Electrostatic Toilet
Entry For and Impressions of NASA's and HeroX's Lunar Loo Challenge

I entered a design competition for a space toilet.

The challenge was presented by HeroX, which is an offshoot of the X Prize Foundation. HeroX is a challenge facilitation platform in which anybody can be a sponsor a challenge with a prize and anybody else can participate in that challenge. It's a cool system. This particular "space toilet" challenge was sponsored by NASA.

I read about the challenge in a news article and thought it sounded like a lot of fun. I figured I'd jot some ideas down, make a submission, and call it a good day-or-two-long project. However, once I started, it was hard not to spend the time to produce a quality submission.

The Challenge

The goal of this challenge was to create a novel design concept for a toilet that could be used on NASA Artemis missions to the Moon. The toilet should overcome limitations of other space toilets, be compact, lightweight, and able to be developed and integrated in the next 2-3 years.

You can read the full guidelines for the challenge here https://www.herox.com/LunarLoo/guidelines

The Forums

I love design competitions. I especially love competitions with broad participation footprints. Interesting and fresh ideas can certainly come from anywhere.

If there is solid evidence for broad international participation in this challenge, it's the HeroX forums. People were posting and conversing from all over the world. Entries were submitted from 100+ countries. I particularly appreciated the other competitors that shared their entries there; it's fun to read other people's thoughts about a problem I'd done some thinking about.

As expected for an internet forum, it had everything. There were lots of up-beat positive posts about enjoying the challenge, looking for a teams to participate with, or discussing possible solutions. There were also plenty of people asking questions that had already been answered by the provided documents and people bragging about how their their design was the smallest, lightest, bestest one that would easily solve all of NASA's problems perfectly (and signs of incredulity that they didn't win a prize when the challenge was over).

Many of the posters struggled a bit with written English, which if it carried over to their entries, I imagine would make judging entries all the more difficult. It's got to be hard to identify great ideas through language barriers. Obviously, if I had to re-write mine in French or Spanish, it would have been way worse.

The best solution I can think of to this language barrier problem is that competitions should have objective criteria. For example, write an algorithm that does this task in the shortest amount of time. Of course, not all challenges lend themselves to that type of evaluation (such as this one). Rubric-based judging is an attempt at objectivity, but I think it largely serves as a mask for subjective evaluation. Perhaps that's an area for future improvement.

Results

In the end, I didn't win any sort of prize. If I had to guess the biggest reason why, based on the talk in the winner's seminar, it's that it would have been pretty difficult to get a toilet like mine mission-ready in 2-3 years, which was a significant part of the judging criteria. Of course, it could also be that my entry was inferior to the other entries in content or delivery.

It's difficult to know, because we can't take a look at any of the other entries unless the authors share it in some other channel.

Furthermore, in this particular challenge, if you're not a winner you don't get any feedback. There are lots of reasons this could be:

  • I'm sure judging was already an enormously onerous task without judges worrying about their comments/scores going public. 2000 entries / 25 or so judges is 80 entries per judge!!!
  • If I worked at NASA or HeroX I wouldn't want to deal with people contacting me and complaining about how their entry was mis-judged and why they should have been given a prize instead of the actual winners. It's aggravating enough to read comments on the forum to this effect.
  • Perhaps there is some legal liability that's avoided by not reporting results, in the same way companies don't often give much feedback about how job applicants performed on an interview.

So I get it, but that doesn't mean that it's not a little disappointing. I spent a fairly substantial amount of time on a project that effectively just went into the void. Of course, the logical side of my brain knew the was the likely outcome going in and I participated anyways :)

As I final critique, the winners webinar (which I enjoyed watching and appreciate Nasa/HeroX putting it together) was scheduled for very soon after the winners were announced. This implies that the winners were notified ahead of time which was a bit anti-climatic for the prize announcement.

Overall, I probably wouldn't do another competition again soon, unless the challenge really spoke to me. It was fun to do, but it took up a ton of time and I have a bunch of other projects I'd rather work on that aren't so "all or nothing".

