closette

@semininja, thanks for the kind words, but others on this thread have done most of the work! The Excel file provided by Kosmo-not was a tour-de-force of data collection I too have been wondering about the 'fuel efficiency' equation assuming zero (or perhaps just a small compared to 'g') acceleration. I\'ll have to think and scribble about this some more though.

Everyone else\'s? Are your designs immune from aerodynamic drag? By the way, on another thread, user Kosmo-not took free-fall velocity data to find that the Kerbal atmosphere has an exponential dependence with altitude between 200 and 40 000m, with a scale height of about 4900m. (The absolute value of density depends on the units that 'mass' and 'thrust' are given in). Link: http://kerbalspaceprogram.com/forum/index.php?topic=5813.msg86098#msg86098 and comments below.

Good idea Darkshadow, and done. Oh, and yes you will have a very slowly decaying orbit between 40 and 70 km. I\'ve been 'stuck' with a periapsis in that range a number of times and no fuel remaining, and it takes forever to come back down.

OK I\'ll keep the challenge up and add one more rule - no waiting around! Yes, the fixed direction makes it more difficult for high orbits, even though you are moving more slowly out there. One explanation from a physics standpoint is that higher orbits have more angular momentum, which the constrained boosting direction can barely change, so to have the same angular momentum at a low periapsis means you have to add a bunch of orbital energy.

Hi Bill, Yes, Kosmo-not\'s work should be published in a more obvious place, it\'s so good. Your Mun experiences are only a few weeks behind mine, and your craft sounds similar, but by all means post the .craft file when you get a chance. Yes, I use RCS to return from the Mun, and usually start with close to 2 tanks left (either a little more or a little less). The 3rd (empty or nearly empty tank) is attached to an ASAS module and the fins, all below the stack decoupler. Shortly after taking off from the surface, I hit the spacebar and jettison these items, leaving me with the command module+parachute on top of 2 RCS tanks. A bit squirrely to maneuver but OK for crude pointing. So if I understand you have a little more than 2 RCS tanks left and follow a similar plan, but somehow don\'t succeed. My first suggestion - move your stack decoupler down so that when tank #3 runs out after take-off, you can get rid of it along with the fins. (You can still land safely back on Kerbin with a couple of nearly empty RCS tanks under your parachute, especially on water. On land, they may explode but your Kerbals will be all right.) My second suggestion: get into Mun orbit in stages, as opposed to one continuous burn. After take off and pitchover to 270 degrees heading at about 15 degrees above the horizon, keep boosting while watching the map view. After a short while you\'ll see your Apoapsis (apomun?) move up and away from the spacecraft. Stop boosting and coast for a minute or so (you can accelerate time if you want). Then when you get close to apoapsis (at about 80% of its altitude), point along your velocity vector and hit 'H' again for 5-10 seconds. The apoapsis will climb up away from your spacecraft again and further around the Mun. If you 'chase' the apoapsis a few times in this manner with small bursts, you will find yourself in a low retrograde orbit around the Mun (12 000 m is typical for me). If you need to, depending on where you started from, orbit the Mun once completely, coasting (switch off RCS so that it won\'t leak on you if you have SAS engaged). Let the orbit take you to just before the 'leading edge' of the Mun\'s grey orbit line around Kerbin. At that point, do a long boost to produce an escape trajectory that should take you around the back of the Mun at about 850 m/s, and will spit you out of the Mun\'s sphere of influence going at about 500 m/s opposite to its orbital direction around Kerbin. If you do this, you will have canceled out most of the Mun\'s motion around Kerbin, and you should fall almost straight down back home. If not, use whatever fuel remains to boost against your velocity and lower your periapsis below 30 000m into Kerbin\'s atmosphere. I usually have ~ 1/2 RCS tank remaining when I come in to land. If more, I just burn it off in the atmosphere. But I save some RCS fuel so that I can reduce my landing speed under the parachute, using the 'H' key for the last 100 m or so. (The Soyuz capsule uses retro-rockets to brake just before landing also). Hope this is clear. If not let me know and I\'ll add some screenshots next time I do it, and good luck!

Haha, lars, I didn\'t think of that! You win. Well, not much interest in this challenge anyway, although it made me put some fairly powerful ships into orbit.

