As a part of the Technical Communications and Experimental Design course, students were placed into teams and tasked with designing and performing a simple experiment. Experimental Project Group Nine consisted of Stephen Johnson, Gunther Maddock, Jaime Flores, Nathan Wells, and Weston Shipman. An experiment was designed to measure the effect of payload mass and number of parachutes on the descent time of a parachute-mass system in free-fall. This web page will covey the motivation for, the data, results, and conclusions for the experiment.
Recent years have seen an increased interest in conducting research in space with NASA’s use of the International Space Station (ISS). Samples are prepared on the ISS and sent down to earth to be analyzed in NASA facilities. A slow impact velocity, the velocity of the payload when impacting the ground, is vital to ensuring the safety of samples. Parachutes are commonly used for reducing descent speed to levels that won’t compromise the integrity of the samples or harm any astronauts on board. Descent speed will be measured using the descent time for a constant distance. If the system has a longer descent time through the same distance, it is traveling at a lower velocity and thus will have a lower impact velocity. The relationship between payload mass, number of parachutes and descent time is to be investigated. Below is an image of the Apollo 13 splashdown with three parachutes .
To investigate the question, a simple experiment was designed using toy parachute soldiers, a set of slotted brass weights, a stopwatch and the Kenneth E. Johnson Technology Center at Olivet Nazarene University. The Technology Center features a second story that overlooks the shop floor. Parachutes would be dropped from the railing of the second floor and their descent time till impact with the shop floor was recorded.The height from the railing to the shop floor was recorded to be 15 feet. Parachutes were purchased off of Ebay. The soldiers were removed so the masses could be attached directly to the parachute. The slotted weights were acquired from Olivet Nazarene University's Physics Lab and returned after use.
The two factors selected for the experiment were one of two parachutes. The three different levels selected were 50g, 70g, or 90g mass attached to the parachute(s). This resulted in 6 different sets of trials.
Two sets of 10 trials were completed to calculate the required number of total trials for the experiment. Each set of ten had a different weight attached to the parachute. It was calculated that 6 trials for each factor at each level was required. To provide room for outliers to be removed and still have sufficient data, the team selected 10 trials per factor and level, resulting in 60 total trials.
To collect the data, one team member was placed at the railing to release the parachute. This member was to ensure that the parachute was released from the same height each time and to release the parachute as close to when the stopwatch was started as possible. Another team member was located on the shop floor with the stopwatch. This team member would announce "3... 2... 1... Drop!" and the team member holding the parachute-mass system would release it. The timer would be stopped when the mass hit the shop floor. The time would then be announced and recorded in a spreadsheet for future analysis.
The following image shows the data without any analysis or processing done to it. There is a very clear outlier, the only time above two seconds. This was due to a slow release and the team immediately knew that the trial would be an outlier. An 2-Factor ANOVA analysis was used to interpret the data.
The fastest descent time, 1.19 seconds, belongs to 90g mass attached to a single parachute. The slowest descent time, 2.09 seconds, belongs to 50g mass attached to two parachutes. This was the outlier and was removed during the ANOVA analysis. The complete table of times is as follows.
The following figure is the results of the ANOVA analysis performed with an alpha level of 0.05.
When a second parachute was added, the average time increased for all payload masses. On average, the 90g payload with one parachute traveled at the highest velocity. The 50g payload with two parachutes traveled at the slowest velocity, with an average time of 1.66 seconds. The average time for the 70g payload was impacted most when a second parachute was attached. The following figure shows a marginal means plot using the average times for each case.
The marginal means plot shows that the levels and factors are independent of each other and thus we can draw clear conclusions from the data. It also shows that the high-mass, single parachute systems have the quickest descent times. The greatest change in time came for the 70g system going from one parachute to two parachutes. Adding the second parachute to the 50g system only reduced the descent time slightly, indicating that additional parachutes have a negligible effect on descent time for low mass systems.
The single parachute systems had seemingly random and uncontrollable flight paths. No two trials fell in a similar way. When the second parachute was attached, the mass would hardly drift in the direction inline with the parachute centers but would still drift in the direction normal to the two parachutes. This indicates that additional parachutes can reduce the drift of the system during its fall. This would be especially useful if it is desired that the system lands in a specific region.
Adding the second parachute decreased the descent time regardless of the mass of the system. Also, increasing the mass of the system decreased the descent time. Thus, if the descent time is to be reduced, the mass of the system should be reduced or the number of parachutes used should be increased.
Possible future experiments to expand upon this experiment include:
- Testing different shapes of parachutes
- How does attaching additional parachutes effect drift during descent?
- Creating a timing and release rig to eliminate human error in descent time recordings.
"Mission Control Celebrates Apollo 13 Splashdown", NASA, 2017. [Online]. Available: https://www.nasa.gov/50th/favpic/missionControlCelebrates.html. [Accessed: 09- Dec- 2017].