Piston Launchers
Overview
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, although the Europeans mostly use fairly short pistons (500 mm or so) 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. European piston launchers tend to have far more internal complexity than US designs, see the drawing below of a Russian piston.
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.
Piston Optimization
Tim Van Milligan, 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.
Thoughts on Launch Towers
by James Duffy
Over the years that I had the good fortune to represent the US at the World Championships, I have noticed that we seem to arrive carrying a motley assortment of launch towers for our rockets. Most of these seem to be reasonably effective at pointing the rocket in the right direction, but many often seem to be flirting with the ragged edge of failure.
For our sport rocketry activities here in the US, “heavy” is usually no big deal, as we can generally throw just about anything we need into the back of the pickup along with the EZ-Up and the cooler as we head out to the local launch. Add in the complexity of international travel, though, and the demands change. Many of us have settled on solutions that use photographic tripods, as they are relatively light and universally available, and provide a (somewhat) sturdy base for our rockets, with a minimal weight penalty. Unfortunately, photo tripods have a weak spot: the tripod head. Attach a tower base with multiple guide rods to the top of a tripod, and you’ve got something that will sway precipitously in anything more than a light , refreshing breeze. The tripod legs are certainly sturdy, the guide rods rigid enough, but that nasty plastic bit connecting the two is perpetually on the brink of catastrophe.
Fortunately, most quality photo tripods offer the user the opportunity to get rid of this offending point of potential calamity. If you have a tripod-based launch support solution, try this: grasp the tripod head and rotate it counter-clockwise to see if it can be removed. If it can, your efforts will reward you by uncovering either a 1/4”-20 (good!) or a 3/8”-16 (better!) attachment point. With the tripod head conveniently out of the picture you’re well on the way to having a more secure launch base for your rockets! Even better, by removing the tripod head you’ve just eliminated One More Damn Thing that you’ll need to schlep to Eastern Europe.
What if the tripod head can’t be removed, though? There are a few budget tripods out there that do not offer the option of head removal, so consider this a golden opportunity to upgrade your gear. Take the old tripod and place it carefully in the trash, or give it to the Russians to hold the Kestrel that they’re borrowing from you. Next, go to the B&H or Adorama website and look for a new tripod, taking care to select one that has a removable head. You’ll note that the better tripod brands offer the opportunity to select tripod legs as a separate item, and you should concentrate on those legs that list a 3/8”-16 attachment point in the specs. To save time, here’s a link to the tripod legs that I use for my current setup (https://www.bhphotovideo.com/c/product/655216-REG/Induro_472_014_Alloy_8M_AT014_Tripod.html).
Now that you’ve bought a good tripod and decapitated it, now what? This vexing challenge used to keep me up at nights, but then I saw the light; a laser light, specifically. Gaining access to a laser cutter has afforded me the ability to fashion all sorts of wonderful goodies, and among my first projects was a tower base for 40mm FAI rockets, followed rapidly by a base for the 1/24 scale Bumper WAC models Jim Filler and I flew in Bulgaria. Both the Bumper WAC and 40mm bases feature mounting points for either 1/4”-20 and 3/8-16” threads, and use the same .505” fiberglass tubes sold under the GlasForms brand by Into the Wind (https://intothewind.com/shop/Repair_and_Kitemaking/Fiberglass_Tubes_for_kites/.370_x_54.5_FG_Tubing). Guide rail support spiders for the top end of the tower were cut from 1/8” acrylic with the laser cutter. If you need a tower base and spider for your 40mm models, get in touch and I’m sure I can help you out.
Here’s the bottom line I’d like to leave you with: a photo tripod is a great support for a launch tower solution, as long as you get rid of the nasty little tripod head that makes it sway like a drunken Russian!
Tim VanMilligan at Apogee Rockets has released a laser-cut tower based on James’ design. More information is available at:
https://www.apogeerockets.com/Launch_Accessories/Launch_Pads/Wedgie_Launch_Pad
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