To fight the feeling that I did unproductive work, I'm publishing my entry here. Hopefully it will either be interesting to my fellow competitors or valuable for somebody doing a similar challenge in the future (NASA has done a bunch of challenges on HeroX).

The Designs!

Winning Designs

If you're interested, you can take a peek at the winning designs here:

My Design

Here is my entry. If I were you, I would just scroll through and look at the pictures :)


The Electrostatic Throne

Attract waste with electrostatic force; use centripetal force to keep it there.

Overview

The design utilizes electrostatic attraction and centripetal force to attract and hold waste.

Both male and female crew members will use the toilet in the same way they use a traditional western earth-bound toilet (sitting-only for males). This results in intuitive use.

The waste is collected in a removable bag that closes automatically when the crew member turns off the toilet or in the event of a power failure. The toilet employs seat-belt-like straps to keep crew members on the toilet without needing to hold on with their hands. The belts can be stowed in two compartments in the toilet's base when not in use; these compartments also hold removable bags and other toilet hygiene products. The base of the toilet bottom is flat so it can be placed on the ground in lunar gravity or attached to some other surface.

Three parts of the toilet base, can, and lid
Toilet and sub-components
Base (left), Can (bottom right), Lid (top right)

Usage procedure

A typical use of the toilet is described below.

Using the toilet

To use the toilet, a crew member:

  1. Removes the attached belt from the storage compartment under the toilet.
  2. "Sits" on the toilet (or, in the case of vomiting, holds the toilet seat with their hands).
  3. Fastens themselves to the toilet with belt.
  4. Powers on the toilet.
  5. Expels waste. Optionally uses hygiene products contained in stowage compartments.
Crew member using the toilet

Cleanup

After expelling waste, a crew member:

  1. Powers off the toilet. Friction slows spinning, and spring-loaded arms seal waste bag.
  2. Opens the hatch on the side of the device.
  3. Removes the can and the attached lid.
  4. Removes the lid from the can.
  5. Ties the bag.
  6. Pulls back and latches the arms. Arms are mechanically bound, so pulling back one arm also moves the three other arms.
  7. Removes the bag from the can and places into storage. Waste could be stored for medical or scientific evaluation or reuse, or ejected.
Cleanup

Preparation for next use

After removing the used bag, the crew member:

  1. Places a new bag inside the can, just like changing a trash bag.
  2. Unlatches the arms.
  3. Attaches the lid to the can.
  4. Slides the lid/can assembly into the base.
  5. Closes the hatch.
  6. Cleans the seat belt (if necessary).
  7. Stores the belt in the compartment at the base of the toilet. When the compartment doors are closed, the belts are sanitized with UV light.
Toilet preparation

Diagram

Detailed view of the underside of the toilet (bottom cover hidden)

Benefits over existing solutions

This design has the following benefits over existing solutions:

  • Lower power consumption
  • Less noisy
  • Eliminates crew exposure to a vacuum
  • Requires no water
  • Relatively compact - volume of 0.11 m3
  • Lightweight - most of the toilet is empty space for accommodating waste
  • Fewer points of failure due to the lack of extensive plumbing and relatively small number of components
Prompt: Please upload an image of your design.


Prompt: Please discuss in detail how your design will: (i) work in both microgravity and lunar gravity, (ii) accommodate female and male crew, (iii) be easy to use and maintain, with low noise, low odor, and fast turnaround time, (iv) allow for transfer of collected waste to storage or external vehicle disposal

Toilet function in microgravity and lunar gravity

Powering on the toilet engages a Van der Graaf machine in the toilet's base to generate a high voltage electric charge. Positive charge will flow to the conducting toilet seat. Negative charge will flow to the base of the can. Since the crew member is in contact with the charged toilet seat, that crew member, as well as his or her waste, will also become positively charged. The voltage gradient between the bottom of the can and the crew member produces an attractive force that will lure the waste into the bag-lined can.