You are a scholar of the highest calibre! I think you meant exponential, not logarithmic, because your data from 2500 to 38 000m almost exactly fits this equation with correlation >0.99: density = 0.01008 kg/m3 e -(height / 4907 m) Above 40km where you are moving slowly through thin air(*) your derived density data gets 'noisy' so it may be constant, or a continuation of the exponential, or l (my guess) inear, but it\'s not a big deal. This is assuming that masses are given in kg (and thrusts given would then have to be in Newtons). I think it is more likely that masses given are close to metric tons and thrusts in kN, based on comparisons with real world command module, parachute masses, and densities of the SRBs, as discussed in this thread: http://kerbalspaceprogram.com/forum/index.php?topic=1769.0. That would scale the surface density up by a factor of 1000, but the exponential scale height would not be affected. And I am still concerned that the presence of an erroneous mass term in KSP\'s drag equation (mentioned in the thread I referred to in my previous post) might mean you\'ll get the same freefall data but different derived drag force and density for, say, a capsule with some additional mass attached (such as an empty RCS tank). But the robust result is that the scale height of Kerbin\'s atmosphere is 4900+/-80m. Sounds about right, but now we KNOW! (*) No idea what the composition of Kerbin\'s atmosphere is, but I use 'air' anyway.

Thank you, and the test you suggest is definitive proof. I\'m not sure how to do it but I think I can come up with a way. It\'s going to be interesting when the drag model is improved - we\'ve all been building ships in the current scheme, and they may not work so well with a more accurate model. That could be quite a drastic change compared to previous version updates - just fine by me though since it will make predicting the drag effects easier when designing.

Let\'s make that periapsis below 30km altitude, because I just tried the challenge myself from an 8.5 million meter orbit (but cheating by jettisoning lots of stages on the way in) to reach a perikerb of 40km altitude. I came screaming in on a hyperbolic path at > 4000 m/s and ended up screaming back out again! I\'ve also come up with a dramatic back-story that some people seem to like with these challenges, which I shall include in the original post.

Hi lars, The SAS will keep you pointing towards your original direction in space, think of it as a gyro stablizer when engaged. That\'s the challenging part about this challenge. If you can\'t accelerate radially fast enough, then after a while you\'ll be pointing 'off center' and will end up zooming past Kerbin, probably on an escape trajectory. And yes, half an orbit later you\'ll be pointing upwards, but you\'ll know you\'ve failed to get home long before then! If you just keep pointing at Kerbin\'s center you can spiral inwards more easily and its less, well, challenging (although not trivial). OK, perigee below 70 km altitude is success for the reasons you describe. I\'ll edit the original post to reflect that.

I\'m confused - does the current model include an incorrect dependence of air drag on the part\'s current mass (assuming that\'s what 'm' is, which can change as fuel is used up). Or is mass correctly canceled out when multiplying the factors together to calculate the force D?

Hi Kosmo, Reverse engineering atmospheric density sounds like a great idea, I had thought of using video capture software to figure out thrust, mass, acceleration (and hence the missing drag term), but was dissuaded from doing so by this slightly confusing thread: http://kerbalspaceprogram.com/forum/index.php?topic=5623.msg73154#msg73154 which might (or might not) imply that the spacecraft mass appears incorrectly in the drag model. Also, the current version of KSP apparently just adds up the (drag coefficient x frontal area) of all parts together, regardless of how they are arranged, so a single stack of 4 tandem fuel tanks ends up with the same number as 4 tanks mounted side-by-side. So perhaps I should enjoy using my 'hip tank' design while I can... In the meantime, I would genuinely not want you to waste your time on reverse engineering an incorrect model, and one which might soon be changed.

Thanks for taking the time to compile these. Most of these .craft files are now in my Ships directory.

No takers yet? The environmentalists could be persuaded to relax the 'no jettison' condition on the challenge, if that helps.

Thanks!

Glad we\'re in the same time zone, thanks for the fast response. I think I can derive that formula, but if you care to provide some of the steps I won\'t mind, and nor will others I\'m sure, since this question has come up quite a few times. Keeping the atmospheric density constant is good enough for now. Not sure whether Kerbin\'s atmosphere is exponential, and the drag model I am told isn\'t quite right (it has a mass term that should not be there), but these are minor details.

Excuse me being dense, but where does that formula come from as a measure of fuel efficiency?

