• Size and dimensions - 45" x 3.1".
• Chute compartment inner dimensions 3 inches by 12 inches.
• Single or dual deployment methods.
• Use of Piston or Deployment Bags.
• Payloads – None.
• Electronics for Deployment if Payload Bays are available.
• Weight at burnout - 29 oz.
• Intended motors - H97J-10.
• Expected Altitude – 2400 feet.
• Deployment velocity – 13.51 mph.
• Descent rate expected or preferred 15 mph or 22 ft/Sec.
• Wind considerations - calm days.

The size and dimension of your rocket are worth knowing as this helps you decide on the kind of chute you like to see on your rocket. Are you going to use an X-Form, or perhaps a multicolored Hemisphere parachute? How much room do you have in your rocket for a large chute such as a hemisphere or just small spaces for only a X-Form chute? The kind of parachute you like to fly may take up some larger space in your rocket. Perhaps your chute is made of less material and that may give you more room for other things like a payload. If your rockets weight is larger, that forces you to use a larger chute; do you have the room in a skinny rocket to pack a larger size chute? The size of the rocket gives you an overall idea of the performance class of your chute. The three main classes are, High-Power, Advanced and Model Rocketry. Each class parachute design ends up being a little different in how your chutes will end up being made. Bigger chutes get more construction changes to meet the needs of larger opening forces.

The Loc Forte' has a generous body diameter of 3.1 inches and a good length of at least 12 inches for parachute and recovery wadding. We could use a medium sized parachute in this space. I would like to use a hemisphere parachute here because of the extra room for a little more material the hemisphere parachute has.

Since this is a Single deployment rocket system, we will not be considering any payloads, electronics or additional parachutes for this example.

If you have a rocket that has a piston deployment system in it you will have to consider the extra room that is being taken up by the shock cord and piston itself in your payload bay. You will have to plan for the loss of chute compartment in your chute selection.

The weight of the rocket is considered after engine burnout. You will be recovering a complete rocket and an empty casing as well so that burnt engine weight must be added into the rocket empty weight. I like to weigh my completed rocket with an old motor before making the chute, this way I can get the exact weight for recovery. But you can use the popular rocketry design software to estimate the weight you could likely have. In our test case the Manufacturer lists the weight at 20 oz but the rocket in the picture came to 29 oz. So we expect the glue and finishing to vary between builders, this would include a motor casing after firing.

The intended motors let the builder know what class of performance this rocket is being flown in and a general idea of the heights being flown to. In our example we picked an Aerotech H97J-10 and this puts us into the High-Power Class of rocketry for first time certifying fliers perhaps. At the specified weight and this size of motor we can expect launch height close to 2400 feet and fairly high speeds after burnout so a good long delay of 10 seconds was reasonable. This analysis put the deployment velocity at 14 mph, which is a really good speed for a safe deployment.

In our case we select a 29 mm case.

Calculating Descent Rate and Chute Size

Everybody has an opinion on the best descent rate for his or her rocket. What I found common for fliers was that they tend to pick a descent speed slightly on the fast side for closer recoveries or perhaps the wind had increased a little more than expected. A fast speed was around the 24 feet per second (ft/sec.), medium was 21-ft/sec. and slow was 15 to 16-ft/sec. I tend to use 20-ft/sec. and have a little damage on my rockets upon landing in grass areas. The slower you descend the farther the drift in windy conditions. 24-ft/sec., left my rockets a little banged up with cracked fin fillets and chipped or busted fins. The 16-ft/sec. choices left me running farther to retrieve my rocket but I never suffered with damage of any kind with this slow decent rate.

The example uses a descent rate of 16 ft/sec. So the question becomes, how big a parachute do I need to carry 29 oz. at a descent rate of 16 ft/sec?

There are plenty of Descent Rate Calculators on the Internet.

Here's one from Jack Anderson. http://www.rocketreviews.com/tool_descent_rate.shtml

Using this calculator I get a parachute size of 26 inches for a 29 oz rocket, for a descent rate of 16.64 when we select the hemisphere parachutes option. Tested against other calculators, this page probably uses a Coefficient of Drag of 1.5 for the Hemisphere Chutes calculations. It is one of the only calculators that do the math directly for the hemisphere parachute. The other calculators just offer calculations for the round parachute.

Other people offer calculators that are software and run as programs on your desktop.

