TU SAT1 -> Tech Specs -> Summary from Paper

Mechanical Components

Design Criteria

Thanks to the present boom in technology, electrical components are faster, more reliable, and smaller than their recent predecessors. In designing the satellite, we took advantage of these characteristics by incorporating the miniaturization technology available on the market into our design. Consequently, we have an incredibly fast, state of the art communications system that is much smaller than any comparable systems.

This is fortunate, for our satellite is restricted to 4 x 4 x 8 inches in order for it to fit into the launch tube provided by Space Systems Development Laboratory (SSDL). In addition, the whole system has to weigh less than 1.5 kg in order for it to meet launch criteria. Designing an operational system of our complexity around these constraints has been difficult. Each new idea, no matter the value of the concept, is met by the question, "How big is it and how much does it weigh?" Through refinement of ideas, we arrived at the design described below.

 

Structural Design

As mentioned above, our two limiting factors in the physical design of this satellite are size and weight.

 

Many of the electrical components are off the shelf and can not be made any smaller or weigh any less than their stripped down versions. As a result, much of the pressure rests upon building a frame that is strong enough to support all the components, spacious enough to fit all the components while still falling within the size limitation. It must also be light enough to keep our mass under an acceptable 1.5kg.

 

 

Our design uses interlocking side, top, and bottom panels milled from 1606-T6 aluminum (Fig. 2a). The panels are a maximum of 0.190" thick along the edges and fashioned with 0.060" thick ribs across the middle to add structural stability (Fig. 2b). These ribs also double as mounting brackets for specific flight hardware. Predrilled holes are set in the correct position

 


Fig. 2a – All of the panels assembled into a complete frame.


Fig. 2b – Detail of a side wall of the frame.

 

to match the electronics boards and the equipment attaches directly to the frame. The panels are completely milled-out in the middle save that of a

0.025" thick shelf that runs along all of the ribs at a distance of 0.100" from each edge. Epoxied to this shelf is a 0.012" thick copper laid PC board that acts as our solar cell backing. All of this results in a lightweight frame with ample strength to not only hold all the components securely, but to also pass the required stress test of 7.7 G’s in the axial direction and .8 G’s in the lateral direction.

Another requirement of our structural design is to deploy needed components at the correct times. There are seven deployables on the satellite that require fail-safe release methods. Although deploying devices are available for sale, none fulfill our task without contributing significantly to the overall weight. Therefore, our own independently designed systems must reliably deploy two solar panels, two 70cm antennae, one yagi antenna, one plasma probe, and a 100 ft. long tether (Fig. 3).

Fig. 3—A computer generated rendering of the satellite.

Solar Wing Design and Deployment

The need for a solar array is unquestionable. The satellite’s numerous electrical systems require electrical power and no batteries can supply it for the estimated 3-4 year lifetime of the satellite. The question is not whether to use solar cells, but how to use them to produce the most power for the least weight.

As with most picosatellite designs, the exterior of our satellite is covered with solar cells. However, the exterior alone is not large enough to produce enough operating power (assuming a worst case eclipse orbit). Our answer to this problem is two wings that unfold to 85-degree angles and serve as extra solar panels. Because our satellite will be spinning within its orbit,

, we placed solar cells on either side of the deployed panels to ensure adequate coverage.


Fig. 4—A close-up of the deployed panel showing the solar cells (B) adhered with silicon adhesive (C) on both sides of the same pc board backing (D).

To avoid casting shadows onto the other arrays we placed the two solar panels up at an 85-degree angle with respect to the top of the satellite. This allows for maximum solar exposure from the top and the sides depending on the position of the satellite (see Fig. 3 for image). Thus, no matter which way the satellite spins or where the sun is relative to the satellite, a sufficient number of solar cells is illuminated to maintain system power.

The actual mechanism we will use to deploy the solar panels works much like a miniature mousetrap. There is a small axle on the top of the satellite, which the panels attach to and rotate around. This panel is inset into the frame just enough for the folded panel to be safe from hitting the sides of the launch tube during initial launch and deployment. On this axle, we wind a spring with a low spring constant. This spring is in tension while the panel is folded and, consequently, pulls up the panel after it is released. In order to keep the solar panel at an 85-degree angle, we position a stop at the top of the satellite to keep the panel from continuing its swing upward. At this point, the spring is still in tension, so it will keep the panel pushed against the stop and lock it in place. There will be some oscillation in the panel at deployment, but this will have little effect on the system as a whole, for the oscillations will quickly die off from the continual pressure by the spring.

Antenna Deployment

 

To effectively communicate with the satellite and provide fast e-mail transmission, we are installing three separate antennae. In order to upload program instructions, the satellite uses a 2m Ham receiver. The satellite transmits its diagnostic data as well as a beacon signal through a 70cm Ham transmitter. The e-mail transmissions and scientific data are transferred on the 900MHz spread spectrum yagi antenna. Given that the primary purpose of the satellite is communications, it is imperative for the antennae to deploy flawlessly in order for the satellite to function properly.