Could you please share the formula you used for 'fuel efficiency', or even if you don\'t mind attachthe spreadsheet. I trust your analysis 100%, I\'m hungry for more details!

I did some calculations in another thread based on a rail-gun 'shooting' straight down at Kerbin from a high circular orbit, http://kerbalspaceprogram.com/forum/index.php?topic=6396.msg85438#msg85438, and it suggested the following challenge: The story is as follows. Our kerbonauts are in a high circular orbit with the spacecraft nose pointing directly at Kerbin\'s center. Suddenly they lose attitude control even though the gyro stabilizers still work. The only way to get home is to boost quickly downwards in that constant direction, and hope that their path will intersect Kerbin. From a near circular orbit around Kerbin, point straight down at Kerbin\'s center, set SAS to fix that direction in space, then try to get back home by boosting in that direction only. What is the highest circular orbit from which you can do that? Rules: - all components must be 100% stock - Once you\'ve set the initial downward direction, you cannot change it, e.g. to continuously point towards Kerbin, otherwise you could easily spiral inwards with enough time and fuel on your hands. - No waiting around. You must start boosting when pointing straight down at Kerbin, and must complete the challenge in less than 1/4 of your original orbital period. - You must stay in Kerbin\'s sphere of influence at all times from orbit to periapsis - the initial circular orbit must have peri- and apokerb within 5% of each other. The perikerb altitude will count and be reported as your attempted orbit height. - Achieving any periapsis below 30 000m is considered 'mission success'. (Edited, see reply 3) - How you get up to orbit is up to you, but thanks to enviromentalists\' protests about orbital debris, NO jettisoning of any tanks or other solid items once you start the radial boost (i.e. any tanks you empty have to stay along for the ride), until you reach Kerbin\'s atmosphere. - Acceptable proof consists of 2 pairs of screenshots - spacecraft view & map view (with Navball) just before boosting, and spacecraft view & map view (with Navball) just after achieving success or partial success. The spacecraft and map views should be within 2 minutes of each other. (We need the before and after spacecraft views to confirm that all the bits are still there). I tried from 350 000 m altitude, it was easy. From 3 million m altitude I failed, but I didn\'t try very hard. Screenshots attached of my failed attempt (sorry, not very good ones, but showing that I couldn\'t get periapsis low enough in time). Did I miss anything? Suggestions (be nice!)? Remember, this is a 100% stock challenge, and it may be more suited for our newer kerbonauts.

Excellent test, and I stand corrected! My own spacecraft are so under-powered that this hasn\'t been an issue for me. Like Hypocee\'s posts which I referenced above, I\'d like to know if there\'s a universal rule-of-thumb for optimal thrust, but your demonstration at least shows that an optimum does exist between low thrust and maximum, with the current drag model.

Cool! Is this with your Mk6 Munar design, or a lightweight powerful orbiter sans lander? Please report back with some comparisons and numbers (throttle settings, fuel consumption, screenshots?) if you have time. The cleanest way to test this would be with purely vertical launches up to say 50km, kind of boring though. I updated my previous reply with a reference to a thread on thrust/weight ratios. Not much evidence in there but it would be interesting to see if your experience accords with the claims made.

Hi Kosmo, I just tried your Mk6 craft, having faithfully followed the previous versions, and got to the Mun and back with plenty of fuel to spare at either end (despite completely messing up the return from the Mun). As always, a very solid, hassle-free and efficient design. It would be useful for newbies to have a bunch of stock craft collected somewhere. I believe it was suggested in the Spacecraft Exchange a while back but few liked the idea. It might be worth proposing again. Searching the boards for 'stock' or '100% stock' or even 'vanilla' is what I did, but that\'s not a perfect solution. My concern is that when v0.14 comes out, all our great spacecraft designs will no longer work, and at least will have to be rebuilt, if not re-designed.

