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About FinalFan

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    Sr. Spacecraft Engineer

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  1. Chalk it up to miscommunication from me to you. Allow me to clarify: your 99% proposal was such a blatant end run around (what I perceived to be) OP's clear intent that I did not think you, an apparently intelligent person, could in good faith actually think a mission using that technique would be accepted as a challenge entry.
  2. Okay, I will assume good faith here. I had thought you were feigning a lack of understanding to highlight your argument, so I'm sorry for misunderstanding. The way I see it is based on intent: you are forbidden to use ions when your motive for the maneuver is to exit the SOI to go to another one. So if your reason for the periapsis kick is to rendezvous with something else in the body's SOI, it is not a violation; if your reason is to leave the SOI, that's an inappropriate use according to the rule. This includes the first of a set of 100 periapsis kicks where the goal is a completed transfer burn. Admittedly, this leaves a gray area where you could use ions for some in-SOI purpose that "just so happens" to leave you in a much higher orbit when you begin your burn. In other words, cheating. There are two answers to this that don't require a blanket ban: one, simply the honor system; two, a modification to the rule saying ion use in-SOI should not leave you in an orbit more than 200km high, or more than 100km in altitude different from before ions were used, or something similar. Would such a rule modification clear up your questions regarding what is and isn't permissible ion usage? (Please note that this is a hypothetical situation, since my interpretation isn't necessarily 100% the OP's intent.) [edit: bayesian_acolyte did a better job than me.]
  3. Well, from context I think it's pretty clear that you would not be allowed to use your 99% idea, as that's directly against the explained motive for the rule. And I also think you're perfectly aware of this. But if someone wanted to use ions for some other purpose unrelated to the main transfer burn it's allowed. I don't see why the ban should be broader than necessary. Wording it so as to be unassailable by the pickiest rules lawyer would indeed be a pain, but in a small contest where the judge is the Creator and available to answer questions I don't see a real problem.
  4. I agree with your decision to avoid individualized bonuses for each achievement, but a totally flat bonus might be going too far in the other direction. Maybe two or three classes of rewards? For instance, Ares I sounds like a fun little touch to add, but I imagine it's much less difficult than Water Park. On the other hand, Beach Bum is the easiest of all with a landing altitude requirement of five hundred kilometers
  5. I suspect that part substitution would end up being one of those ideas that sounds simple but is actually very problematic. Locking, though, I have hope could be done. (Toggle the travel settings instead of changing the part.)
  6. @Zhetaan Wow! Thank you for the extremely educational post. I had been secretly wondering what the harmonic mean was. Referring to the original question, you did answer it. I believe I had been misled by reading too much into my three example data points that had multiple variables. Clearly the biggest difference between my oversimplified model and the correct one is illustrated in extreme cases that expose the difference in limits that are approached, 2a in one case and b/2 in the other. The most non-intuitive effect (for me) of doing things the correct way comes in if the high thrust and high Isp engines are the same, because I don't instinctively consider fuel flow and as conditions get more extreme the fuel flow on low Isp (higher fuel flow) engines will tend to overwhelm the high Isp (lower fuel flow) engine even if it's the lower thrust one. But I suppose this is what causes the upper limit on combined Isp in the first place. I read your post at work, thought about it from time to time, and I'm back home now. I can't remember the last time I rewrote a post so many times based on increased understanding, even if it's partly due to divided attention.
  7. Wow! I'll have to remember that somehow. But is it fair to say that the error (produced by doing it the wrong way) increases dramatically, perhaps exponentially, as the Isp difference increases? I compared the same engines (Terrier + Reliant) using the method I had formerly thought was accurate and came up with 317 (vs. 316, tiny difference). Comparing a Mastodon + Wolfhound setup seems to yield about 310 versus 316.5 the wrong way. (6.5 is still relatively tiny compared to 90.4 difference between NERV/Terrier calculations despite the Isp difference being over 25% as large)
  8. Sorry, but you presume incorrectly. The Cubs are much smaller and narrower than the Thuds, allowing three to be crammed in approximately the same space as a single Thud. Maybe I'll try it again with dead weight to make up the difference; the 3-Cub formation is actually at a small launchpad thrust disadvantage versus a Thud, so it'll be interesting to see how that goes.
