Pahimarus

Hi all, Thanks for the advice. I did, in fact, go to space today. It still handles like a pig until you get high into the atmosphere and I had to fly it perfectly to make it to space but it did it. Improvements and flight profile essentially following IronMaiden's advice to the best of my ability. Further suggestions on how to improve are welcome. Redesign

Hi all, I'm a fairly experienced kerbal player but novice when it comes to aircraft. I watched Scott Manley's playthrough and am attempting to build my first spaceplane. So far I haven't been able to get my apoapsis above about 45k. My flight profile is to climb to 10,000m, accelerate until I am about 1100 m/s and my whiplash thrust is >300KN. I then pitch up and climb at about 20 degrees activating my rocket engines when my whiplash thrust drops below 150 KN and then close the intakes and shutdown around flameout. I'm also having trouble with some roll instability all throughout and overall flight instability between about 20 and 35km. Thanks in advance. Design

Thanks everyone. Was able to generate intercepts with Eeloo for both spacecraft within my delta-v budget and replicate the process several times. The additional tips on throttle controls make me confident I will be able to successfully complete the burns when the time comes. I play my game with multiple missions running simultaneously (makes the long missions feel like they are long) so it may be some time before my little craft enter Eeloo orbit.

Ok that is super helpful, I guess I really didn't understand what people were describing with mid-course corrections. So I went back and followed Sir Padget's directions again and managed to generate an intercept this time without losing my node. First of all. <pauses> Huzzah! I've got about 2700 m/s of DV left in the tanks. It looks like that won't be enough to execute this maneuver and still have enough DV for a capture burn (estimates I've seen 1000-1500 m/s for Eeloo capture). Should I keep that node and just go for the flyby or could I expect that an earlier mid-course correction as described by bewing might be able to generate an encounter with less DV? **Further updates** Using same process as described by Sir Padget managed to generate an intercept with my nuclear rocket as well as well. I setup my plane change node a little closer to Kerbin (about halfway as suggested by some posters). This allowed me to generate an intercept with an acceptable amount of DV ~500 m/s which leaves me enough to make a capture. With this experience and orbit for reference, I feel confident I could move the node on my other rocket and replicate a similar lower DV maneuver. So to recap: 1.) Periapsis kicking worked beautifully to make my escape burns more manageable. 2.) Adjusting the throttle limiter was also quite helpful for #1 as well as for fine-tuning orbital maneuvering. 3.) Sir Padget's method appears to be (as he mentioned in his post) somewhat wasteful of DV but it was critical to helping me visualize what I needed to do to the orbit in order to make the intercept with Eeloo happen. I'm still trying to get some clarity in my mind around "mid-course corrections." Bewing's explanation was helpful. Would it be accurate to state that Sir Padget's "plane change node" (described in #4 of his step-by-step instructions) is, in essence, doing the same thing as a "mid-course correction"? It seems that because the DV required is higher when this is positioned closer to the intercept point that the maneuver nodes become usable for visualizing it. Whereas when the mid-course correction is far from the maneuver node the amount of DV is so small that it is too fiddly to visualize using the maneuver node? Thanks again everyone for your helpful input. Your advice was super helpful, any failures were purely end user error.