Nice coverage of the fuel budget considerations, Kosmo. And yes I forgot to mention struts for structural stability (not quite the same as aerodynamic stability). If I may comment on one statement you made in the 'balancing game': From posts elsewhere, my understanding is that for Kerbin\'s gravity and atmosphere, the balance is tilted strongly in favor of 'get as high as you can as soon as you can'. Yes the higher speeds will increase drag, which is why I use SRBs to help push through it, and then jettison them asap. I would love to say 'GO at throttle-up' after reaching some altitude, but every spacecraft I\'ve tried is most efficient if thrust is at a maximum from launch to at least 20K. (By which time you\'re above most of the atmosphere and deciding on pitch angles). That\'s true even for my fairly draggy Munandback131A craft, but I would be pleased to learn of counter-examples. I read somewhere(*) that if/when a more accurate drag model is implemented in KSP, this will skew us even more towards 'power hard'. (*) Found it, FWIW. Read Hypocee\'s replies in this thread: http://kerbalspaceprogram.com/forum/index.php?topic=6008.0

Glad to hear you had a good flight, tarspaceheel! It will come as no surprise that my Munandback131A design was the result of many iterations (about 14) and that was after trying out some less-than-satisfactory stock Mun designs I found here on the message boards. Even then it has its shortcomings - if you try to pitchover too early in the atmosphere it will go haywire on you and you\'ll spend all your RCS fuel (needed for return from the Mun) trying to get back near vertical. It sounds like many of us newbies have stability problems with the first 'lifting' stage. Symmetry is important (although the tool does have a few bugs, watch out for them), but here are some stability/controllability considerations I took into account: - rotational inertia. a long skinny rocket is much more reluctant to change direction than a shorter fat one of the same mass, because in the former case much of the mass is far from the pivot point. (Grab a long stick at its center and wiggle it back and forth, then break the stick into 4-6 pieces and wiggle the bundle back and forth, it will feel much easier). Since v0.13.1 we can attach liquid fuel tanks 'sideways', and route fuel lines between them, which is a great help in this department, especially for orbital maneuvering. - center of mass (CM) or center of gravity (CG). In free flight, a rocket will naturally rotate around its CG. (For example, throw a hammer a you\'ll see it rotate around its CG). For a Mun lander, a low CG will make it less likely to topple over, a good thing! On the other hand, for lifting the stack through the atmosphere, you don\'t want the combined CG to be too low, because for stable flight it should remain above the center of pressure (CP), see below. As liquid fuel tanks drain from top to bottom during the lift, the rocket gets less massive, but be aware that the CG gets lower due to the fuel remaining in the bottom of the tanks. That is one reason why a big liquid rocket can be stable at launch but then go nuts some time later. The other reason is - center of Pressure (CP). This is the point on the rocket through which the sum of all the aerodynamic forces can be said to act. For a vertical rocket, if the CP is below the CG, the rocket will naturally want to climb nose up and will be stable to small deviations from vertical. Ever wonder why fins are placed on the tail of a rocket and not usually near the top? Adding more surface area lower down, causes the CP to stay below the CG even when fuel has almost run out. Also, if the fins are adjustable, placing them near the bottom (and far from the CG) will allow them to exert more torque (or turning effect) to control your rocket. For a given geometry (i.e. not jettisoning tanks or other bits) the CP stays more or less in the same place as long as you stay nearly vertical. However, if your rocket ends up pitching over too far so that it\'s hitting the air flow sideways, the CP will move close to the rocket\'s mid-point, possibly above the rocket\'s CG. And you\'ll have one heck of an unstable beast (less ladylike words usually fly out of my mouth when this happens!). A good explanation of rocket stability and CP and CG for model rockets can be found here: http://exploration.grc.nasa.gov/education/rocket/rktstab.html Finally in KSP, if your rocket still needs a little help pointing straight, we have - SAS modules. Throw a few into your lower stage 'stack'. I used three. if they max out during flight, and you already have fins and RCS, consider adding more. You only need one Advanced SAS (ASAS) module though, since it acts like a 'brain' to control the others. Conveniently, users have found that the ASAS module provides a pretty good crumple zone for surviving crashes, too, which is why I place mine under the command module. As to how many fins/SAS modules/fuel tanks and where to put \'em - trial and error! LOTS of error! Hope some of the above is useful in keeping the sharp end pointing upwards.

Nice ship, Darkshadow, and thanks for the plug! I remember how tough it was for me to get to my first orbit (not that long ago) and this would have made it much easier. Very robust, economic design and easy to maneuver around orbit, without the 'overhead' of a Mun lander dictating the configuration of the lower stages. I\'ve been using it to try for the 'most circular orbit' challenge, and also to learn a bit about inclination changes and spot landings (I\'m aiming for some snow-covered terrain, or for a return to the Space Center).