By design, waste that comes into contact the can will rapidly begin to lose its positive charge. Like household trash bags, waste bags in this toilet are made out of polyethylene. The dielectric strength of low density polyethylene is 21.7 MV/m. A bag thickness of 1 mil (0.0254 mm) gives a breakdown voltage of just over 500V, several orders of magnitude below the voltage produced by Van der Graaf machine (typically in the 100s of kV). Therefore, the waste bags should not meaningfully inhibit conduction between the charged terminals.

The benefit of not lining the base of the can with a better insulator is that it prevents excessive static charge build up between charged terminals that may pose an electrical safety hazard. The drawback, of course, is that waste that reaches the bottom of the can loses its charge and therefore most of its attractive force. Although human waste is largely made up of water which will maintain a small attractive force to charged objects because of its polarity, this force will likely not be sufficient to hold the waste safely.

To secure the waste, the toilet utilizes centripetal force and a can with concave sides. Powering on the device will spin the can along its vertical axis. The spinning produces an outward force on any waste that comes into contact with the can. Waste will then flow to the outside of the can and remain contained due to the can's shape. Because the can is closed on the bottom, the shape of the can allows operation in both microgravity and lunar gravity. This shape provides 13 liters of waste storage in micro gravity and even more in lunar gravity.

Correction: The below graphic is requires a correction. Centripetal force is an inward directed force, so the purple arrows are in the wrong direction. Alternatively I could have specified centrifugal force, which is an apparent force in the outward direction. The operation principle is the same because both of these terms refer to same force in different reference frames. I made this error in one of the other images below as well.
Cross section of can operation in different gravitational environments

Female and male crew

Both male and female crew members will use the toilet in the same way they use a traditional western toilet (sitting-only for males). The spray pattern difference between sexes sitting on the toilet should not affect the toilet's function.

There is not any difference in male and female urine, feces, or vomit particle size and composition. Menses have a similar enough particle size and composition to urine, feces, and diarrhea, so because the toilet can capture these waste types, it can also capture menses.

Easy to use and maintain

A traditional western toilet is familiar to crew members and therefore will be easy to use.

The toilet has relatively few components, so it is unlikely to require maintenance.

Noise

Elimination of a traditional space-toilet vacuum component results in major noise reduction. Since the toilet doesn't need to drive a fan, the motors that produce the centripetal force and that engage the Van der Graaf generator turn at much lower RPM (rotations per minute), reducing noise.

Though I was unable to find an exact statistic for the noise level of a Van der Graaf generator, videos suggest that their operational volume (without shock noises) are well below human conversation level (60db). If required, further sound reduction could be achieved by encasing the generator with sound insulating materials.

Odor

Waste bags are changed after each use. Since, with normal use, the bag is the only part of the toilet that comes into contact with waste, odor between uses should be very small. In case of accidents (bag tears, spray) the parts of the toilet most at risk from being soiled - the can and the lid - can be detached from the toilet for cleaning.

Electrostatic force has an outsized effect on smaller particles that can cause odor; you can verify this mathematically with the calculator in the next section. This is why electrostatic filters such as N95 masks are effective at filtering small particles. The active electrostatic system should attract small waste particles and reduce odor during active toilet use.

When the toilet is powered off, the shrinking centripetal force allows the spring loaded arms attached to the can to seal the waste bag and prevent waste and odor from escaping the toilet. The sealed waste bag should effectively contain all waste and odor.

Turnaround time

It takes me about 90 seconds to empty the small trash bag next to my desk and replace it with a fresh one; my wife says I'm slow.

Prompt: Please discuss in detail how your design will: (i) capture and contain urine, feces, vomit, diarrhea, and menses, (ii) Stabilize urine, (iii) accommodate simultaneous urination and defecation, (iv) accommodate the needs of 2 crew members for 14 days, (v) accommodates the use of toilet hygiene products, (vi) clears previous waste content prior to next use, (vii) defines how often the collections system must be replaced or disposed of in the mission

Waste capture and containment

Capture

Three forces can draw waste products into the can:

  1. Waste expulsion force
  2. Gravity
  3. Electrostatic force

Waste expulsion force is not reliable for all waste types. Gravity is only relevant in a lunar environment. Therefore, the design depends upon electrostatic force.