John Powell of ParaCalc V1.0b, writes, "ParaCalc is designed to help one approximate the size to make a parachute for one's rocket. However, it is not designed to replace actual trial and error." His web site is http://www.angelfire.com/co/m2rules/paracalc.html

Your RockSim Software has a deployment velocity calculation that works great. The same rocket with the parameters listed above in a LOC Forte' sim file estimates the descent rate a little differently. It accounts for a 2.5-inch spill hole. Weight of 29 oz and a parachute size of 26 inches with a Cd of 1.5 calculate the descent rate at 17.56 ft/sec. I like this calculator as it's built right into the simulator and you can test fly a bunch of chute sizes to see what you like best. As well, you can adjust the elevation and atmosphere in the software's initial settings to get conditions close to what they are like in your areas.

What is interesting about this software is the ability to do drift calculations and adjust the wind conditions with the height of your launch to get the downwind drift performance of your whole project.

Many descent rate calculators have a section that allows you to change the Cd. Cd is the coefficient of drag. It's part of the Drag Equation.

Drag = Cd1/2 Ro V^2S

Your parachute performance is a function of the Coefficient of drag, (Cd) the atmosphere, (1/2 Ro ) the descent velocity, (V^2) and the size of parachute in area (S)

Listed below is a chart of parachute types that have a listing of Cd's that are used for each parachute. Many people take for granted that a formed parachute like the hemisphere has a better Cd than a flat parachute. In many calculations this value is often picked as high as Cd of 1.5 but I have my concerns that nothing performs that good.

According to the chart below the same hemispherical parachute has a value between 0.62 to 0.77 Cd.

Dc/Do: ratio of canopy constructed dia (sewn) to the nominal dia (designed)
Dp/Do: ratio of canopy projected dia (inflated) to the nominal dia (designed)
Cd: parachute drag coefficient
Cx: parachute opening force coefficient
Oscillation: how much canopy swings or oscillates during inflated descent

Fig 1: From the Parachute Recovery System Design Manual by Theo Knacke, 1992, Table 5-1, page 5-3.

However, a parachute can have a mode of flight that permits gliding and this lateral movement through the air allowing the parachute generate lift like a wing in flight.

This additional movement can create a low-pressure zone over the canopy and further reduce the descent velocity. We can reduce the Cd from .66 up to 1.5 to compensate for such conditions. Be aware that this condition may not exist and is predicated on many variables that are very difficult to account for in real conditions. The selection of a Cd for accuracy is not an easy task. To be on the conservative side perhaps a Cd of 1.2 is a good choice for our purposes.

A recalculation of our information gives a new descent rate for a 26-inch parachute and a 29 oz rocket at 18.63 ft/sec. Certainly there is nothing wrong with a descent rate of 18.63 ft/sec.

The selection of Cd for your particular project is a subject that many people will argue over. Cd's are quoted within a range anywhere between .60 all the way up to 1.89, but verifying the high values as accurate is a serious undertaking.

Taken from the NACA Technical Note # 1315 June 27, 1947.
Free Falls and Parachute Descents In The Standard Atmosphere, by

A.P. Webster, Bureau of Medicine and Surgery, Navy Department, on Pg 14. An analysis of parachute drops was conducted using various parachutes and materials to arrive at the Cd for those types of parachutes.

The mean – observed – descent times give the following average Drag Coefficients.
28–foot Silk 0.828
28-foot Nylon 0.730

Full report: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082042_1993082042.pdf

Consider that the information in this report is for full size test subjects and equipment and have a different scale factor than our rocketry parachutes, still is some testimony to the range of Cd used in parachute operations.

Other considerations that will affect the operation of your parachute are the effect of elevation and outside air temperature. Colder or warmer conditions will give a marked change in the descent speed of your designs. Use the Cd value in your calculations in a reasonable fashion and you will have parachutes that operate as you expect them to do. So lets go back to our calculations and re consider the use of a Drag value as high as 1.5, I think that we should consider using a Cd of .85 so I loaded this value into the parachute section of RockSim software and see what we get. The first picture shows a Cd of 1.4 where the descent rate for a 30-inch chute is 15.8 ft/s.

In this next calculation we changed nothing except the Cd to .85 and a descent rate of 20.3 ft/s results.

The RockSim software is a good tool to use to help you finalize the last stages of parachute size and descent rate.

The final parachute size for this project will be,

30 inch 8 panel parachute

2.5-inch spill hole.

---