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Random Altitude


For NARAM-51, a version of Precision Altitude, named Random Altitude is being held.

Random Altitude has the challenge of building and flying a model that can fly as close as possible, to a Target Altitude which is not announced until shortly before the event is held.

Only one flight is allowed. If the track is lost, does not close, or if there is a Cato, a reflight will be allowed.

The Target Altitude will be Randomly chosen at the field on Tuesday morning, the day of the event.

Engine power - Anything up to 160 N-Seconds total (G class), with Contest Certified engines.

The model can not be radio controlled or contain any device whose purpose is to limit the altitude of the model (e.g., a wire or string).

For the full rules for this event, please see the Precision Altitude Rules on the NAR web page. Take note that Random Altitude is listed in rule 22.2.3

Scoring - For Random Altitude, the Model's tracked Altitude is calculated against the Target Altitude to produce a percentage error. Lowest percentage error wins.

For example, let's say the Target Altitude chosen for the day was 200 meters. If one model flies to 180 meters, and another model flies to 220 meters, both being 20 meters off from the target of 200, both would get a score of 10%. A model that flies to 196 meters, or to 204 meters would get a 2% error. Remember that the 200 meter target altitude is ONLY an example here.

Design considerations - It is not practical to produce a list of plans or kits, since this is so wide open. You certainly do not have to build a special model for this event, you can make use of an existing sport model if you want. The trick is in trying to make it fly as close to the Target Altitude as possible.

Strategies - I believe this is the first time this event has been flown at a NARAM. It almost defies a strategy.

If this was Predicted Altitude, you could just test or practice fly your model at some other launch, get it tracked (or use an altimeter), and use the altitude it flew to as your prediction.

If this was Set Altitude (Target altitude announced with the contest info), then the first step would be to use computer Flight Simulation software. The Flight Sim would give you a ballpark idea how high various rockets would fly with different engines, choose a combination that would fly close to the "Set" target altitude, and test fly it. You could test it at a launch, find out how high it went (tracked or altimeter), then make adjustments such as adding some extra mass to make it fly lower, or even changing to a different model that could fly higher.

But for Random Altitude, you cannot prepare very much in advance like above, since the Target Altitude will not be known until the morning the event is held.

If you do have any models that you know how high they fly, great, take them. If you are lucky, the Target altitude might happen to be close to an altitude you know those models fly.

If not, it pretty much comes down to computer flight simulations. If you can bring a laptop to the field, that would help, but that may not be very practical for most people.

But, there definitely will be some who will bring laptops to the field for this very reason, to run flight sims once the target altitude is known. Those who are well prepared will bring an assortment of rockets, so that if the Target Altitude is too much mid-way between what a certain rocket would fly on a B or a C engine, they will have several different rockets that would fly to different altitudes on a given engine. For example, one rocket that would fly to 189 meters on a B, another to 228 meters on a B, another would fly to 264 meters on a B. So they could choose the appropriate one to work on adjusting from (By using one that is known to fly higher than the Target Altitude and add some mass to make it fly lower, with Flight Sim software to help indicate how much).

So, at the least, bring a variety of rockets and engines. If you do have flight sims, but cannot bring a laptop, then run the sims at home and make a record of how high the models should fly for a given engine and given mass. Make some runs with some extra mass, to se the effect on the altitude, and include that in the records you keep. So, you will have some idea to work with.

Now, unfortunately, flight sims and real world altitudes do not tend to be the same. So, if your flight sim says your model should fly to 202 meters, and if the Target Altitude was announced as 200 meters, do not do the victory dance just yet. It might turn out that the flight sim is wrong. In general, flight sims tend to over-estimate the altitude. So, if a flight sim said 202 meters, I would not be surprised for the real altitude to be tracked to 183 meters, or something like that.

Also, keep in mind that the tracking is at EJECTION, not apogee. So for flight sims, use the option to tell you the altitude at ejection, not at apogee.

Last tip - ASK! If you have no better idea, you can ask some fellow competitors for help. Someone might have a decent idea of how high a rocket like yours is likely to go on a given engine. Especially if you have a rocket that is a fairly popular model for the event since many people will tend to use kits for this.

Some additional advice from other competitors:

From Greg Poehlein:

The only thing I can think of is pretty obvious - build a payload model and fly it several times on each motor with no weight in the payload section and then with a couple other known weights. Given three or four data points for each motor, you should be able to guestimate the amount of weight needed in the payload section to get the required altitude. Of course, that means a number of test flights with full tracking, but I suppose even an Estes Altitrack would be better than nothing at all.

Excerpts from Lee James, from the TCC NAR Competition Primer page for Precision Altitude:

Almost any kit and contest approved motor will work.

Use computer simulations to determine altitude for various rocket and motor combinations. Free altitude simulation:


wRASP 32 -

Make multiple flights with various models and motors using NAR competition type theodolites ( USMRSC 14 Altitude Data ) to determine the altitude your model achieves. Add weight, such as tracking powder, and/or use a different motor to reach the altitude you want. Make practice flights using theodolites to measure altitude and/or computer simulation to find how much weight to add and/or which motor to use.

You can use tracking powder to vary the launch weight of your entry, but its main purpose is to make the model more visible to the trackers (theodolite operators). Chalk for a carpenter's chalk line is available at most hardware stores. For more info on tracking powder see rmr FAQ part 9.4.i.1:

Tracking Powder - It is highly recommended to use tracking powder in your model. This produces a small "cloud" at ejection which the tracking crew looks for. Without tracking powder, it is not likely your model will get tracked.

Dry Tempera paint, or a fine powdered Fluorescent Dye, are often used for tracking powder. Some contestants used to rely on powdered chalk, but it is clumpy and does not really produce much of a tracking cloud for the volume/weight of the powder. Red is a good color choice for tracking powder, though some like to use black if there is a high overcast or hazy "white" sky. Fellow competitors are often willing to share tracking powder.

Here's a good way to install tracking powder. After installing wadding, pack the parachute and shock cord into the model, and push them down into the tube to leave room for the tracking powder in the upper part of the tube. Use a piece of wadding or plain paper to make up a long narrow "cup" than will easily slide inside the body tube. Press that cup into the tube, then pour in the tracking powder to fill the cup. About 1" or so depth of powder is a good ballpark. Using tracking powder can require greater forces to expel everything out of the body, which sometimes results in the engine kicking out instead (however, the cup method reduces this problem a bit compared to just dumping powder into the tube). Make sure the engine is secured in the rocket extra-tight. Some people like to attach the fins a bit above the bottom of the body tube so they can apply a "collar" wrap of tape to the bottom of the tube and the engine. This helps prevent the engine from ejecting.

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Above: Example of a tracking powder cloud, using Red powder, on a B Eggloft Flight.

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 Last Updated   7/3/2009