I'm working on a project that will be launched inside a Blue Origin New Shepard rocket and need to know the details of how our experiment will be grounded. The experiment will have a 5 V USB 3.0 power supply and will be powering a Raspberry Pi Zero W. How would I ground our electronics in this capsule?
Talk with the spacecraft designers, they have to have someone coordinating electrical specs for on board equipment and you will probably need to meet their specifications.
You should probably connect the ground on the RPi to the enclosure for electrical shielding. If there are other experiments in the payload you should have some guidelines from the systems engineer on EMC matters. Just randomly, here is an example of what you should get (that link is for a CubeSat). For example, outgassing is particularly important if there are optics involved in any part of the payload(s). There will be ICDs.
If the environment is going to be bereft of air you will have to ensure an adequate thermal design to get the heat out without allowing the chips' junction temperatures to rise too high, and the G-forces and vibration during launch are typically fairly severe so there will likely be a rather solid thermal and mechanical connection from the PCB to the housing.
You need to bring a container full of earth, and connect it to circuit ground.
No, you don't. Earth as ground has little special meaning if a) earth is not used as a return conductor in a power grid, b) there is no need to ensure conductive parts of equipment do not have a dangerous potential relative to earth (which makes an accident very likely if people are expected to stand on earth).
For advice on how to do it - as mentioned in other answers, do whatever the launch vendor tell you to do.
How would I ground our electronics in this capsule?
You ground it to the enclosure, however realize that the enclosure will be 'floating' and can change voltage, which will not make a difference for electronics inside of the enclosure if the enclosure is conductive, if the enclosure is not continuous, this could create problems with charging.
However, there can be problems for any 'exposed potentials' because these collect surface charges from the space environment (which includes plasma's) The International Space Station can change voltage to something like -10 to 25V with respect to a neutral charge. So I wouldn't worry too much if you don't have any exposed potentials, if you have solar cells or sensors on the outside the net surface charging will need to be accounted for.
The spacetime environment is a nasty place which includes radiation (solar, and cosmic), plasmas and vacuum. This can create problems for commercial electronics because of temperature. Vacuum can actually erode some materials (like PVC) so it is important to make sure materials are vacuum compatible. The battery also needs to be vacuum compatible. Most IC epoxies are vacuum compatible, but this needs to be checked.
Side note: Kapton is a vacuum compatible and has a wide temperature span, great for use on spacecraft, it is a wonder material.
It would also be advantageous to test your project in a vacuum for several days to make sure it still works. You'll also need to make sure the enclosure is not air tight (a pressure vessel) which is generally not allowed.
You'll also want to make sure the temperature of the electronics will be within the operating temperature as things can get very hot or cold as there is no air to equilibrate the temperature.
You might want to reach out to a sounding rocket community, there are students that launch projects all the time on rockets.
A voltage is not an absolute number. A voltage is a difference in potential between two points. If you say a voltage is "0V" then what that means is that the node you are measuring is at the same potential as your "ground" (the difference in potential between the node and ground is 0V). If you say the voltage is "12V", that means that the node you are measuring has a potential 12 volts higher than the ground. Once again, voltage is a difference, and thus your reference point ("ground") is arbitrary. You can put it anywhere in your circuit, and all that means is that any other voltage you measure is the potential difference between that chosen ground and the node you are measuring.
On the CubeSat projects I worked on, the aluminum structure was used as the grounding point. And every subsystem had their grounds matched with a PC104 connector.
It is best to talk to a power system or project system engineer.
In part, what you can do for a ground may depend on the surrounding specifications of the launch, you will need to check. I make the presumption that the experiment will be launched self-contained into free space. In all probability, you should clamp the negative terminal of the battery (I presume) is powering your 5v USB power supply to the suitable metal chassis or case of the experiment and use that as your reference. Any equipment requiring grounding can be connected to the chassis. If your experiment makes transmissions I make the presumption that it has some kind of antenna arrangement outside of the case.
As many have said, this problem has already been solved for you. The project techs will have the grounding specifications for you. However, to help you understand what's going on....
Grounding in space works in the same way you ground an automobile. The "chassis" represents the grounded electrical plane and the "negative battery post" (the reference potential for the rocket's power supply) is connected to it. This is necessary because the combination of those beautiful tires that we take for granted in our modern age and asphalt insulates the car from a traditional "earth ground."
Remember, your circuit operates on the difference in electrical potentials. Let's assume it's running on 1.1v. It doesn't matter if the actual ground and source voltages are 0v and 1.1v or 10,000v and 10,001.1v, the Rasberry will still work.
It's important to remember that calling ground "0v" is a mathematical concept, not necessarily an issue of reality. It makes the math easy, which is why we assume it. So long as we have a predictable way to get the difference we need between the two references, we don't really care what those references actually are.
A very long time ago I helped design Futurebus circuits, which were designed to operate in satellites. Those chips included circuits connecting the two planes to insure transient voltages on one plane (ground or source) also happened on the opposite plane to ensure the operating potential difference inside the chip was always the same, thereby stabilizing the chip's operation in an environment where a traditional "earth ground" didn't exist.
This was important because the sheet-rho of the ground and source planes and the resistance of the wires connecting the chip to the rest of the world was great enough to cause local variations of voltage on the planes (causing the potential difference to vary). On earth, you somewhat ignore that because replacing the chip on the million-to-one chance it might fail is not big deal. Space isn't that accomodating. (Note that this was a long time ago. Modern material science may have metals that no longer require that level of paranoia.)
Grounding in your case means connecting to the average charge of the largest available mass (that being the body of the rocket instead of Earth). That will be the ground value reference and the rest will be a voltage difference when compared to that.
protected by W5VO♦ Feb 21 '18 at 21:49
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