Power Appliance Guide by /u/Leuku
Though the DM and the Player should feel free to come up with their own Power Appliances, here is a general guideline on the basic principles behind some of Power Appliances above.
Each engine corresponds to a specific type of function. Flame can heat as well as thrust. Lightning can provide fine control and motor functions. Cold absorbs energy and slows processes.
The bigger a power appliance is, the more minimum number of engines it will require. Things that are handheld, like a Power Tool, or things that are no greater than size medium, like a Refrigerator, are likely to need no more than 1 engine. Increasing the size of a Power Appliance, such as with Rocket Barding, may warrant additional engines.
How complex a power appliance is may also affect the minimum number of engines it requires. The Grapnel Gun, despite being handheld, requires a minimum of 2 engines, due to the complexity of its motor mechanism. In comparison, the Refrigerator, despite being much larger,, has a very simple function and thus does not warrant any additional engines. The Portable Forge requires not only tremendous heat but a means of carefully regulating that heat, hence why it requires both a flame and a cold engine.
Sustained vs. Temporary
Some of the Power Appliances can be separated into two categories - “Sustained” and “Temporary”. Power Tool and Power Torch are examples of the former. All of the power appliances with the word “rocket” in their name are examples of the latter.
Sustained power appliances have no limit on their duration. However, if they grant you improved performance on something like Power Tool does, then they may warrant a chance to break. Other sustained power appliances, like the Home Brewery, may not warrant a break chance at all. If the benefit, size, and complexity of a sustained power appliance is small, such as with the Power Tool, then the chance to break should be small, too. You might put some kind of check against a set DC every time the power appliance is used, with a failed check resulting in it breaking. For the Power Tool, I created a variant mechanic of this by having a very easy DC of 5 be set against the advantage the tool gives you. It is rather unlikely for the greater of two d20 rolls to be 5 or less. If you happen to make a larger and/or more complex sustained power appliance that grants advantage on a check, consider using this variant mechanic with an increased DC.
Temporary power appliances trade a chance to break with a limited duration. Doing so may warrant consuming an additional resource like spellslots. All of the rocket series power appliances are from this category. There is no chance that they might suddenly explode, but they are only usable for a certain amount of time before you have to feed another spellslot into them. Note how attempting to control them warrants an ability check against a medium DC of 15. Temporary power appliances have more powerful effects compared to sustained power appliances, but are often more unwieldy as a result. Additional, different types of engines can improve the performance and handling of such power appliances. Lightning engines can be used to improve handling by reducing the DC to control it. Cold engines can improve duration.
There is a lot of flexibility to power appliance design.
Take a careful look at the Rocket-Powered Vehicle vs. Rocket
Carriage/Wagon. They all use two flame engines as a
minimum, but the latter two are size Huge while the
former is only size Large. *
Then notice how the
Rocket-Powered Vehicle requires a 1st level slot at minimum,
compared to the Rocket Carriage/Wagon requiring a
3rd level slot. I wanted to make Rocket Wagon/Carriage
accessible slightly sooner, so instead of increasing the
number of minimum required engines to 3, I increased the minimum spellslot level consumed to 3rd. Flavor-wise, this is because I assert that bigger objects need more base energy to be moved, which can compensate
for fewer engines. In this way you can make effects
accessible sooner by increasing costs. You may then
later allow a Rocket Carriage/Wagon with 3 minimum
flame engines that consumes a minimum 1st level slot
Wherever you can, try having power appliances mimic existing mechanics and resources. Besides the Hand Rocket, all of the rocket series power appliances’ speeds are based on draft and riding horses.**
* No longer accurate, needs updating