  9. Thank you very much. That makes much more sense than what I was imagining. I was thinking of the old "hang things off the side" construction even though it's obviously not necessary with radially attachable engines. Three tries were not enough for me to replicate your success, no doubt due to pilot error. I also had terrible issues with wobble from the upper portion. I tried a version using a 1.875m payload and it seemed to be flyable, but I still did not immediately succeed in orbiting without help from the payload. My "what about Cubs instead?" version was easily able to do it with 1.875m and 2.5m payloads, replacing the 3rd, 4th, and 5th stage Thumpers with groups of 3 Cubs per Thumper. Fins were not necessary; these tests produced some of the smoothest no-control gravity turns I've ever done. (I discovered this when trying to counteract SAS oversteering; the Cubs have really outrageous gimbal for this vessel.) Unfortunately uncontrolled gravity turns tended to not be shallow enough for best fuel efficiency so my successful tests were manually controlled. In the end I set all the Cubs to 50% gimbal. Initial tests made orbit with between 14% (1.875m) and 19% (2.5m) fuel remaining in the last launch stage. However, there was a problem: My 13.88t payload vessel had less launch weight than your vessel, biasing the test in favor of the Cubs. I then adjusted it for a 76.323t total weight (14.683t payload), with 2.16t difference due to the Cubs weighing less than the Thumpers they replaced and 0.001t lost due to inaccuracy. It made orbit with 13.5% fuel remaining. The Cubs' weight advantage may have helped, but the biggest difference was probably the Isp advantage.
  10. I have a lot of trouble picturing your craft. My attempt to recreate it did not have the right tonnage. In any case, I think you may have misunderstood my comment about the Skipper. Clearly the Cub has much less raw thrust, but their performance relative to size is pretty similar: —Engine TWR ASL/VAC is 19.33/22.09 versus 19.12/22.66 —Isp ASL/VAC is 280/320 versus 270/320 In fact, you can slap 16 of them around a 2.5m stack and have almost the equivalent of a Skipper (with extreme vectoring) ... for 16,000 funds instead of 5,300, but c'est la vie. If cost is no object, I believe Cubs would be better than Thuds outside of the bottom stages. There are two main difference between the Cub and the Thud: you need 3 Cubs to be as strong as 1 Thud; and the Cub has decent vacuum Isp. The Cub also has much higher engine TWR. 3 Cubs also have the same VAC thrust but about 7 less ASL; and Isp that is a bit worse ASL but evens out at just 2400m.
  11. Aren't the aerodynamic issues an even bigger reason? It's true there aren't all that many radial engines but you can get reasonable quality out of the ones that are there: the Cub from MH has comparable performance to the Skipper in TWR and Isp.
  12. I have been using the Mun because it's a shorter trip than Minmus, but that was predicated on the moon in question being the refueling point. (And really, Mun orbit seems to this noob like an all-around more sensible staging point than Minmus orbit for interplanetary operations with its shorter orbital period and lack of inclination.) The extra fuel spent shuttling the miners to and from the surface wasn't a big loss for me since they weren't exactly operating at capacity anyway. However, if we're talking about filling a tanker that goes to LKO with no tight schedule, then clearly Minmus operations are better. I haven't been doing that—not systematically, anyway—but it seems like a good plan. Time to start shifting gears in my operations.
  13. re: 3. I think your pedantry is slipping! "SSTO to wherever" is simply a further restriction; it not only gets T.O. but also to elsewhere. If you really want to go down that road, SSTO is arguably not a thing but a property of a thing, i.e. you can have an SSTO rocket, and that rocket will be SSTO, but you arguably don't have "an SSTO" per se. However, SSTO also meaning a rocket or spaceplane that is SSTO is good enough for me.
  14. I understand the story you're telling. The moral is, "if you've got oxidizer, burn it." I agree—I think I did the math back when I first sent a spaceplane beyond LKO. But I don't see how it's in any way relevant to the OP.
  15. Well, now you've got me curious, @Rayder. Two 2.5m ore tanks and one full Mk1 liquid tank makes the same 32 tons of fuel, but 4.25t dry weight instead of 4. For fun, let's put an empty T400 on there to match the NERV's liquid tank, and with both engines that's 8t. But there's also the converter—a 250, of course, to avoid wastage—but since we didn't calculate probe cores or anything for the original vessel I'll ignore cooling now. So 12.25t dry and 44.25t wet. Plug it in and... Just over 10,000 delta-V. If we had ignored the different weight ratio of ore tanks and the need to add a converter, then it would have been extremely simple to figure out, and 7700 would be obviously way too low: if the ONLY difference was 800 isp versus 345 isp, then we'd have 800/345 times the Terrier's ~5,600 m/s, over twice as much. Periapsis kicking on a burn that's already less than two minutes from periapsis on each side? Not my flavor of crazy. It's come in handy, though, most notably when my first ion probe shipped out. (Thankfully, all it has to do is hit a circular solar orbit for an asteroid detecting contract: a relatively easy target to hit.) I like that fuel tanker idea, though. It's certainly better than sending the tanker after a ship that's already outbound, which I did once.