The performance of a piston launcher employing an operational design modification was compared to the standard zero volume piston launcher. Test results showed that the modified launcher performed 34% better than the standard piston.
The design modification was derived from a deficiency noted in the standard piston after a theoretical consideration of the principles of its operation. This deficiency is responsible for the deceleration of a model prior to its separation from the launcher, resulting in significant velocity loss. The modified launcher employs a floating piston head to overcome this deficiency. The piston head can float freely off the support rod allowing the forward motion of the piston and model to continue until the model separates from the pressurized tube. This minimizes velocity loss.
The performance of the piston launcher was determined by measuring the velocity of the model at a fixed point in flight. Velocity was determined by a novel method utilizing an infrared emitter/detector pair mounted in the path of a passing fin. The time required for a fin of known chord to traverse the IR beam was measured by a computer and velocities were directly calculated. The method was expedient and trouble-free and should prove extremely useful in future research projects where velocity measurements are required.
Other performance benefits observed with the floating
head piston launcher include a greater degree of reproducibility
and little incidence of model tip-off that is associated
with the standard design. It is recommended that these
benefits along with the optimization of the floating head
design be investigated in future research and development
Email: cbweiss at frontiernet dot net
Slingerlands, NY 12159
Email: jeffvincent at verizon dot net
Since its invention in the early '70s, there have been a number of studies of optimization of the zero-volume piston launcher (hereafter referred to as the standard piston). Thoelen, Bauer, and Porzio studied the effect of varying piston length on altitude performance. The Zunofark Team reported improved performance with their brass-head piston design. These improvements have generally resulted from optimizing a particular parameter of the standard piston's design.
In this study, the performance of a piston launcher employing a basic operational modification is compared to that of a comparable standard piston. The modified piston utilizes a floating head, which minimizes the deceleration due to the piston tube impacting on the standard piston head. The objective of this report is to demonstrate the performance benefit of the floating head piston over the standard piston design. The measurement of this performance involved the creation of a new method of determining the launch velocity of model rockets.
A piston launcher consists of the support rod, the piston
head, the piston tube, and the guide-stop (see Figure 1 -
Piston Nomenclature). The model rocket's engine is friction-fitted
into the top of the piston tube so the engine will
thrust into the chamber formed by the piston tube and head
(see Figure 2A). Upon ignition, the exhaust gases created by
the engine pressurize this chamber and cause the piston tube
and model to rise. This relationship may be explained by the
Ideal Gas Law:
---- = R,
where P is pressure, V is volume, n is the mass of the gases in moles, T is the temperature, and R is the universal gas constant. As the engine thrusts, the mass of gases and the temperature increase. Since the tube is free to move, it rises, increasing the volume, allowing the pressure to remain constant, and maintaining a state of equilibrium. This continues until the guide-stop approaches the piston head (Figure 2B). When the volume of the piston chamber cannot expand at a rate sufficient to compensate for the increase in gas mass and temperature, the internal chamber pressure will rise. This increase in pressure acts to separate the model from the piston.
When the guide-stop of the standard piston approaches the piston head the rate of increase of the volume rapidly decreases to zero (i.e.- volume becomes constant). This results in a pressure increase, which coupled with the model's inertia, acts to overcome the friction-fit of the engine and separates the model from the piston tube. It is at this point that the deficiency in the standard piston design arises. The mechanism responsible for the separation of the model from the piston also creates a rapid deceleration of the piston and attached model. We may reason that the model will also experience some of this deceleration prior to its separation from the piston, reducing its launch velocity and altitude performance. It may also be reasoned that the friction-fit of the engine is critical to the standard piston. If the fit is too tight, excessive deceleration will occur prior to separation. If the fit is too loose, the model will be separated prematurely, not realizing the full potential of the piston's assistance. However this critical fit is usually determined by the judgement of the operator, which can vary significantly from one modeler to another. It is these two problems: piston (and model) deceleration and criticality of the friction-fit, which the floating head piston was designed to remedy.
To alleviate these problems, the modified piston utilizes a floating head. As the name implies, the piston head is not firmly attached to the support rod and is free to "float" with the impact of the guide-stop. The operation of this piston is illustrated in Figure 3. During the ascent of the piston tube, the head is held firmly atop the support rod by the internal chamber pressure of the piston. When the guide-stop reaches the piston head (Figure 3B), the free floating head is lifted by the guide-stop and carried with the coasting piston tube. The presence of the head on the stop reduces the rate of volume increase to zero (volume becomes constant) and the internal pressure increases. This pressure increase overcomes the engine's friction-fit and the model is separated.
