Solar Sail Ripple Pointing Effects Experiment
(SSPREE): Phase 1
Meredith Larson
Wendi Pickett
William Pratt
DeAnn Redlin
Aerospace Engineering
University of Colorado at Boulder
Abstract
The Solar Sail Ripple Pointing Effects Experiment (SSPREE) was designed and built by undergraduates at the University of Colorado to replicate, verify, and measure “pointing ripples” as seen on recent Russian solar sail experiments. Current solar sail experiments utilize large vacuum chambers due to concerns regarding aerodynamic effects. SSPREE aims to show that the “rippling” phenomenon can be seen in ground-based experiments without the need for complicated experimental setups. SSPREE will be completed in three phases, with Phase 1 consisting of qualitatively spinning a solar sail test article in a 1-g, 1-atm environment. Future phases increase the complexity of the project by adding reorientation of the test article and quantitative measurement of the rippling effects. This paper presents the experimental results from Phase 1 testing. During testing, large standing waves resulted from weak, imperfect flow from the air support system. In addition, as the air support system lowered, aerodynamic suction caused the sail to adhere to the air table. By strengthening and evening out the airflow from the support system, it should be possible to successfully spin a simulated solar sail in a 1-g, 1-atm environment.
1.0 Introduction
1.1. Background
Propulsion continues to be one of the driving factors in designing long-range space missions. One of the most feasible propulsion concepts for long-range missions, solar sails use the sun’s radiation pressure to generate thrust [1]. While the generated thrust is low on the sail, the continuous pressure and corresponding acceleration make solar sailing attractive for the future. Solar sails with sizes ranging from 4 meters to 100 meters have been proposed to replace conventional fuel propulsion systems. However, solar sails must be made from extremely lightweight, highly efficient support structures and sail film substances a few microns or less thick. These very lightweight “wimpy” structures will be extremely difficult to manufacture, deploy, and control. Currently, solar sail technology has three major issues: deployment, pointing effects, and structural integrity.
The Solar Sail Ripple Pointing Effects Experiment (SSPREE) deals with the pointing control aspect of solar sails. A rotating solar sail requires only a fraction of the structural mass of a non-rotating sail, but pointing becomes a serious issue due to the resistance encountered from reorienting the sail’s angular momentum vector. SSPREE stems from research completed by Russian scientists Melnikov and Koshelev, who created a mathematical model for describing a solar sail phenomenon known as “rippling” as shown in Figure 1 [2]. The “rippling” is described by the following equation:
(1)
This shows the amplitude of the outer edge ripple (α), depends on sheet spin rate (w), orientation rate (W), the total radius of the sheet (Rk), and the radius of a central rigid disk(R0). This assumes the radius of the central disk to be much smaller than the radius of the entire sheet at around a factor of 10. Sparse ground based and space based experimental results are available, but the data sets were obtained from experiments not designed to exclusively study the rippling effect, so another experiment is required to study the effects of rippling due to pointing torques. SSPREE is an experiment devoted entirely to studying the effects of rippling due to reorientation of a spinning solar sail.
1.2. Phase Goals
Designed and built by undergraduates at the University of Colorado, SSPREE aims to recreate, observe, and measure this rippling phenomenon without the use of a zero gravity environment or large vacuum chamber. This project will be completed in three major phases:
Phase 1: stably spin a solar sail test article in a 1-g, 1-atm environment
Phase 2: reorient the spinning test article and qualitatively observe rippling phenomenon
Phase 3: successfully quantitatively measure and analyze rippling phenomenon
2.0 SSPREE Phase 1 Configuration
2.1. Requirements
Phase 1 of SSPREE includes qualitatively verifying that a solar sail test article can stably spin freely under normal gravitational and atmospheric conditions. Each SSPREE Phase 1 subsystem can be adapted for use during subsequent phases. To fulfill the requirements of Phase 1, SSPREE must have the ability to freely spin the test article at a rate high enough to allow the centrifugal forces to counteract the forces of gravity and give a wrinkle-free, flat, spinning sail.
2.2. Description and Function
The Phase 1 SSPREE configuration consists of four major subsystems:
- Support structure with draft cover
- Test article
- Spin motor
- Air support system
Phase 1 is the most difficult part of SSPREE to implement since the test article must spin freely so that the centrifugal forces overcome the forces of gravity and the sail spins flat with no wrinkles.
Acting as the “backbone” of the experiment, the support structure suspends the test article and spin motor during Phase 1 testing. For future phases, the support structure supports the measurement system and provides the reorientation axis necessary to create the rippling effect. Constructed from UniStrut and attached to a fixed backstop and rigid truss structure, the support structure experiences negligible static deflections and vibrations. The UniStrut rigid truss side of the support structure can be seen in Figure 2. In addition, a plastic wrap draft cover eliminates air currents that might interfere with the behavior of the thin test article as seen on the left side of the picture in Figure 2.
The 0.6-meter diameter test article simulates an actual solar sail in proportions and behavior. Constructed from 12 μm thick Kapton, the test article film attaches to two rigid inner disks made from old compact discs (CDs). To manufacture the test articles, Kapton sheets are stretched between a set of acrylic molds, the film trimmed, and the CDs attached. A test article is stored inside the molds, shown in Figure 3, until used for testing. After using a test article, the molds are reused to manufacture another test article. To help even out the effects of gravity, the test article attaches to a spin motor and hangs from the support structure. A high-speed, low-torque motor shown in Figure 4 spins the test article to at least 500 rpm; a rate sufficient for counteracting the forces of gravity acting on it.