Electrostatic force

The toilet consists of two separated charged surfaces, similar to an electrical capacitor. Therefore, we can approximate the force between the disk-shaped terminals using this formula:

0AV2 / 2d2 ) (1 + 2d/ D)

ε0 - permeability of freespace (8.85 * 10-12 Nm/C2)

A - area of the plates (0.785 m2)

V - voltage between the plates (160kV)

d - distance between the plates (0.28 m)

D - diameter of the surfaces (0.5 m)

Non-constant values are derived from the geometry of the toilet, except for the voltage.

Commercial Van der Graaf generators produce ~350kV. An ad-hoc 125kV Van der Graaf generator has proven operational on the ISS. So, 160kV was chosen as an achievable voltage resulting in worst-case capacitor discharge energy at ~0.32 joules (see Safety section).

Therefore, the toilet generates 2.4 Newtons.

This is an approximation of the force acting on the human, but not of the forces acting on the waste. Waste has a different capacitance for charge based on its size and shape. Once expelled, the electrostatic charge applies two distinct forces to the waste:

  1. Repulsive force: positive charges in the body and waste repel each other, forcing the waste into the can.
  2. Attractive force: negatively charged can attracts the positively charged waste.

Because of the principal of superposition, these forces can be summed to determine the net force acting on the waste.

Electrostatic forces acting on waste

Peak force is applied at the two most important moments: right after waste leaves the crew member (so it moves in the correct direction) and right before it makes contact with the can (to ensure good contact with the can's bottom).

Using Coulomb's law, the equation for capacitance of an isolated sphere, and some simplifying assumptions, we can estimate the force on waste particles of any size at different voltage levels and at various positions within the toilet.

Simplifying Assumptions:

  • Waste is conducting
  • Density of waste is the same as the density of water
  • All charges behave as point charges. In reality, we have a disk charge (can base), an approximate point charge (waste), and a whatever-shape-a-human-bottom is charge.
  • Waste is a spherical droplet. This is accurate for smaller waste (e.g., urine droplets), but less accurate for larger waste (e.g., feces).

The calculator below quantifies electrostatic forces for waste particles.

After experimenting with the calculator, we draw two conclusions:

  1. The acceleration is greater on smaller particles than on larger particles.
  2. There is relatively little force when the waste is in the middle of the can (furthest from the charged terminals), but moderately significant force at the ends. For example, 1cm radius waste sphere will experience ~1.2 m/s2 of acceleration when within 1cm of the crew member or from the base of the can. That's ~12% of Earth's gravitational force, which plays a significant role in the motion of the waste, especially with limited gravitational forces at play.

Containment

Once waste comes into contact with the can's bottom, friction causes the waste particles to move in the same direction as the spinning can. This imparts a centripetal force on the waste and traps it on the outside of the can.

Splashback is a concern, but it is mitigated by two factors:

  1. Splashback magnitude is reduced when liquid approaches a contact surface at an angle. The angle from the crew member to the can is almost always not 90 degrees because of the angle at which humans urinate.
  2. Although waste will begin to lose its positive charge when it contacts the can's surface, it will still maintain some attraction to the charged surface of the can because of water's polarity.
Top-down view of waste coming into contact with the side of the can

Though the centripetal action is the primary force holding the waste, the cohesion and adhesion of the waste and the bag will also assist.

Urine stabilization

The toilet mixes waste materials, but during long space flights, separation and reuse may be essential. The design allows for separation using the principles of a separation toilet via three modifications:

1. An additional inner can for dedicated collection of non-urine waste.

Inner can with additional arms

2. A toilet seat insert which positions the user properly to ensure waste ends up in the proper can.

Top down view of toilet with separation modifications

3. Updated crew member operations. Crew members will:

  1. Add urine stabilizing chemical agents to the urine compartment after powering on the toilet but before using it. This can be automated.
  2. Seal waste bags two times when emptying.
Modified emptying procedure:
A) Waste deposited in two separate compartments
B) Arms close, sealing compartments from each other and the exterior; first seal is made
C) Top arms are opened; second seal is made
D) Bottom arms are opened; waste bags removed for transfer to waste processing/storage

Other considerations

Electrostatic forces are sufficient to attract both urine and feces, supporting simultaneous urination and defecation.