This design change creates a number of advantages. First, the piston and the model are not significantly decelerated by the impact of the guide-stop. The only deceleration encountered during this "coast phase" is due to momentum lost in accelerating the piston head and typical losses due to gravity, aerodynamic drag, and the friction of any guidance devices. This deceleration should be significantly less than that experienced by the standard design. We may conclude that launch velocity shall be greater due to this fact, and that overall altitude will be increased. Secondly, the tightness of the engine fit should be far less critical on the new piston. Since there will be far less deceleration prior to separation, an instantaneous separation is not critical. For these reasons, we feel the floating head piston should show a significant increase in performance over the standard piston design.
One of the more difficult aspects of this project was devising a method of testing the performance of the floating head piston against a control piston. The best method would be tracking the altitude achieved by the test models. However, due to limited manpower and a lack of tracking theodolites, this was not a feasible path. A second parameter which could demonstrate the performance of the pistons was launch velocity. We first investigated the use of a movie camera, but found that processing time for movie film in our area was a prohibitive factor. We also examined the use of video tape, but consultation with several professionals revealed that for truly accurate results we would have to get a Beta-format camera and an editing video cassette recorder. The daily rental on these items was in the hundreds of dollars, ruling out their use. We then began experimenting with an infra-red (IR) sensor and a computer-based timing program. This proved to be the method we utilized for our report.
The speed measurement device is based upon an IR emitter/detector pair which are mounted in the path of the fin. If the fin is of a known length, the time it takes the model to traverse this distance can be used to calculate the speed of the model. The IR sensor is attached to the joystick port of an Atari 800XL 8-bit computer. A hybrid Atari BASIC/6502 machine language program runs a timing loop which determines the length of time the IR beam is cut by the fin. The timing device has shown an accuracy of +/-0.05% over long (100 second) tests and has a computed accuracy of +/-0.22% at the model velocities encountered in this test. It is strongly recommended that anyone interested in further details of this device consult the Appendix.
A special apparatus was required to hold the piston launcher and the sensor in the proper orientation. The guide for the piston tube was similar to a three-pole tower launcher. The piston support rod was centered between the poles by a center-drilled wooden cylinder. A masking tape stop held the support rod in this cylinder and provided consistent launch heights. The apparatus also had two adjustable fin guide rails constructed from 3/4" x 2" pine. These rails were mounted adjacent and parallel to the piston guide poles. The IR sensor was mounted in these rails 18" above the position of the base of the model at rest. The rails were spaced 0.13" apart (approx. two fin widths) and the inner surface was highly polished to reduce friction. Initially, the detector was being triggered by ambient IR light, leading to unreliable readings. This problem was solved by coloring the inside of the rails adjacent to the sensor with a black magic marker, reducing reflections.
The two piston launchers were identical in construction, except for the modifications necessary to implement the floating head. Both piston heads were cut from a single piece of hard maple, turned to the proper diameter on a modeler's lathe. The diameter selected was such that the piston head could fall freely within a tube without binding, insuring low friction. This resulted in a diameter of 0.526", +/-0.002". The middle two-thirds of the length of each head had its diameter reduced to create a top and bottom seal ring, lowering friction. The base of the floating head had a 17/64" hole drilled into it to mount it atop the 1/4" support rod and allow it to separate easily. (All center-drilled holes were drilled with a modeler's lathe to insure correct alignment.) The floating head has two 1/32" holes for the ignitor to mount in the terminals in the support rod. The standard head has a 1/4" hole drilled completely through it which accommodates the support rod. The piston guide-stops had a 9/32" hole drilled through them, allowing free travel on the support rod and avoiding any binding due to misalignment. The support rods were 1/4" brass. The piston tubes are CMR RB-50S phenolic tubes, polished internally with steel wool to reduce friction. The completed mass of the two piston tube/guide-stop assemblies were within 1.7% of each other.
Two models (designated A and B) were constructed to conduct the flight test program. The models were constructed identically from Estes BT-5 tubing and BNC-5 nose cones. The fins were constructed from 1/16" basswood for durability. The fins were rectangular with a span of 2.0" and a chord of 1.5". One fin on each model was selected as the fin designated for the test and this fin was colored black for identification. The chord of the designated fins was 1.495" (A) and 1.510", +/-0.002". Each model utilized a small plastic streamer for recovery purposes.
The experimental design employed was intended to minimize error due to differences in the test models or preparation procedures. Each model was flown an equal number of times from each piston. A total of eight test flights were made from each piston. All test flights were flown between l2:00 and 7:00 p.m. on the same day. Weather conditions were warm and cloudy with occasional short periods of sunshine. The temperature ranged from 80 to 85°F. The model/piston combinations were cycled through the day to even out any environmental biases.
