Jump to content


  • Posts

  • Joined

  • Last visited


116 Excellent

Profile Information

  • About me
    Spacecraft Engineer

Recent Profile Visitors

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

  1. A major challenge for designing low cost staged launchers is the cost of staging itself. You might think that a Reliant and FL-T800 fuel tank makes an efficient lower stage for a small launcher, and in mass/fuel efficiency terms, it is. If you add in the cost of decoupling it, though, the balance goes in a funny direction. A Reliant and a dry FL-T800 tank combined weigh 1.75 T. A TR-18A stack decoupler, while only weighing 0.05 T, seems a convenient way to stage it off to allow a lighter, more efficient upper stage. That coupler, though, costs 400. If you look at the kind of cost-to-orbit that is achievable in the game, it's likely to be cheaper to not stage those off and pay the extra fuel to bring the whole thing to orbit. That's one of the reasons why the ultra-cheap launchers I described earlier in the thread were specifically built around the idea of keeping the decoupling to an absolute minimum. I did have a go to see if it was possible to make one fly where I ditched the decoupler entirely, put the ant on the top, pushing backwards, but I didn't achieve orbit, though it was quite funny pushing an empty Hammer backwards.
  2. Fuel for the plane (not counting the orbiter's tank) is 270. While getting confused about some odd cost numbers, I have now noticed that it appears the fairing itself, not just the "base" has a cost associated with it. For example a Hammer costs 400, a fairing base 300 and a tiny decoupler 300, so you would expect the cost of the rocket in my earlier post to go down by 1000 if I remove those (those being the "lifer costs". In fact it goes down by 1038 because that's the cost of the fairing itself. So call the cost-of-launch (assuming I can figure out how to recover the plane) about 310 (its fairing is a bit longer). Right, now I have to go and clear out all of these "2Cheap" "SuperCheap" and "BargainBasement" satellites from my LKO before they go all Kessler on me.
  3. Same old Ant. I have a suspicion that the Ant may have its physics modelled as if it did have a fairing even if one doesn't show. For the sake of completeness, I thought I'd see if I could do any better in the SPH. I knocked together this little spaceplane That thing in the middle is a diverterless intake that also holds liquid fuel for the whiplash, and there are a pair of Sparks each with 3 Oscar Bs on the sides. The payload, at the front, is the same orbiter as before, still with its ant and single oscar B. I couldn't get the plane to orbit, but I achieved a suborbital flight with an Ap a little over 85 km. I ditched the fairing and launched the payload near the edge of space on the way up, took control of the payload and flew it to orbit while the plane began its re-entry. I achieved orbit with he plane still in the 60's, went back to the plane and attempted to fly it back to base. Because it was suborbital, re-entry heating was no problem, but on the return flight I ran out of fuel a little to the east of the islands with the island runway, so had to ditch in the ocean (which I did successfully with no damage). I reckon with bit of optimising and practice I could get something for that kind of price that would make it back to the runway. For such a light plane with a whiplash on the back it accelerates like you wouldn't believe. I set the undercarriage so that it sits with a nose-up attitude on the runway, and it practically flies itself. Obviously it only pays off if you get the plane recovery money.
  4. My understanding is the radial battery, like the thermometer, just adds its mass to the part it's attached to, so doesn't unbalance the craft. It's inside the fairing (just), so doesn't have any aerodynamic issues. And it contains enough charge to transmit the all important science even on the night side of Kerbin. I've had a bit of a play around, and I think the key issue is the step from 0.625 m parts to the 1.25 m Hammer. I just launched a satellite using the 1.25 to 0.625 m structural part and a 0.625 m nose cone instead of a fairing. While the radial battery is OK from a mass distribution point of view, it's no good aerodynamically, so I had to use an inline one instead. I achieved orbit, but with a wafer thin fuel margin (less than 1 unit left in a less than 80 km orbit) and it cost more than the last one.