The toilet supports disposal of toilet hygiene objects, which have smaller attractive forces than waste particles but have reliable downward force from the crew member's hand. The asymmetry caused by adding toilet hygiene objects or feces to the spinning can will result in vibration, but this is acceptable because the can spins at a low RPM.

This toilet can accommodate use by 2 crew members for 14 days. Waste is stored in bags, so mission length is not limited by toilet capacity, but rather by external waste storage capacity.

Previous waste content is removed by the crew member via waste bag prior to next use. Waste bags can be stored for medical or scientific evaluation. They can also be ejected.

Prompt: Please discuss the safety measures in place to ensure that during nominal use or in the event of a system failure: (i) crew handling of waste materials during maintenance or system use is minimized, (ii) crew members are not exposed to vacuum

Crew handling of waste

The arms of the can are slightly off-center. The off-centered arms allow spinning to produce an outward force on the four arms. Because the arms are on the outside of the waste bag, the whole assembly acts as centripetal "valve" on the bag that is open when the can is spinning and closed when the can is not. In case of unexpected power loss (and lack of spin), the centripetal valve will close off the waste bag to the outside world. Even in nominal use, powering off the toilet seals the waste bag so crew members do not handle the waste materials.

Toilet can in the closed configuration (left, not spinning) and open configuration (right, spinning). Bag not pictured.

Exposure to vacuum

Crew members cannot be exposed to a vacuum.

Other hazards

Electrical hazards

In lieu (ha ha) of the vacuum, the electrostatic system produces some hazards of its own. The high voltages, but low current, produced by the Van Der Graaf generator itself are not considered medically dangerous to healthy adults. However, there are a few important safety issues to address.

Catastrophic capacitor discharge hazard

First and foremost, static charge from the generator built up in a high voltage capacitor (like our toilet setup) can be dangerous (e.g., a Leyden jar). Estimates for the maximum safe electrical energy discharge via capacitor vary from 1 joule to 10 joules. For reference, AEDs discharge ~150-360 joules. TAZERs discharge 0.36 - 1.76 joules depending on the model (though with other characteristics designed to incapacitate, such as proprietary waveform, multiples discharges, etc.). Using the capacitor model described in the previous section, the energy discharged by an empty electrostatic toilet in a worst-case scenario (complete breakdown of electric insulation, fully charged capacitor, with a human in the current loop), is estimated using the following equations:

Capacitance (C) = ε0A/d = (8.85 *10-12)(0.785)/(0.28) = 24.8*10-12 farads

Energy (E) = 1/2 * C V2 = (0.5)( 24.81*10-12)(1600002) = 0.32 joules

Note: Variable definitions are described in the previous section

The resulting capacitance for this device is ~25 picofarads, which puts the maximum energy discharge at ~0.32 joules. This amount of energy is very unlikely to pose and electrical health hazard.

Routine usage capacitor discharge and continuous current hazard

Even shocks that aren't dangerous can be uncomfortable. If sufficient air remains between the "human terminal" and the "can terminal" to prevent the electrical breakdown of air, there is minimal shock risk from using the toilet. If a person completes the circuit by bridging the terminals with their waste, that person is at increased risk for receiving a shock.

Aside: In particular, urine is the most likely to form a bridge. However, note that a continuous stream of charged urine is unlikely, as repulsive electrostatic forces will quickly split a charged stream into smaller droplets, though this effect is not an important part of toilet operation.

To mitigate this risk, the bottom of the can is insulated from the negative terminal of the Van der Graaf generator by a 1MOhm resistor. Such a resistor is common in electrical grounding devices to distribute the electrical discharge over time. After the initial electrical discharge into the resistor, if the circuit remains closed, the user will still be subject to continuous current from the generator. Commercial Van der Graaf generators produce on the order of 10 micro amps of current. This is well below the threshold of perception for humans (~ 1mA), so crew members should not experience any tingling or electrical sensations.

