Jump to content

herbal space program

  • Posts

  • Joined

  • Last visited


505 Excellent


Profile Information

  • About me
    Test Chimpanzee Cage Leader
  • Location
    San Francisco Bay Area, USA
  • Interests
    Thinking about other types of science as a dilletante to avoid thinking about molecular biology as a professional.

Recent Profile Visitors

4,143 profile views
  1. Well I looked it up just now and it was actually 1.25 at launch, but that was for reasons that have nothing to do with the most mass or energy-efficient way to get to orbit. The F1 engines could not be throttled, so they were like SRBs are in this game. If you put that together with the totally un-KSP mass ratios in real life, you've got a rocket for which the chief limiting factor is too much acceleration at the end of the burn. They dealt with this on the Saturn V by shutting off the center engine when they reached 4g, since they couldn't throttle back. As @GoSlash27 pointed out (and I acknowledge), that sort of situation is pretty much the one case where it actually makes sense to have rocket take off with a TWR as low as 1.2. The STS took off at a much higher TWR of ~1.75 1.6, because that is more efficient and the higher payload fraction and the ability to throttle the RS-25 engines mitigated against too much acceleration at the end of the burn. I've taken off just fine with rockets that barely lifted off, so that is definitely not an issue for me. I have tested this pretty exhaustively, both on Kerbin and on Eve, and based on that and a number of discussions in this forum, I think what you say here is just not true. Unless you push your TWR to implausible extremes or fly an excessively flat ascent profile, gravity losses far outweigh drag losses for a well-designed craft. You just have to keep it pointed prograde the whole time. Pretty much the only way I ever use SRBs is to provide that extra oomph at launch. I start with my core stage at full throttle and a TWR of ~1.8, getting my speed to around 200m/s as quickly as ever I can, then throttle back to keep things from getting out of control as I do my gravity turn. Both STS and now SLS pretty much do the same thing.
  2. Well on the upside, I think that with the panels facing backwards you could still probably manage to keep it below that hard limit for the entire propeller phase of the Eve flight. I'm pretty sure my plane was only going 160 m/s or so at 20km on Eve, although OP's craft looks like it has relatively less wing area than mine does. I might give it a try later just to see...
  3. Sorry, I did figure out that's what you meant, but not until after I had replied. True, but it also means you can ditch all that extra fuel you needed to power the fuel cells, probably in turn significantly reducing the number of rotors/blades you'll need, not to mention all those batteries! In general, it also looks like your plane has more torque than it needs. When I'm flying the one above, my 10 small rotors don't get near their max RPM until I am approaching the top of my envelope, which is around 20km for that plane. It's able to do that using just the power from the OX-Stat panels you see, although the sun angle does need to be fairly high for that. It also carries only ~6K in electric charge. Of course it is nothing like the behemoth that yours is, but at 82 tons it's no paper airplane either! And although I understand your issue about part count, the small rotors have 20 times less torque than the big ones, but weigh almost 40 times less. I never actually found a situation where the bigger rotors performed better. I'm pretty sure hypothesis 2 is correct. When I had the panels mounted backwards behind the nose cones, I was not actually able to do anything that snapped them off, although I didn't push it all that hard. I imagine a large yaw excursion could be problematic, but OTOH maybe having that nosecone in front just reduces the panels' drag beyond all reason due to how the aero model works. I will see how far I can push it later today. I'll also mention that you don't really need to go all that fast to use props to lift you out of the soup on Eve. I unfortunately didn't take a snap, but I don't think I was flying at even 200m/s when I was at 20 km in that plane. Absolutely! Indeed. You might also consider replacing 2 Vectors and their tank(s) with a Twin Boar if you don't want to move up to 3.75M fuselages, although you'll take a bit of a hit on ISP See my above comments about the rotors. When was making my first prop-driven Kerbin SSTO, I found that 6 large turbine blades on a small rotor gave me the best performance overall, with max RPM happening only near my operational ceiling. Based on that, you should have something like 120 blades on those big rotors! FWIW, I also found that offsetting my blades outwards a bit gave them more lift per blade at the expense of requiring more torque, with the caution that you'll need a bigger nose cone because exposing the blade bases to the wind creates terrible drag. Absolutely!