  5. By the time the fins burned off the craft was flying in a straight line anyway, and as they were attached to the discarded booster, they aren't really that important. Right, just built a HECS, 1 Oscar B, 1 Ant, 1 Z-100 Battery, 1 Communotron 16 and 2 OS-Stat solar panels craft and put it on a Hammer under a fairing with a TR-2V decoupler and achieved orbit. Mine only cost 3298. The experienced KSP players will be shouting at their computers "fix your staging!" around about now. So the question is why did this work when my earlier HECS + Hammer tests went so badly wrong? The answer, it appears, is aerodynamics. I did a few test flights All had the Hammer at 40% and the flight trajectory was straight up. Hammer + HECS: unstable. Hammer + HECS + 3 fins reached a little over 20 km. Hamm, HECS, 3 fins and a 0.625 m nose cone: a bit better, reached 26 km. Hammer + HECS + 3 fins + 1.25 m nose cone. Similar, under 30 km. Hammer, Hecs, 3 fins, 1.25 m fairing: holy cow! Not only does this thing go up much much faster than the others, with the engine not burning out till over 40 km, it leaves the atmosphere with an Ap of over 280 km. Also, the drag reduction at the front means the fins can be ditched and a stable flight maintained. So I conclude that we all need to forget the old days of "fairings are just for aesthetics".
  6. The problem with this analysis is it doesn't consider actually getting out of the atmosphere. If you go with a single SRB as a launch stage, you have to accept it's sub-optimal burn profile (constant thrust until the fuel is gone) and the need to get out of the thermally and aerodynamically challenging part of the atmosphere. I haven't chosen the Thumper for either it's thrust or it's dV, but for the fact that it can get out of the lower atmosphere. I've just had a go at a basic test: I put a HECS probe core on a Hammer, nothing else. That was a total failure: it's aerodynamically unstable. I added 3 fins. After a reasonably involved test campaign with different thrust settings and attempted launch trajectories, I never managed more than 22 km. I'll grant that I didn't find the optimum, but I reckon 25 km is the maximum you'll get out of a Hammer. I don't approve of "explosive decoupling, but for The Sake of ScienceTM I had a go with a Hammer + Flea combo. Result: no decoupling As a result of extensive testing, all I can presume is that this "technique" has been removed from the game. Anyway, I moved on to Hammer, TR18-A decoupler and Flea as first and second stage, with a simple HECS in control. I did manage to achieve sub-orbital flight with an Ap of 99 km as my best shot, but the Flea + Hammer + decoupler costs 1000 while a single Thumper is 850, so I'm not sure where the benefit comes from. When I first created this concept, I didn't want to be ridiculously unrealistic, so I used a fairing, but experiments suggest it's important anyway (see below). Your points on optimising the orbiter are good ones. So I have run some tests as follows: Communotron 16, HECS, 3x OX Stat, 1x Oscar B, Ant, TR-2V decoupler, Thumper, 3x Basic fins. I found the result became unstable in the transonic regime. I'm not sure if this is due to aerodynamic forces or mass distribution. I added the 1.25 m fairing back in, and the result flew nicely. I found that if I flew a trajectory that resulted in a Thumper burn-out with an Ap of less than about 200 km (so more horizontal velocity), atmospheric heating becomes high enough to destroy parts (the fins all burn off, but the fairing protects the orbiter). The result is a successful orbit, but without the external battery the satellite has insufficient energy storage to actually transmit a temperature reading, so can't manage to bring any science home. Anyway, that launch cost me 3748 total, so that is a new personal best for "cost to orbit" for a minimum orbiter.
  7. I'm home from work now and have re-created this launcher and flown a test flight. There is so much wrong with this particular launcher that it is borderline un-flyable. Anything approaching an efficient flight profile is just not happening. The first issue is that, while the Thumper can get this load out of the atmosphere, you don't have enough margin to do a very effective gravity turn. In the event, though, that's not much a problem. The real problem comes from the Ant. It gives a new definition to "feeble". The burn time required to get any kind of useful dV is insane. If you do make a nice gravity turn so you leave the atmosphere with an Ap of perhaps 85 km and a the rest of the energy in prograde orbital velocity, because the Ant is so feeble, you will go up to Ap, come down the other side and burn up on re-entry long before you are anywhere near orbital velocity. The only way I can make this thing fly at all is basically to go straight up. The objective is to get as high an Ap as possible from the Thumper to maximise the time I have between leaving the atmosphere and re-entering in the hope that the Ant can burn its six little legs off and achieve a stable orbit. The launch went something like this: Go straight up. The Thumper quits at about 40 km altitude, with an Ap of around 200 km. Once I hit 60 km, I ditched the fairing and Thumper, pointed between 25 and 30 degrees above the horizon and burned that Ant on 100%. All the way up. And back down again. I passed through Ap at about 220 km, and finally got the Pe above 70 km when I was at 140 km altitude coming down the other side. I flew this with the Thumper at 50% thrust, but didn't put too much effort into optimising it. The rundown of the whole craft is as follows: 1 communotron 16: cost 300, mass 0.005 1 Probodobodyne QBE: cost 360, mass 0.07 2x OX 4L 1x6 solar panels: cost 2x380, mass 2x 