Electronic device hazard

The static charge buildup is a concern for crew comfort as well as for other electronic devices crew members may encounter after using the toilet. After use, crew members should take care to ground themselves and the toilet to either the space craft's hull (as in the ISS) or to a planetary/lunar body. Grounding prevents crew members from carrying a charge with them after they use the toilet. This could happen automatically if the toilet is attached to a ground.

Furthermore, opening the toilet hatch, which should happen after each use when replacing the waste bag, mechanically connects the two charged terminals inside the toilet to eliminate voltage gradients between uses. This post-use discharge prevents shocks while replacing bags and while performing any toilet maintenance.

Belt contamination hazard

The seat belt makes toilet use more convenient for crew members by allowing hands free operation. However, for faster turnaround times, the same belt is shared between crew members. To promote cleanliness and limit bacterial or viral transfer between crew members, belts stowed in the base of the toilet are bombarded with UV light between uses. To prevent crew exposure, this bombardment only occurs for a short period of time after the lower storage doors are closed.

Prompt: Please discuss the technical maturity of your proposed toilet design. What TRL would you assign it? Please provide a supporting rationale and/or evidence for this rating. Why do you believe this could be developed and integrated into a lunar rover in the next 2-3 years?

The highest TRL that I can justify for this toilet is in the range of 3-4.

Let's start climbing the ladder:

TRL 1

This design is based on existing technologies and well understood principles: electrostatic attraction/repulsion and centripetal force. Electrostatic force is governed by Coulomb's Law and centripetal force is a consequence of Newtonian physics. These phenomenon have been used in engineering applications for at least the last two centuries.

TRL 2

This level is satisfied by this design. This document describes a practical use for electrostatic and centripetal forces as a means to attract and contain human waste in a low-g or zero-g environment. It includes high level analysis that suggests its feasibility and enumerates the benefits of such a design.

TRL 3

There is experimental affirmation of this design's primary technological component: the effect of static electrical charges on water droplets in a micro-gravity environment. On expedition 30 to the ISS, Astronaut Don Pettit demonstrated the behavior of these forces using knitting needles, water particles, and the Triboelectric effect:

This video and its corresponding research paper show the feasibility of charging water particles via a Van der Graaf generator and show that such a charge produces a meaningful amount of force on the droplets.

There are also demonstrations of centripetal force being used to develop air pockets in the center of liquid/solid materials, the same effect used by the spinning can to contain the waste materials. Astronaut Jeff Williams (skip to 2:37 to see the demonstration):

Separation toilets have been tried and tested in gravitational environments. The major difficulties that researchers encountered are sociological: men not willing to sit, children misunderstanding toilet use, unwillingness to sit on public toilet seats. This should not be a problem for trained crew members.

Example separation toilet

To me, these demonstrations constitute "experimental results validating predictions" that satisfy the exit criteria for TLR3. These experiments were performed in a relevant zero-g environment, which further strengthens this design. However, the climb stops here. For hardware, TRL4 requires that a "low fidelity system/component breadboard is built and operated". No integrated system prototypes of this specific toilet design exist, to my knowledge. So, based on the rubric, the highest technology level that I can justify for this toilet is in the range of 3-4.

Other considerations

There are relatively few components compared with, say, this diagram of toilet used on the Shuttle. Fewer components require less lead time, design work, and testing to implement, which substantially increases the chance that such a design can be developed and integrated in the next 2-3 years.

Areas of future focus

Though there is a rich scientific and mathematical history of attempting to understand charges and the forces they produce, it can be difficult to analytically calculate these forces when the geometry is complex. The Don Pettit video's research paper suggest that finite element analysis can accurately predict these forces, but that has not been performed in this design.

Developing a safe device seems viable, as described by the high-level analysis in this document. However, this dimension needs more detailed attention. For example, can some unexpected component of the toilet produce a charge capacitance that might prove harmful to the crew?