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This craft is a 1:1 scale full stock replica of the H-4 Hercules, more popularly known as the Spruce Goose. This craft, while relatively simple in its design, ended up being one of my more involved replicas, with a large amount of work and experimentation put into replicating the design. The craft is powered entirely by 8 of my R-4360 turboprops, which combined produce 3200 kN of thrust. The wings of this replica represent my first attempt at fully constructing a custom aerofoil. The technique used is effective, but I have since improved upon it,and you can expect more advanced versions of it in my future replicas. One interesting quirk of these wings, is that the enormous amount of drag and lift they produce means that this craft flies very smoothly at its top speed of 26 m/s, yup. Performance wise, this craft did not meet up to my expectations. I had intended for this craft to be capable of water takeoff and landing. With this in mind I kept the craft as light as possible, being about half the weight of my other replicas of similar size and complexity. Unfortunately, this craft was not able to land, nor take off from the water, meaning that this replica is basically completely aesthetic. While I am somewhat disappointed by this craft, I learned a great deal whilst making it, and it was ultimately a valuable experience. Download: https://kerbalx.com/Kronus_Aerospace/Kronus-H-4-Hercules-Spruce-Goose Craft Mass: 310.95 tonnes Part Count: 2141 parts I would love any suggestions about what *big* plane I should build next!
Before Reading I'd recommend that you'd watch Bradley Whistance's video on his stock prop speed test. The method he outlines is not relevant to the actual math work, and simply affects the values I will be plugging in. https://www.youtube.com/watch?v=J7oc1FLnWlY&t=438s I will only discuss the resulting data in this thread to keep things simple, if you have specific questions about the math, let me know. With that out of the way, let me preface this discussion. The dominant method of powering stock props is by using RTGs, which makes sense. They continuously generate power, allowing stock props to run forever, this is great for Duna, Eve, and Laythe exploration. However, I commonly see this used on props built for fighter craft, transport aircraft, and others. While there are some cases where endless flight is desirable on Kerbin, such applications certainly do not qualify. Most people don't fly a single stock prop fighter for hours on end (without crashing). This leads to the question, are RTGs the most efficient way of powering these props? Short answer, no, but I'll explain in more detail. I am taking efficiency to mean weight in this conversation, although other uses of the term, such as part count, may also be referenced. First we must establish all of the possible methods of powering a stock prop: RTGs (duh), batteries, and fuel cells. RTGs we already discussed. Batteries would simply run the engine using their stored electric charge, and the flight would end once they run out. Fuel cells would burn LFO (Liquid Fuel + Oxidizer), continuously generating the engines power needs whilst draining LFO from on board tanks. Of course, I wouldn't even consider the fuel cells to be an option unless I had a good reason. Doing some simple math, we can find that all batteries hold 20,000 units of electric charge per tonne. How much electric charge, then, is LFO equivalent to per tonne? It depends on which of the fuel cells you are using, but for the small fuel cells this comes out to 79,934 units of electric charge per tonne of LFO, and for the large fuel cells it is 81000 units per tonne. Looking at the raw data, fuel cells are the obvious winner. However, the additional weight associated with fuel cells throws a wrench into the works, and in either case this tells us nothing about how RTGs stack up. So I will analyze these three choices in an applied setting, where I will test their mettle in a hypothetical prop that utilizes 10 of the 1.25 meter reaction wheels with 2 dumpling fuel tanks used as bearings. I did these calculations presuming that this engine would also utilize the trick described and demonstrated by Bradley Whistance's video. Using this method the reaction wheels consume roughly 2.73 times their normal power (according to my own testing), I'm using ball park numbers so any small discrepancy is irrelevant. RTGS: The 10 reaction wheels consume 13.65 units of electric charge per second, as such the engine would normally require 19 RTGs to run continuously, which themselves would weigh 1.52 tonnes. BATTERIES: To run this engine for 1 hour you would need 49,140 units of electric charge. This would require 2.457 tonnes of batteries, so those certainly aren't the best solution, although using the largest battery bank available, this would only require 13 batteries. Since the weight of the batteries required scales directly with time of flight, they are likely the most efficient in very short flights. FUEL CELLS: The smaller fuel cells generate 79,934 units of electric charge per tonne of LFO, meaning only .615 tonnes of LFO is required to run the engine for 1 hour. This amount of LFO is almost perfectly held by the 2 dumplings in the bearing, plus two oscar tanks which altogether hold .62 tonnes of LFO. In total the tanks would weigh .698 tonnes (including dead weight), and this engine would require 10 of the small fuel cells to run continuously. This adds another .5 tonnes to the total weight, bringing it up to 1.198 tonnes. As well as 14 parts, but really that's 12 since the dumplings have to be there regardless. In every possible way, the fuel cells are more efficient, while the use of dumpling bearings may seem to bias the fuel cells, in reality this does not affect the weight, and only affects part count. Using a different bearing type would only add 1 additional part, making it on par with batteries and superior to RTGs. None the less, it is far superior to both in terms of weight. This leads me to conclude that Fuel cells are the superior method for powering stockprops intended for short to medium flight times. It should also be added that it is of course possible to mix these methods together. In this one instance, it is actually beneficial. While 10 fuel cells are required to meet the continuous power generation needs, this is only by a small amount, the raw value is 9.1 fuel cells. This adds the equivalent of 540 additional electric charge generation required over the hour of flight. Since fuel cells come with 50 units of electric charge storage each, this means that 9 of them would have 450 units total, leaving us 90 units short. These 90 units can be accounted for with a single of the smallest battery pack (which has 100 units). Weighing in at a mere .005 tonnes, this change leaves us with the original part count of 12 and a reduced mass of 1.153 tonnes. So yes, it is possible that adding batteries will lead to a net increase in efficiency, however, the instance described above is the only type of scenario where this is actually the case. So limited is this possibility that even if the real value turned out to be just 9.3 instead of 9.1, then using batteries would result in a net increase in weight. Again, these results may be different for engines of differing sizes. however I can only see fuel cells being knocked from their throne with very different total flight times. For instance, short flight times where batteries' ability to instantly deliver power gives them a major advantage, and long flight times where RTGs' endless power generation eventually overcomes their initial mass costs. This may be obvious already, but just to clarify, the fact that the math above uses reaction wheels affected by Bradley Whistance's technique IN NO WAY affects the actual math or conclusions. The math above is roughly equivalent to if you had simply used 27 reaction wheels functioning under normal conditions. The objective of this thread was to demonstrate that fuel cell powered props are a lighter and more part efficient approach to powering certain stockprops. While it took a decent amount of complex work to arrive at this conclusion, in actual application fuel cell powered props are no more difficult to build or use then RTG props. There's also the small fact that a fuel cell powered prop will not only start out lighter than a RTG powered prop, but it will also get even lighter as LFO is drained throughout the flight, further increasing performance. I appreciate any questions or feedback you may have. If I made any mistakes, please let me know, and I would love to hear your thoughts on the topic. @klond I think this may interest you.
This is a stock 1:1 scale replica of the Pratt & Whitney R-4360 Wasp Major propeller engine. This engine utilizes 40 1.25 meter reaction wheels and produces a modest 230 kN of thrust. The bearing is very strong, and the engine is extremely smooth when operating. Its root-part is a large girder, allowing it to be easily attached to another craft. It should be noted that the thrust value mentioned is when the blades are set to maximize stationary thrust. Download Link: https://kerbalx.com/Kronus_Aerospace/Kronus-R-4360-Propeller-Engine Part Count: 94 (93 without decoupler) Mass: 11.42 tonnes