  4. Perhaps you could mount them to small nosecones embedded in the trailing edges of your wings so that they extend backwards? If they're mounted vertically, then pitching up probably wouldn't snap them off, although turning most likely would. I'm kind of curious about it now, so I might try fooling around with that type of design myself in the sandbox later. One thing I can tell you, although you probably already know it, is that the aero forces low in Eve's atmo are just brutal. I've often snapped off wings that were strutted out the wazoo by turning just a little bit too aggressively. (edit)... And it actually seems to work! I tried mounting a pair of Gigantors to my existing Eve plane, first facing outwards. In that configuration, they caused massive drag and snapped off right at 100m/s: If OTOH I mounted them to the backs of my rotors facing backwards, they caused a lot less drag, and I couldn't get them to snap off even at >140m/s: Teleporting the test to Eve didn't seem to change anything either: So I'm gonna say, yes, you could actually do it that way!
  5. I don't think it's ever good for a rocket to have a TWR of less than around 1.4 at takeoff. You'll just be throwing a bunch of dV down the gravity hole. In fact, I'd say my most efficient lifters generally start at around 1.8. With the aero model as it is, what you shave off in gravity losses that way far outweighs what you'll lose to drag in the lower atmo. If you've hit it right, your limiting factors should be staying in control through max Q, followed immediately by not burning up before you get above 25km. At least in Stock. I have no idea how RSS might alter that calculation.
  6. Making a regular SSTO plane that can fly to Duna and back is actually pretty easy because the low surface pressure and gravity make it possible to take off on just Nervs. The main thing is just to have a lot of wing area: I also found it helpful to mount drogue chutes on the trailing edges of the wings for landing, to mitigate against the high stall speed and the tendency to bounce in the low gravity:
  7. It all really depends on what your constraints are. Early in the (career) game, part count is a major limitation, so landers that can make it all the way from LMO to landing to home are the best. Later in the game, having autonomous transfer stages, aka "motherships", that drop off and pick up landers at various target bodies is clearly the most efficient way to go. When I was milking Mun for all its science in my current career game, I had an orbiting transfer stage/mothership module that refueled the lander module four or five times so that it could collect surface science from different biomes.
  8. Oh there was definitely some discussion about it. It's absolutely a problem, especially with the legs since you can't tweak them like the wheels.
  9. How do you make those rotors without the BG DLC?? (edit)...So each propeller is a separate craft that you undock into some kind of cage and then switch to and spin up with reaction wheels? Boy that must have been hard to get working. I'm impressed!
  10. I spent some time trying to make a propeller-driven SSTO plane that could operate on Duna and ended up scrapping the idea, mostly because the operational ceiling on Duna is too low for the props to make much of a contribution. Just making something that can take off and land on props is not so tough however. Just keep it light and have lots of wing area. As to how to get it there, landing it from orbit should not be a problem. It's putting enough engines and tanks on it so that it can get back to orbit and still fly reasonably on props that's pretty tough.
  11. What I call a transfer stage is the whole part of the ship that does all the pushing from LKO to low orbit of whatever the target body or bodies are. It can have any number of engines, but they will always all be firing together, as that is clearly the most efficient way to do things. As my TWR gets higher than what I need, I'll generally drop them in pairs to trade that excess TWR for more dV. So if I have a core stack surrounded by three pairs of asparagused side boosters, is that one stage or four? I think what you are talking about only really applies to stages that are stacked on top of each other, which is quite clearly not the most efficient way to do things in general because the upper stages are all dead weight until they fire. Which raises the question of what exactly do you mean by maximizing performance anyway? What I am talking about is optimizing the tradeoff between having enough TWR to get off the ground and do your other burns relatively efficiently, and also having enough dV to get where you need to go. Any other measure of "performance" is pretty much only of academic interest in my book.
