by Trip Barber, NAR 4322
Beginning model rocket competition fliers use the classic launch lug/launch rod technique for launching their rockets. For glider rockets (boost glide and rocket glide), even more advanced rocketeers generally use this approach. The disadvantage of using a launch lug is that the lug stays with the rocket throughout flight, and its aerodynamic drag can contribute significantly to the overall drag of the rocket, thereby leading to lower flight altitudes and lower performance in events where maximizing altitude performance is key to success.
One good way to eliminate a launch lug is to use a “tower” launcher. This is a launching device that has multiple rails positioned symmetrically around the center where the rocket is located. These rails guide the rocket like a launch rod would, until the rocket reaches a velocity where its fins provide enough corrective force to stabilize it for flight. These rails have to be rigid, generally by being secured in position at both the top and the bottom, so that they do not spread apart and allow the rocket to fall out sideways between them while on its way up the launcher. Generally a tower launcher uses as many rails as the rocket it supports has fins. Since almost all competition rockets (except in Scale) use three fins, most tower launchers use three guide rails. While it is possible to build tower launchers with fixed rails that serve only a single body diameter (this applies in FAI international flying, where all rockets must be 40mm diameter), NAR competition uses a wide range of body diameters so it is desirable to have rails whose position (distance) from the centerline of the launcher can be varied to match the body diameter of the rocket being launched. Unfortunately, there are no tower launcher kits currently on the market since the discontinuation of the old Apogee/Balsa Machining Service “Medalist” tower kit, and there have been few articles published with good plans for a tower launcher. Tower launchers work best with rockets whose diameter is constant along their length. Rockets such as egglofters with tapering body diameters can rattle around inside a tower and come out of it with a significant nonvertical trajectory, unless the smaller-diamteter tail sits in a slding shoe or sabot of some kind that prevents the rattling around while inside the tower but is discarded as the rocket separates from the tower.
A second approach to launching devices is the “piston” launcher. A piston launcher is a device that the rocket sits on at ignition, that uses the hot exhaust gases from the rocket motor to pressurize a moving piston system that does mechanical work with these gases, accelerating the rocket that sits on the piston to velocities above what the rocket would have attained in its few feet of free flight coming off a standard launch pad. The rocket pulls the piston along with it during this early portion of the flight, so the piston must be lightweight or its added weight will offset the mechanical advantage derived from its use. And its optimum length is specific to the type of motor being used and the weight of the rocket being launched because by the time that the rocket motor builds up thrust to beyond a certain level, the rocket is better off transitioning to free flight; the value of the piston is obtained from the first tenth-second or so of motor operation, when in a well-designed piston its gases give more velocity to the rocket by doing mechanical work in that piston than they would have done solely by pushing the rocket through the air in free flight. An optimum-length piston launcher can add as much as 0.5 N-sec to the power imparted to the rocket by the motor; a piston of non-optimum length may add no useful power and in the worst case may actually detract from performance. For US competition models the optimum length is often fairly short (one to two feet). With European motors in lighter FAI models, the optimum length may be quite a bit longer; see the R&D report below.
Pistons generally need to start with limited free volume that requires pressurization before the rocket can start moving, and they need to use pistons that are no greater in diameter than the rocket motor, to minimize the amount of volume that must continue to be pressurized by the gases in order to continue doing useful mechanical work as the rocket moves upward. The “zero volume” piston launcher (article below) achieves this. The piston-motor connection (generally achieved by friction-fit although Europeans use some form of clamping device for repeatability) needs to be tight enough to hold in the gases that pressurize the piston, but not so tight as to bring the rocket to a stop (thereby negating the velocity advantage provide by the piston) when the piston reaches the limit of its travel and the rocket transitions to free flight. The “floating head” piston launcher deals with this tradeoff in an elegant manner that has proved to be successful in competition.
Piston launchers are used fairly regularly by serious competition fliers, particularly in lower-power events where the effect of the performance boost that they can provide is a more significant percentage of overall flight performance. A well-designed piston that is the right length can increase the altitude of a lower-power competition model (C motor and below) by over ten percent. Almost all competitors of all nations in all events in FAI competition use piston launchers. Rockets do tend to come off pistons with some form of sideways vector due to uneven separation, and the resulting nonvertical boost can negate their advantage, so some competitiors use lightweight “fingers” (thin carbon rod or spruce) a few inches long at the top of the piston to help keep the separation straight, while others build their pistons inside tower launchers to achieve this effect even better, although at the cost of significant complexity. Piston tubes collect rocket motor combustion degree on the inside so they must be either cleaned or replaced between each use. Most European fliers use fiberglass or carbon tubes that can simply be rinsed with water. US fliers tend to use paper tubes, which require a test-tube brush to clean the inside or must simply be replaced between flights.
Tim VanMilligan, a US Team member who runs Apogee Components, published an excellent article in his company “Peak of Flight” newsletter concerning piston launcher design optimization, based on research work done by Patrick Peterson. Click here to download the newsletter from the Apogee website. In addition Patrick (as a member of the “Neutron Fusion” Team) subsequently did an R&D report on optimizing piston length and diameter for FAI models with European motors, which is attached below.
- Floating head piston launcher kit – Sunward Aerospace (also sold by Apogee Components)
- Piston launcher kit – Qualified Competition Rockets
|Collapsible Tower Launchers (Johnson)||May 29, 2014, 6:35 pm||477 KB|
|Piston Launchers (Stevens)||May 29, 2014, 6:35 pm||1 MB|
|Zero-Volume Piston Launcher (Burzynski)||May 29, 2014, 6:35 pm||738 KB|
|Neutron Fusion NARAM-56 RD FAI pistons||October 10, 2014, 9:12 pm||830 KB|