The test program was conducted by the following procedure. Flights were conducted in rounds, with both models being prepped for a given round. The recovery device was first inserted in the model. The engine was selected (all engines were Estes 1/2A3-4Ts - batch 27Ql) and fitted in the model with 0.5" protruding from the base of the model. The engine was then shimmed with masking tape to achieve the desired piston fit. All fittings were done by the same operator to achieve uniformity. The engines were fitted, by the best judgement of the operator to be the optimum for a standard piston design. The models were then weighed and the weight was adjusted if necessary to keep within 1% of an established value. The pistons were then equipped with ignitors cut to a size to provide a piston stroke of 9.125", +/-0.125". The models were then refitted to their respective pistons and taken to the launcher. Each model was loaded by turn into the alignment apparatus so that the designated fin rode in the fin guide rail and the piston was at the predetermined rest point. The continuity of the launch system was then checked. The exact value of the particular model's fin chord was then entered into the program for the speed calculation. The computer was then placed in the IR scan mode where it sounds a tone if ambient IR is detected. Next, the battery was connected to the IR LED and the tone would confirm the presence of the IR beam. The computer was then put into the test mode, and the model was launched. Immediately after the launch, the duration that the beam was cut, the model's speed, and the maximum timer error were displayed on the TV screen. These values were recorded and the model was recovered.
|Standard piston||Test piston|
Over eight trials, the standard piston achieved a mean launch velocity of 22.51 ft/sec with a standard deviation of 2.37 ft/sec (10.5%). The mean launch velocity of the floating head piston was 30.09 ft/sec with a standard deviation of 1.72 ft/sec (5.7%) for the same number of flights. The average launch mass for the standard piston was 16.76 grams with a standard deviation of 0.166 gm (0.99%). The average launch mass for the modified piston was 16.77 gm with a standard deviation of 0.113 gm (0.68%).
The data shows a 33.7% increase in average launch velocity for the floating head piston, a significant difference. The average launch mass showed no significant deviation. These assertions are confirmed by a two-tailed T test at a 99.9% confidence level. Due to the experimental design and the care taken in the design and preparation of the equipment used, we are confident that the performance increase measured cannot be attributed to bias error. A comparison of the relative standard deviation of the pistons (10.5% for the standard and 5.7% for the floating head) reveals a higher degree of reproducibility for the floating head piston. This increased precision may be related to the frictional fit of the engine in the piston tube.
During the visual observation of the test flights, a discernible difference was noted in the motion of the two pistons. The greater deceleration predicted for the standard piston was apparent. No visible deceleration was observed in the action of the floating head piston. The phenomenon of tip-off was not observed during the test or development flights of the floating head design. The radical deceleration and lower launch velocity characteristic of the standard piston may be responsible for tip-off.
This project served as an operational test for the equipment and method used for velocity measurement. The ease of operation and the results obtained verify the usefulness and validity of this technique. No difficulties or inconveniences related to the equipment or method were experienced.
The objective of this project was to demonstrate the performance benefit of the floating head piston over the standard piston design. The results show that this operational modification achieved a substantial increase (34%) in launch velocity over the standard piston. The tests also revealed a number of other potential benefits of the floating head design. The higher precision of performance of the floating head piston suggests it is more forgiving of operator error in the friction-fit of the engine. Observations also suggest that the problem of tip-off associated with the standard piston will occur less frequently with the floating head piston. This study also demonstrated a new method of determining launch velocity.
The floating head piston offers a number of possibilities for future research. First, optimization studies of the floating head piston design may be conducted. Parameters such as piston head size and weight and the length and diameter of the tube may be investigated. Since the theory of the operation of the floating head suggests that higher launch velocities can be obtained through a tighter friction-fit, this relationship should be studied. Further refinements of the speed measurement device may be devised.
The total expenditure on this project is $55.00, broken down as follows:
|Parts for piston launchers||$5.00|
|Parts for piston/fin guidance apparatus||$25.00|
|Four packs of Estes 1/2A3-4T engines||$10.00|
|Parts for test models||$5.00|
|IR sensor hardware||$5.00|
All other items cited on the Equipment List, such as the computer equipment, TV, lathe, and measurement devices were items available at hand.
Atari 400/800 Hardware Manual. Sunnyvale, CA: Atari, Inc., 1980.
Chadwick, Ian. Mapping The Atari. Greensboro, NC: COMPUTE! Publications, Inc., 1983.
Halliday, David and Resnick, Robert. Physics. New York: John Wiley & Sons, Inc., 1960.
Mason, Robert D. Statistical Techniques in Business and Economics. Homewood, IL: Richard D. Irwin, Inc., 1967.
Scanlon, Leo J. 6502 Software Design. Indianapolis, IN: Howard W. Sams & Co., Inc., 1980.
Thoelen, Bauer, & Porzio. "Optimization of the Zero Volume Piston Launcher", NAR Technical Review, Vol. 2, No. 1, Fall 1974.
Zunofark Team (Steele, M. & Gassaway, G.), oral presentation of "The Brass Head Piston Launcher" at NARAM-26.