. Typically, an altimeter, timer or magnetic apogee detector would be used to fire a black powder or pyrodex charge to deploy a parachute.
Costs
Apart from the smallest model rockets, all amateur rocketry is quite expensive. A BP motor ranges from around a pound for an A to about six pounds for an E motor, AP motors range from around 20-25 pounds for a G reload plus 20-30 pounds for the case to almost 300 pounds per launch for an M reload plus 200 pounds for the case. Hybrid motors cost around 35 pounds for a G motor, plus six pounds per reload plus perhaps a couple of pounds for the nitrous oxide. An M-class motor costs around 300-400 pounds to buy, and each reload varies from 60-100 pounds. Nitrous oxide for this size motor would be 15-20 pounds per flight. In addition, the ground-support equipment for a hybrid motor costs around 500-600 pounds, more if gaseous oxygen is needed. Apart from the BP motors, the motor is the item you most want to get back, so it is common to attach the parachute to the motor or motor mount - it's also the strongest part of the rocket. If you were wondering what the eye bolt on the front of the Pentamax was for, now you know...
Demonstration hybrid motor
Of course, it's not usually possible to stand close to a rocket motor when it's firing. With hybrid motors in particular, you need to be at least 30 metres away if the motor is in the open, However, that doesn't allow anyone to see how they actually work. We have built a safe motor that has a see-through case and fuel grain that we can fire in the lab, and people can see exactly what's happening. This can be done because the motor operates at low pressure and under complete control. We use this motor for demonstrations to students, and also to applicants at our open days. The motor is made from plexiglas (PMMA or polymethyl-methacrylate) and uses gaseous oxygen as the oxidiser. Strong bolts and careful calculations ensure the pressure stays where it is supposed to.