0.0175 (yes, I could get by with the OX stat for lest cost and weight, but I prefer bigger panels) 1x 2HOT thermometer: cost 900, mass 0.005 (redundant, there to demonstrate the satellite can complete contracts and do science) 1x Z200 0.925 m inline battery: cost 360, mass 0.01 (a pair of Z100 batteries would be cheaper for the same charge/weight) 1x small inline reaction wheel: cost 600, mass 0.05 3x Oscar B fuel tanks: cost 3x 70, mass 3x 0.225 1x Ant engine: cost 110, mass 0.02 1x TR-2V stack decoupler: cost 300 mass 0.015 1x AE-FF1 Airstream protective shell containing all of the above, cost 300, mass 0.075 1x Thumper SRB: cost 850, mass 7.56 3x Basic Fin: cost 3x 80, mass 3x 0.01 1x TT18-A launch stability enhancer: cost 200 (recovered), mass irrelevant. The orbiter itself has a dry mass of 0.27 and a cost of 3545 (dry), 3600 (wet). Total cost of launch (inclusive of payload): 5392 (5192 after launch clamp recovery). That's a total cost to orbit of 1647 plus a little over 2 Oscar B's worth of fuel (I had a little less than 1 tank left for establishing the target orbit, but that's plenty for completing most simple contracts). There are plenty of ways to make the craft more efficient if you think purely in terms of fuel burn, but they all add to the cost significantly more than they save. A kickback alone costs 2700, which is more than the total cost to orbit of this launch, so that's a non-starter. A Hammer lacks the dV to get the Ap out of the atmosphere, so would need to be combined with some other sustainer engine. That means a TR-18A stack decoupler. A Hammer plus a decoupler cost 800 combined before you add the cost of the sustainer engine. That's only 50 cheaper than a Thumper, but even a sepatron costs 75, and that's the cheapest engine in the game. In an earlier iteration of this launcher I used a Spark, which can achieve the same orbit with only 2 Oscar B's worth of fuel, and a much easier flight profile because it has something like 10 times the thrust. But a Spark costs 240 (so 130 more than an Ant) and the third Oscar B only costs 70 wet, so where's the saving?
  8. I haven't done any detailed study, but for small payloads to LKO my feeling is that for minimizing cost with a disposable launch system, a system based on an SRB is the way to go. I've made serious money on LKO career contracts using a QBE core, a couple of solar panels, a communotron 16, 0.975 inline battery and reaction wheel as the payload, with an ant and 3 oscar Bs as an orbital engine on a 0.975 m decoupled, on top of a 1.25 m fairing, Thumper SRB with three basic fins and a launch clamp. For that king of payload, the cost to orbit isn't that far off the fuel burn alone for an SSTO capable of delivering the same payload.
  9. One of the features I have always found a little frustrating is the lack of science from space. There are tons of biomes on planets, and a few biome specific experiments in low orbit, but mostly we just have space low and space high. I find there's a bit of a science drought between getting to Kerbin orbit and landing on Mun, after which science comes in huge quantities. Given that satellite contracts are offered differentiating things like equatorial, retrograde, polar, tundra and Molnya orbits, it seems to me there might be an interesting way of making actual space flight worthwhile beyond being simply a method of getting from one celestial body to another if being in different categories of orbit could grant science as separate biomes. It would give an incentive to actually do something other than launch to and equatorial orbit and just head straight for Mun.
  10. I have my fair share of "Commsat" variants, as well as usually having a KSS. The other name I frequently use is "Moho" for my first ship to orbit as a nod to the Mercury program.
  11. Presumably if you turn cross feed off and use fuel lines you can do asparagus the old fashioned way? I prefer to eat my words with fava beans and a nice chianti
  12. Yeah, but that is such a massive improvement, I don't think it counts as a "little thing". I have years of muscle memory to unlearn now. When I launch my fingers just automatically hit T-Z-M. It'll take many launches before I can unlearn the M.
  13. We've all read about the "big news" features in 1.2, things like communications net and balanced fuel flow. How about the little things? What new features have you found that you have noticed that make life a little easier? for me it's "When a Kerbal plants a flag, credit is given to all landed/splashed Kerbals in physics range. Prevents the need to have flag-planting parties with large crews."
  14. A good suggestion. I've also found problems with planes being uncontrollable on the ground when the undercarriage is not strong enough for the weight of the plane.
  15. I've encountered this problem when the wheels aren't quite straight. In the SPH, select the rotate tool, set rotation to absolute and make sure angle snap is on. Give them a wiggle and make sure they're set so the wheels are vertical and aligned axially with the plane. Most times that fixes the problem.
  • Create New...