FinalFan

Members
  • Content Count

    246
  • Joined

  • Last visited

Community Reputation

151 Excellent

About FinalFan

  • Rank
    Spacecraft Engineer

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. FinalFan

    The Landing Legs & Gears Is Killing Me!

    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.)
  2. FinalFan

    Oberth > Isp?

    @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.
  3. FinalFan

    Oberth > Isp?

    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)
  4. FinalFan

    Efficient Ratio Booster/Payload Weight for LKO ?

    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.
  5. FinalFan

    Efficient Ratio Booster/Payload Weight for LKO ?

    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.
  6. FinalFan

    Efficient Ratio Booster/Payload Weight for LKO ?

    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.
  7. FinalFan

    Efficient Ratio Booster/Payload Weight for LKO ?

    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.
  8. FinalFan

    Ideal Local Mining Operation?

    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.
  9. FinalFan

    Having trouble building a good heavy SSTO.

    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.
  10. FinalFan

    Oberth > Isp?

    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.
  11. FinalFan

    Oberth > Isp?

    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.
  12. FinalFan

    Oberth > Isp?

    Everything you said seems basically true (I got slightly but not significantly different delta-V numbers when I checked you), but it's a very different situation than either my actual situation or my hypothetical NERV + other scenario. The reason that burning both engines (or even the Terrier alone!) is better than NERV-only in your scenario is, as you said, because you eliminate a bunch of mass in the form of oxidizer. But in the scenario, the question was whether ore should be refined exclusively into liquid for the NERV or partially into oxidizer for the LFO engine as well. There should be no oxidizer left over since you simply wouldn't refine any that you weren't planning to use, and thus both should end at the same dry weight. And in my actual scenario, of course, there was no issue of deadweight unusable fuel at all. In both of these cases, clearly the efficient engine will always win out unless there is some other factor at play like a TWR requirement (such as on Tylo) or the Oberth effect as in the OP. I'm sure you were aware of the following, but it's worth noting that while the NERV and Terrier in your example make for a very simple average Isp (since they have the same thrust, you just add them up and divide by two), it can mislead readers into thinking it always works this simply. In fact, you have to weight them by thrust. For example, one Rhino (2000 thrust, 340 Isp) and one Vector (1000/315) average to ((2000 x 340)+(1000 x 315))/3000 = 331.67 Isp for the whole 3000-thrust combination. [edit: @Zhetaan's post below gives the real, somewhat more involved, way to calculate average Isp. The struck-through method will give you an approximation that I think should be fairly close as long as the engines' Isp isn't too radically different (i.e. Dawn or NERV vs. anything other than themselves).]
  13. FinalFan

    Oberth > Isp?

    Did a little math, and I reckon that, with the difference in Isp, the less efficient engines "wasted" enough fuel to run the efficient engines for about 1.875 seconds. So the question becomes "is the difference in Oberth effect worth more than 1.875 seconds of free thrust?" Based on my foggy recollection of the maneuver's total cost, and given Brikoleur's "single digit" estimate ... no, engine efficiency was better unless I completely screwed up my math or Brikoleur is off by a lot. I calculated close to 20m/s so a minor mistake in the maneuver dV doesn't change the outcome. But there were a lot of moving parts in that calculation and it's not inconceivable that mistakes accumulated or my logic was flawed. Or Brikoleur could be wrong. Is there any relatively easy Oberth estimator around? Either way, thanks for inspiring me to think about this. [edit: got home and I think I messed up the fuel flow difference. I think it's more like over 4 seconds of free thrust.]
  14. FinalFan

    Oberth > Isp?

    Like I said in my reply to Brikoleur, I'll need it at the destination. The extra engines are limited application but higher TWR compared to the main engines which are heavier but more efficient. If we take as a given the single stage nature of the craft, is there still a trick I'm missing?
  15. FinalFan

    Oberth > Isp?

    Thanks everybody for all the answers! It sounds like someone in low orbit (as opposed to a flyby) has considerable leeway on either side of periapsis in terms of taking advantage of the Oberth effect, so it probably helped to keep the less efficient engines turned off. Brikoleur, the less efficient engines were added for extra punch on the Tylo landing I wanted this vessel to be capable of. It should have a local TWR of 1.45 fully loaded as I recall. Tylo and Laythe are the only moons I'd need that kind of thrust for. That said, I am very silly and the thing is a gigantic beast. Its name is "Admiral Perry" because of the muscular exploration it's meant to do. It's designed to deliver up to two dozen kerbals in style to anywhere short of Eve within its 5,500 deltaV reach and carries enough ISRU to do it all over again without having to wait a year. The more and less efficient engines in question are Rhinos and Vectors. @Rayder, I guess I was insufficiently clear when I said "transfer burn from Kerbin to Jool". What I meant was the actual burn that made me leave Kerbin and cross Jool's path. But your information was very helpful. I will certainly keep it in mind for my future maneuvers around Jool's orbit and other places.