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# HPR Design

## Articles to Read:

Search \[topic] + "apogee newsletter" and you can often find great results

* Specific articles coming soon

## Design Check:

* Make sure you allocated enough space for your parachutes. Too much space is better than not enough space!
  * TODO: how to determine/estimate packing size
* Do you have rail buttons? (Most likely you should have 1010 rail buttons)
* Does your motor mount fit the motors you plan on using?
  * The motor can be longer than the motor mount, but then the motor may take up parachute space!
  * A lot of H motors are 38mm.
  * A lot of J motors are 54mm (but the BAR L2 special \[the J350W] is 38mm).
* Ground hit speed should be <20ft/s. Is your main parachute big enough?
* (For dual deployment) Speed at main deployment should be <70-80ft/s. Is your drogue big enough? (TODO: should I say <50 ft/s instead?)
* Velocity off the rail (our rails are at least 6 ft) should be >50 ft/s.

## Design

* Stability = (CG - CP) / rocket diameter, with CG and CP measured from the bottom of the rocket. It is unitless, but usually written as a number of “cal”s.
  * Stability should be between 1.5 cal to 2.0 cal. Stability of 1.8 cal is good.
  * Consider adding a mass to the nosecone-payload tube region if the rocket is under-stable for a large motor
* Tubing:
  * Fiberglass is expensive (?).
  * Carbon fiber is more expensive (?).
  * Blue tube is strong and relatively cheap.
  * Don’t use cardboard for HPR.
* Shoulders:
  * Recommended shoulder length (i.e. extent of coupler tube) is about tube diameter (i.e. 1 cal).
* Nose cones:
  * 4:1 tangent ogive is nearly optimal. (?)
* Fins:
  * Plywood is good up to around 330 ft/s. Above this, there is significant risk of fin flutter, which can break your fins right off the rocket.
  * G-10 fiberglass has flutter speeds of over 1 Km/s (\~3330 ft/s), so for L1/L2 it should be fine.
* Tube size:
  * Hands tend to be less than 4” across -- so if you want to be able to reach all the way inside your rocket, 4” should be big enough.
* Fin mounting:
  * Sandwich between bulkheads.
  * Epoxy the shit out of the fins.
* Bulkheads:
  * 0.25” plywood is a good choice.
* Motor mount:
  * Usually a phenolic tube; may be fiberglass.
  * Design around the length of the motor(s) you plan on using.
  * If using a threaded motor retainer, allocate enough room for this by having enough of the motor mount tube extended beyond the aft centering ring


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