Safety

Recovery Hardware (June 2023)

High power rockets often experience high shock loads during recovery deployments. Many rocketeers now utilize nylon or Kevlar webbing to provide adequate shock cord strength during the deployments. A commonly observed weakness during safety inspections at the Student Launch Initiative and the Spaceport America Cup is the adequacy of the hardware used to connect the shock cord to the rocket.

Bent wire eyebolts often provided with high power rocket kits may not always be the safest component because they could straighten out and release the shock cord when exposed to high shock loads. The bent wire eyebolts can be used if the “eye” is welded closed but, unless space is a problem, better alternatives exist. Forged eyebolts are an excellent alternative often available from hardware and home improvement stores. “U-bolts” are another acceptable alternative and have an advantage that they cannot rotate free from the rocket. Some form of thread lock is recommended for all of the threaded fasteners used to retain the connection hardware and is especially important if the termination can rotate, e.g. eyebolts. Thread lock methods can include thread lock compounds, e.g. Loctite, nuts with nylon locking inserts, staking nuts with epoxy, and safety wire.

Often overlooked is the use of large diameter “fender” washers or similar parts (e.g. the rectangular plate often provided with U-bolts) underneath the anchor device. The intent is to distribute the loads on the bulkhead to which the shock cord is attached. Small diameter washers normally placed under the threaded fastener concentrate the shock load stresses over a smaller area. This can cause localized cracking/deformation of the bulkhead and, in severe cases, bulkhead failure.

Steve Lubliner
NAR Safety Chairman

Flight Simulations – They need the correct inputs! (May 2023)

Simulation programs to predict hobby rocket performance have been available for over 50 years.  Fliers often refer to their simulation results to justify safe launch performance, stability, and apogee predictions.

Users need to provide correct inputs to get accurate flight predictions.  The expected ambient temperature and local ground elevation (or ambient pressure) at the launch site need to be entered.  The planned launch rod or rail length needs to be entered to properly calculate the exit velocity from the launch pad.  The actual launch weight of the rocket needs to be used in the simulation to accurately evaluate acceleration and apogee performance.  Rockets experience weight “growth” because finishing materials, adhesives, and small items (e.g. quick links, switches) were not accounted for.  Inaccurate material descriptions within the parts list can significantly change the calculated weight.  A good discipline is to weigh all components being used in the rocket and either verify the program calculations or utilize the mass overrides many programs will allow.  Program calculations for the center of gravity (CG) should be taken as suggestions since recovery components will shift within the airframe during acceleration.  The CG is easily verified during rocket inspections by determining where the rocket balances.  External part dimensions and placements need to be as accurate as possible to permit accurate center of pressure (CP) calculations and the subsequent evaluation of static stability.

Some programs will allow the user to override the calculated coefficient of drag (CD).  While determining the actual CD is somewhat of an art and is often refined after test flights, users should consider if the CD seems reasonable for the model’s finish and construction.

Stephen Lubliner
NAR Safety Committee Chairman

Igniter Installation (April 2023)

One of the differences between model and high power rocket operations concerns rocket motor igniter installations. The NAR Model Rocket Safety Code makes no mention of when or where igniters may be installed in the rocket motor(s). Most NAR sections permit model rockets to be presented at check-in with their igniters installed. This is considered a safe practice when using igniters produced by the manufacturers of model rocket motors. Consult the Range Safety Officer at your launch if you intend to use igniters that require very low currents or are static sensitive for any special use precautions.

The NAR High Power Safety Code states that the motor igniters will be installed after the rocket is at the launching or prepping area. Some additional steps are suggested prior to installing the rocket motor igniters. First, the rocket should be mounted on the launch pad and positioned vertically. Second, all internal avionics, including payloads if installed, should be powered, confirmed to have passed any power-on self-tests, and are indicating flight readiness. Finally, the number of personnel at the launch pad should be the minimum required to install the igniters. The igniters may be installed at this time (staged rockets are a separate discussion due to likely complexity or sensitivities of the igniter installation).

The launch pad controllers should be verified to be in a safe state prior to connecting the ignition leads. If the controller has no audible/visual indicators of safe status briefly contact the leads and observe no sparks. Once satisfied that the leads are not energized, connect them to the igniters and clear the launch area of all personnel.

Steve Lubliner
NAR Safety Committee Chairman

Shelf Life (March 2023)

Many of us are familiar with the “Use Before” dates on our food stuffs. What we may not think about is the use before dates or shelf life on the materials that we use for building models. Adhesives may age by solvent evaporation and/or polymerization of their ingredients. This can affect their strength or viscosity and create application problems. They may not properly cure and achieve the structural strength expected when used. Paints may separate, color shift, have solvent evaporation, and some chemistries may also polymerize. In addition, spray cans may leak their propellant gas over time. Like adhesives, these issues can create curing and application problems.

Typical spray paints, e.g. Krylon and Rustoleum, have a manufacturers’ specified shelf life of two to three years. A common 5 minute epoxy by Devcon has a shelf life of three years, West Systems epoxies have “expiration dates” two years after manufacture, and Titebond III has a minimum shelf life of one year. Your experiences may be different. Environmental conditions, typically storage temperature and humidity, will affect shelf life. If the storage environment is comfortable for you, it will probably extend the shelf life of your products. Also, most shelf lives are stated for unopened products. Exposure to oxygen and humidity from broken seals can shorten shelf lives. Aerosol cans, once used, are more likely to leak their propellant over time.

Some materials will allow the addition of solvents to thin them to ease application if they have become too thick to apply. Agitation to remix separated ingredients may help. Follow the manufacturers’ recommendations. The best strategy, if in doubt about a paint or adhesive, is to apply a sample to spare or scrap items and evaluate whether it is acceptable.

Steve Lubliner
NAR Safety Committee Chairman