A homemade 1 x 1 meter “air support table” placed underneath the test article uses compressed air to “levitate” the test article, creating a nearly frictionless environment while spinning from rest. The air support table is raised and lowered using a jack. Figure 5 shows a close view of the air support table beneath the test article, while Figure 6 shows an edge-on view of an inner disk “floating” on the air table. Figure 7 presents a diagram for the SSPREE experimental setup with all the SSPREE subsystems. The high-torque motor and laser measurement system are included in future phases of the SSPREE project.
2.3. Phase 1 Experimental Procedure
Phase 1 qualitative testing to stably spin the test article utilizes the following steps:
- Attach test article to spin motor by screwing inner disks onto motor shaft
- Slide in air support table below the test article and turn on air flow
- Remove test article molds
- Turn up compressed air to 40 psi and begin to spin motor
- Gradually turn up air pressure and lower table as motor speed increases
- Once motor reaches over 500 rpm, slowly turn off air pressure and remove air table
- If successful, the test article is now spinning free and flat against the forces of gravity
3.0 Phase 1 Results
Phase 1 testing utilized all the subsystems presented in Section 2.2. The air support table successfully levitated the test article and created a frictionless environment. An optical encoder inside the spin motor provided accurate readings of the spin rates, which had the capability to be to be well over the required 500 rpm. Initial observations showed that the spin axis was relatively perpendicular, with no significant “wobbling”.
At the beginning of each test, the air table was placed within two inches of the test article as seen in Figure 8 and the air turned to 40 psi so the sail film floated freely on a cushion of air (see Figure 9). As the motor began to spin, the sail quickly evened out to the desired configuration as shown in Figure 10. At this point, attempting to lower the air table caused significant problems. If the spin rate of the sail was relatively low, around 200 rpm, the sail film successfully separated from the air table surface and spun freely (see Figure 11a). However, the spin behavior was not uniform or stable, and the large standing waves did not damp out after a reasonable period of time as seen in Figure 11b and actually became more dramatic as the spin rate increased. Every attempt to reach 500 rpm at this point resulted in failure in the form of crumpling around 250 rpm. If the spin rate of the sail was close to 500 rpm as the table was lowered, the test article actually “sucked down” to the table to cause failure. The suction effect occurred as the air table dropped from beneath the sail. As the table moved, the edges of the sail film “suck down” to the table surface and stick to it despite the increase in air pressure through the table as seen in Figure 12. Even if the air flow though the table reaches maximum at 80 psi of air pressure, this is still not enough to overcome the suction effect. Varying combinations of table drop rates, air pressure, and spin rates all yielded the same results of either unstable spinning at less than 500 rpm, or incredible suction at 500 rpm.
4.0 Discussion
Two major problems were observed during Phase 1 testing: large standing waves and unstable spin geometry that cause failure before reaching the required 500 rpm, and an apparent “suction effect” as the table lowered with the sail at high speed. Both problems must be corrected in order to successfully complete Phase 1 and allow continuation of the SSPREE project.
The large standing waves appeared in tests where the air table started out at more than one inch from the surface of the sail and could be the result of many different issues. In theory, these standing waves will disappear once the test article reaches the ideal theoretical spin value of 500 rpm. However, these waves actually cause failure before that spin rate. If a standing wave or instability is created in the sail, it causes the otherwise perfectly edge-on sail to become less aerodynamic. As the spin rate increases, the standing wave acts as a barrier to airflow, thus further increasing the size of the wave. At approximately half the required speed, 250 rpm, the size of the standing wave degrades the aerodynamics so much that the sail winds up around the motor shaft. Many different factors possibly play a role in the formation of the waves, yet the presence of the air table itself is the most likely explanation.
The air table supports the sail while it is being spun to 500 rpm. In theory, this would not cause problems, however this assumes an even distribution of airflow over the entire surface. Greater pressure in one section of the table as opposed to another lifts that section of the sail higher. As shown in Figure 9, varying flow rates across the table result in large initial deformations of the sail film. Here the test article is not spinning, and the sail film is clearly uneven at different positions on the air table. During each test, the same sections of the sail started off “higher” than the rest of the film. If the table is kept within one inch of the sail while the spin rate is increased, it appears to spin evenly because the speed of the spinning sail creates aerodynamic flow underneath that effectively “squashes” any inconsistencies from the table. However, if the table distance at the start of the test is not within one inch of the sail, the speed of the sail can no longer “squish” the air underneath and uneven airflow once again becomes an issue.
In addition, the test article experiences a suction effect proportional to the spin speed as the table is lowered from beneath the sail. Aerodynamic research has shown that if a spinning disk is placed next to a large flat surface, such as a wall, this actually creates a void between the disk and the wall that acts an efficient pump [3]. Only increasing the distance from the wall or slowing the spin rate of the disk decreases the suction. This phenomenon is the most likely cause of the suction problems during Phase 1 testing. The spinning film is essentially a rigid disk spinning against a wall and increasing the distance causes the “pump” to suck the sail down to the table as shown in Figure 12. Evidence for this is demonstrated in that not just the outer edges of the sail are sucked to the table; it is the entire surface of the sail. In some testing, it was possible to spin the sail up to 500 rpm and then lower the table so much that one could observed a “volcano shape” taking place in the sail before failure. Since the spin rate of the sail must be at least 500 rpm, the initial space between the table and the sail must be larger at the start of the test.
If the table remains within one inch of the sail at all times, the required spin rate of 500 rpm can be achieved. However, such a fast spin rate dramatically increases the suction effect so that the air table cannot be moved without the sail suctioning to the table. It is critical that the table can be moved to allow for the sail to be reoriented during future phases. If the test starts with the table farther away from the test article the suction effect is eliminated, but imperfect flow rates over the table cause standing waves that prevent the spin rate from reaching 500 rpm.