I recently moved into a travel trailer to lessen the southern California cost of living (and because I like the idea of portable structures as an answer to housing scarcity). This living arrangement sparks my engineering creativity, which is the motivation for this post. Here I discuss RV living from a mechanical and software engineer’s viewpoint.
The following sections explore renting computing power since I no longer can carry a large desktop, modeling the trailer’s computational fluid dynamics (CFD) and mechanical dynamics, setting up an online registry of good boondocking sites, examining the 12 VDC system and figuring out how to add solar and wind power generation to enable off-grid living, improving the insulation and adding a thermostat to the air conditioner, and learning how to get on without my engineering textbooks (which do not fit in the RV).
Renting Computing Power
I no longer have room for a large desktop computer; instead I’m working with a laptop. This is a severe reduction in computing power, so whenever I’m working with a high computation, memory, or disk load I’ll rent computing power from Amazon’s Web Services. Amazon EC2 instances are inexpensive and easy to set up so this strategy is very practical.
Computational Fluid Dynamics
My truck manual limits the amount of surface area perpendicular to the direction of travel that can be towed, presumably due to drag forces. So I tried to use computational fluid dynamics (CFD) to find out if making the front of a trailer more aerodynamic buys one more surface area. I was unsuccessful at this experiment (other than generating cool pictures, below) since I couldn’t figure out how to get the OpenFOAM CFD package to work with compressible flow. First I ran incompressible CFD on a block-shaped trailer model:
Then I ran incompressible CFD on a trailer model where the front is arced:
The pressure at the front appears to be slightly less for the arced model, which says that it is more aerodynamic. However, because this modeled flow is incompressible, I could not really say much about whether towing performance would be improved by using a trailer with a curved shape at the front. Nonetheless, I bought a trailer with an angled front assuming it will reduce towing drag.
Online Boondocking Site Registry
I won’t be boondocking (parking somewhere for the night without utility hookup) anytime soon since I have a day job that requires 40 hours/week presence. I’m staying in an RV park where there are showers and a laundry facility. However, I’ve been thinking of the needs of boondockers, and plan to create an online registry of good boondocking sites, if one does not exist.
The web application for such a registry will have fields for site location and ratings that users can fill in. I’ll have it generate maps if possible. For the infrastructure of the web application, I envision writing it in Python using Django, with MySQL as the supporting database. However, it may be better to use a NoSQL database like a document store—I’ll have to investigate this more thoroughly. For hosting the application, I’ll likely use an Amazon EC2 instance.
Part of my motivation for creating such a site is that I can see myself boondocking in the future, if I find myself between jobs.
Vehicle Mechanical Dynamics
While towing the trailer, I noticed that whenever a semi-truck passed me, the front of my truck was pulled toward the passing semi, and I had to turn the steering wheel to keep my truck straight. This prompted me to explore the dynamics of towing:
In this very simple model, a wind gust from the semi-truck is modeled as force Fair, which rotates the trailer clockwise (about the center of mass which is between the wheels). The force at the hitch pin is also modeled such that it rotates the trailer clockwise when it is positive. For this “back of the envelop” calculation, I ignored the forces caused by the tires meeting the pavement to simplify things—assuming that they slip when lateral forces are applied.
On the tow vehicle, I again ignored the forces at the tires and joined the vehicle to the trailer with the hitch pin forces. I then constructed the above equations of motion and solved for the angular velocity of the trailer. This shows that when the trailer is forced to rotate clockwise by a wind gust from a passing semi, my truck is rotated counter-clockwise toward the passing semi, as observed.
The RV’s lights and fan run on 12 Volts DC, so they can run off the grid, but there are no 12 VDC receptacles inside the unit for running arbitrary 12 Volt appliances. However, I have 12 VDC tools that I would like to be able to run off the grid. I can get around this by removing the fuse box cover and attaching power clips to the DC input terminals for the converter:
The problem with this arrangement is that the negative terminal is dangerously close to a positive one, so that if for example my cat bumps a clip, it might cause a short circuit and potentially a fire. I could insulate the terminal with electrical tape but that is too much of a “hack” for my taste. If I start boondocking regularly I’ll wire in real 12 VDC receptacles, and be sure to add fuses to them to prevent high current loads.
I added a second marine battery when I purchased the RV, so the overall amp-hours I can power is double the original amount (the batteries are identical, both new, and wired in parallel). A power converter below the fuse box converts grid 120 VAC to 12 VDC to keep the batteries charged. I check the water level in the batteries weekly to ensure the water is not boiled away during charging.
My RV does not have solar panels installed, which I’ll add if I start boondocking regularly. I currently have a small six Watt panel (pictured below) which works as little more than a trickle charger. Since I have no charge controller I have to manually check the batteries while using it, and have to disconnect it at night to prevent battery discharge through it (unless I add a diode to the circuit, which has the downside of lowering the current supplied to the batteries during the day). This six Watt panel certainly does not produce enough charge during the day to recover a night’s worth of DC power use.
For true off-the-grid use I’ll buy two 130 Watt panels along with a charge controller. If I can find a charge controller that feeds power back to the grid when I’m plugged in—even better. (It will have to have an inverter built in to do this). I expect the whole package to cost about 1200 USD overall. I’m not ready to commit those funds right now while I’m living in an RV park and have easy grid access, but plan to research the equipment needed now so that I’m ready to buy in case my living arrangements change.
It may be best to design and build a charge controller from scratch, given that I may want to mix solar, grid, and wind power (below) in one system, and I’m not sure if existing controllers can accommodate all three very well. Designing such a controller would be a fun project, but would take a substantial amount of time. If I do this I’ll use an Arduino or Raspberry Pi platform for computing, and use optical relays to separate the computing circuits from the power delivery circuits.
I have never seen an RV with a wind generator installed, but see no reason why a small turbine such as the Primus Air 30 (pictured) cannot be used.
Depending on where I’m staying, I’ll perhaps want a marine-grade turbine. However, my RV is made of aluminum, which is not marine grade, so staying near the coast may be a bad idea no matter what turbine I buy.
I envision mounting it on the bumper and the upper rear of the trailer, as shown in the cheesy image below, with attachments that allow easy removal for when the RV is in motion.
Living in San Diego County requires little use of air conditioning or heating, but while traveling from Texas to San Diego things got hot in the trailer. The “R value” for the wall, floor, ceiling is R-7, which leaves much to be desired. The next time I take a trip across the desert I’ll cover the trailer’s windows with reflective bubble insulation while driving. My thinking is that these window covers can be held on with Velcro for easy removal.
Air Conditioner Thermostat
My trailer’s air conditioner has no thermostat to stop it from running once a desired temperature is reached. This causes problems in the middle of the night when things get cold but I don’t want to get up to turn the machine off. Furthermore, continuing to blow past the point of comfort wastes electricity. To deal with this matter I plan to splice in a household thermostat as soon as I measure the voltage at the AC’s on/off switch, assuming I can make the voltages match. If a household thermostat proves unsuitable, I’ll design a controller from scratch, again using an Arduino or Raspberry Pi system.
Relying on Wikipedia and the Web for Engineering Knowledge
To fit into the RV, I had to get rid of all my engineering and science textbooks. Now I’m relying on the internet whenever I need to look something up.