  12. The first part of this may be roughly true for ascent stages, but I think transfer stages with their much lower TWR requirements should pretty clearly have more dV than the ascent stages for best performance. As to the last part, it kind of depends on what you consider a stage. Asparagus staging is pretty clearly the most mass-efficient way to get to orbit, because it allows you to shed superfluous dry mass at the highest rate that your constraints of TWR will allow. My ascent stacks therefore typically have 4-6 side boosters that are dropped in pairs from a core stage so that the overall TWR starts pretty high and gets lower as the ascent progresses and gravity losses become less of a factor.
  13. I'm not really so keen on having everything be scalable, as that's not particularly realistic wrt real world rocket engines/pods, and it would also be a huge headache (i.e bugs, delays) for the devs to implement. Procedural wings and control surfaces OTOH are something I would consider a must-have. I am tired of all my planes looking like they were slapped together out of stuff found lying by the side of the road! I think also that letting the user change just the length of the tanks of different cross sections, perhaps as well as being able to freely distribute their available capacity between LF and Ox, would neither be very hard to implement nor unrealistic. Similarly, allowing the lengths of different adapters and girders to be user-adjustable seems reasonable to me.
  14. Uhhhm, don't build such a ridiculously gigantic ship? But seriously, the same rules of thumb wrt TWR and dV apply on all scales.
  15. As has been said several times, every stage you add will increase overall dV unless it has a TWR <1 for its full burn while still on the ground. For my part, when I'm designing rockets I have two key considerations in mind: the ultimate dV required to deliver the payload to all its destinations and the TWR required to get through each of the phases of that process efficiently. In general, the more TWR you have in a given stage the less dV that stage will have, so I try to only have a high TWR when I really need it, which is for takeoff and landing, and use a lower TWR for all other phases of the mission. So to illustrate, let's say I'm doing a Tylo flag-planting mission with a Tylo orbit rendezvous design. I'll start by building my lander ascent stage so that it has a TWR of around 1.4 (on Tylo) at takeoff, maintaining that or higher TWR for at least the first third of the total dV required to reach orbit, which for Tylo would be something like 800 m/s . For the second third, I would start at a TWR of around 1, topping out at maybe 1.4 again, and for the last third I might start with a TWR as low as 0.5, topping out around 0.8. To this I would then add a descent stage that has the same overall dV as the ascent one, but starts at a TWR of at least 0.5 and ends at a TWR of at least 1.4. Once I have that built, I would assemble that package to a transfer stage that has enough dV to take the full lander from LKO to LTO and then just the empty ascent stage from LTO back to a Kerbin encounter, with a TWR that remains somewhere between 0.2 and 0.4 for its whole journey. Usually I'll do this by assembling my near-empty Tylo ascent stage to the 2.5m core of the transfer stage with enough tanks and Nervs on the bottom for a TWR of 0.25 and something like 2.8 km/s dV in that configuration. Then I will fill the tanks and add my descent stage to that, and take note of the dV with those added. I'll then start adding paired, asparagused side stacks of Mk1 fuselages and Nervs to this, until I've added another 2.8 km/s of dV to whatever the display showed before I started adding them. In this process, I'll try to keep the TWR between 0.2 and 0.4 for the whole burn time of each asparagus stage, setting it up so that each stage will run out of fuel right as the overall TWR reaches 0.4. Once all that's built, I'll usually mount it on top of a 3.75m core stage with a Mammoth engine that has a starting TWR of right around 1 and maybe 1.8km/s total dV. I will then put an asparagused pair of Mainsails on the sides of that, so that I'm adding another 700 m/s or so of dV at a starting TWR of maybe 1.2, and then finish it with a pair of giant SRBs to add another 700-800m/s of dV at a starting TWR of >=1.4. Anyway, that was rather a complex narrative, but if you follow that sequence you should end up with a vessel that can get the job done with a reasonable if not perfectly optimal level of both efficiency and flyability, and without getting fancy about gravity assists.
  • Create New...