 

 

The 70cm antenna we are using will serve as both a 70cm transmitting antenna and a 2m receiving antenna, allowing communication with the satellite during its initial orbits. With this link, we will be able to tell how the satellite is operating (via the beacon) and then initialize the start-up sequence for its fully functioning capacity when the satellite is over Taylor. Therefore, this antenna must deploy first. However, the design for this antenna also has to allow for some form of communication with the ground even if the deployment fails. Our design was to have two, eight and a half inch long sections of conductive material that point out of the bottom of the satellite at 71.35º diagonals. The angle at which these are positioned is quite conducive to broadcasting. The highest gain is perpendicular to the line of the antennae, which is also parallel to the line of the other antenna. This is beneficial because along the line of the antennae is where the signal has to go through the most atmosphere thus needing the greatest possible gain. The angle also means that the broadcast signal is uninterrupted even if there is some wobble in the satellite as it orbits. In order to deploy these antennae, we made each section fold up across the sides of the satellite not being used by the solar flaps. The hinge mechanism is very similar to that used by the solar flaps. It incorporates the use of a spring in tension as well as a physical stop to open and lock the antenna in place once the burn wire releases. The beauty of this design is that if everything goes wrong and nothing deploys, we will still have contact with the satellite. The antennae along the side will still be able to broadcast prior to deployment. They will be broadcasting out to the sides of the satellite, but some of the signal will still be able to reach earth. Thus, through the beacon transmitted over this antenna, we will be able to tell what is wrong and how best to fix it.

The spread spectrum antenna is a two-element 33cm yagi. Once deployed, it looks like an inverted "T" with two top crosses instead of one. The antenna is made out of a strong wire that has high tensile strength. We can "fold up" the cross pieces and put the whole antenna into a round tube inside the satellite. Once the burn wire is released, the antenna is pushed out by a spring and the crosspieces merely unfold to their natural position.

Gravity Gradient Boom

There are two main forms of satellite stabilization: active and passive. Active stabilization includes such things as thrusters and dynamic electronic control whereas passive stabilization involves such things as gravity gradients and viscous dampers. We chose to use the passive stabilization of a gravity gradient boom because it is cheaper, less complex than burning fuels for thrust or writing AI code, and weighs much less than other methods. The boom utilizes the fundamental law of gravity to achieve its stabilized state. The basic principle behind the gravity gradient boom is that the strength of pull that gravity exerts on an object is proportional to the distance the object is away from the earth. We spool out 100 feet of line with an attached mass on the end. If deployed correctly, the mass will be 100 feet closer to the earth than the satellite and thus be feeling a stronger gravitational force. The result is that the tip mass, in essence, orients the bottom of the satellite down towards the earth causing the satellite to remain upright and in a stable attitude. Also, the tip mass will be at a lower altitude than the satellite and will therefore have a faster orbit. This will pull the satellite along and force it to always point towards the earth.

The actual deployment of the tether is very similar to the old clothes dryers that wrung out the water by pulling the clothes through two rollers. Instead of clothes, we are pulling a thin titanium wire (about seven thousandths of an inch in diameter) off a spool and through two wheels which act as rollers. One of the "wheels" is attached to a stepping motor that rotates 15 degrees for every digital pulse it receives. The potential problem with this design is that the boom may be deployed at the wrong time. If this happens, instead of being stable with the bottom facing the earth, the satellite will act as the tip mass and be oriented upside-down. We therefore had to build a retractable tether in case this happened. To do this, we merely attached a small dc motor to the spool. If deployed incorrectly, the motor could wind up the tether and then deploy it again properly. The stepping motor attached to the roller would create enough tension to allow the spool to be wound compactly and properly.

Weight Distribution

In order to reduce any wobble and to increase the stability as the satellite spins the center of mass needs to be as close to the actual center of the satellite as possible. Therefore, we need to distribute the weight evenly throughout the interior of the satellite. This is not an easy task because there are several components that must be placed in specific locations regardless of their weight (i.e. the gravity gradient boom). To start off, we mount all the electrical components along the sides of the frame making sure that each of the side panels has approximately the same amount of mass on it. We know that our plasma probe, antennae, and tether system all have to be on the bottom of the satellite. Therefore, in order to counteract this we put the heavy batteries up at the top. The result was a well-balanced satellite that had its center of mass (and thus its center of rotation) very close to its center.

Thermal

There are also a few things to consider regarding the thermal aspects of the satellite. The first is matching the coefficient of expansion of each of our materials, especially when it comes to our more fragile pieces. For example, if the coefficient of expansion for our solar cell backing did not match the coefficient of expansion for our solar cells themselves we have a problem. This would mean anytime the satellite rotated into our out of the sun, the solar cells would not be able to withstand the force of the backing moving at a different rate and would crack. For some of the larger pieces and connections between components and frame, the different coefficients of expansion would have very little to no effect at all.

There are also many components on the satellite that will be producing heat as they operate. A prime example of this is our battery pack. There is no atmosphere (or very little) at the 650 km altitude that we will be orbiting at, so we can not rely on the principle of convection to keep our satellite cool. The only way to make sure we do not overheat the satellite is to make use of the metal frame each of the components attaches to. As the component heats up, the heat dissipates throughout the frame itself. The frame is quite thin in parts with a good deal of exposed surface area. Consequently, the heat radiates off the frame and into the colder space. This process can only

work on the side of the satellite that is facing away from the sun for the heat from the sun would reverse the heat radiation. There will be a little bit of heat conduction throughout the frame, which will allow the heat to transfer to the cooler side of the satellite and radiate into space. However, by spinning the satellite we accomplish this much more effectively and thus maintain thermal stability. Spinning also reduces the amount of direct sunlight incident on each panel by 1/6.

The final variable in terms of the thermal aspect of the satellite is the sun. As the satellite flies through space, it has no atmosphere to protect it from the harsh heat the sun puts out. To counteract this, we will be positioning optical solar reflective tape (OSRT) to the outside of the satellite to reflect most of the thermal energy from the sun. However, the solar cells will be absorbing a large flux of solar energy. Our cells are about 19% efficient and reflect only about 8% of the incident electromagnetic radiation. That means that 73% of the incoming light turns into heat. This can pose quite a thermal problem for our satellite. However, the solar cells that we are using are space grade and have a unique coverglass that not only protects the cells but also radiates the heat out into space rather than back into the satellite.