Y'all I have to be missing something. Here is an update I mostly tried to follow Sir Padget's detailed advice. So I actually have two vessels in orbit of Kerbin with intent to launch to Eeloo. The first the one contains commsats and is the one that I discussed in my initial post. I was unimpressed with the DV that I got from the NERV and so for the second one I switched back to a pure chemical rocket carrying an orbiter and a lander. Side note, I didn't realize that the NERV burned only fuel and not oxidizer (makes sense based on how a NERV is supposed to work but I didn't figure the designers would make it that opaque). Good news is that I was able to crossfeed the oxidizer into my partially emptied 3rd stage tank that will get discarded after I start running the NERV transfer stage so I "found" about 600 DV with that move. I decided to try to setup the intercept with the pure chemical rocket as it avoids some of the problems I was having with the low-thrust awkwardly staged one previously described. I figure if I can learn a process to get an intercept with this one, then I can apply that back to the other one. 1.) I setup the initial escape burn using the numbers from the Alexmoon transfer planner. With this I was able to generate a closest approach of about 25,000,000 km. 2.) I scaled the burn a back to about 1/3 of the total DV. 3.) I executed a periapsis kick burn around the scaled down maneuver node leaving me with about 1400 m/s to go for the full burn. 4.) I setup the escape node again. 5.) I setup a plane change node on the AN of the orbit, out near Ap. I fiddled until I got an Eeloo encounter with a periapsis of about 50,000 km which was the lowest I could get. I honestly can't remember which handles I had to pull to manage that or how much DV it was. Update: Here is before the escape burn. 6.) I executed the escape burn per Sir Padget's description above and achieved this orbit. (Note: I also used the trick shared of changing my max throttle to 0.5% to tweak the burn to get the absolute closest encounter that I could while burning prograde/retrograde only). 7.) I didn't quite have an Eeloo encounter any more and so I started fiddling with different midcourse correction nodes. Somewhere I lost the plane change node that got me closest to Eeloo. 8.) I fiddled and fiddled and fiddled and fiddled and fiddled................................................................................................................................................................................................................. 9.) Fiddled some more. 10.) Managed to get a midcourse correction node near Ap that got me an Eeloo encounter but with an unacceptably high DV (like almost 2k more DV and wouldn't have enough left to capture then). 11.) Fiddled and fiddled. More fiddling. 12.) Managed to replicate the node from #10. Yep, still unacceptably high DV. 13.) Fiddled and fiddled and fiddled. I am just utterly lost on what I am supposed to be doing when constructing a "mid course correction node." I would think it would be similar to trying to rendezvous with a spacecraft that changing inclination at the AN or DN to match would help me get closer but nope anytime I change that I lose my closest approach altogether. The only successful node had me pulling mostly radially at apopsis. Anyway, feeling lost and super frustrated. All the advice that got me to this point is appreciated. Any further advice would be much appreciated.

**Update added to thread** Hi all, Playing the deepest into career mode that I've ever gotten and I'm trying to pull off an Eeloo intercept and failing, miserably. I've read a bunch of guides and posts but I haven't been able to sort out my issues with them. I basically have three questions: 1.) How to get to Eeloo generally but ideally under my conditions. 2.) How to match the maneuver node trajectory more precisely. 3.) Is there a better way (mod?) to more precisely plot interplanetary transfers? What I have attempted: It is day 257 02:00 I am within 1 hour of the transfer window from Kerbal alarm clock. One day late for the optimal transfer window in AlexMoon's launch planner. I plot a prograde burn with the listed DV (2054m/s). I then eyeball the ejection angle listed (109 degrees to prograde). This generates and change of SOI with jool past apoapsis. I fiddle the DV slightly and manage generate an intercept with Eeloo at 25 million km closest approach. This seems far so I fiddle for an hour and never come up with anything better. My idea based on other guides is to call this good enough and then try to adjust it closer by matching inclinations at the AN/DN. I then attempt the burn 5 times. Each time I can generate the change of SOI with Jool but I can't match the trajectory precisely enough to generate an Eeloo intercept. Reasons I am having trouble matching the maneuver node's trajectory. My rocket has 971m/s remaining in a chemical stage and then 1778 m/s in a NERV stage. The chemical stage burns out in 55s and the nuclear stage burns for almost 8 minutes. I have extensive experience flying around the kerbin system and I usually point toward the maneuver node and try to divide my burn evenly around either side of the maneuver node. I've struggled to do that here for several reasons. 1.) Do I want to divide my DV or my burn time around the maneuver node? I thought logically DV but when I do that the trajectory has been more off than when I try to divide time. 2.) Do I want to burn prograde or toward the maneuver node? I've tried both and I can't tell. I'm limited in my ability to burn toward the maneuver node too far in advance of the maneuver because I end up burning down into the atmo since I am so far around the orbit from the maneuver. So, any advice on how to sort this out would we welcome. OR Did I make some fatal mistake in my rocket DV or timing and I should convert this to a Jool bound mission instead? I have 3600m/s of DV remaining on the craft and I am in LKO 80km x 80km currently. I ask is there a better way to do this because well honestly I love playing KSP, even somewhat tedious, repetitive stuff like orbital rendezvous but this is my second attempt at interplanetary intercept (have 3 craft on their way to successful Duna encounters) and holy cow is planning a maneuver using the standard maneuver nodes just agonizing and all around not fun. Thanks in advance!

So I've done some additional testing and settled on a "final" design (do we every really stop iterating lol). This one flies pretty darn straight all things considered. It still has a slight tendency to roll a touch but it is stable enough until I get to an altitude where the aerodynamic forces are reduced such that I don't mind turning the SAS back on and correcting any misalignment. Some key points (many of the troubleshooting ideas I got from the thread Streetwind linked): 1.) You and others were right, the side booster fins were causing a significant problem. I couldn't tell before because it was just one problem among many but as the design got more refined and I solved other problems this became apparent. Basically what I think is happening is that because the rocket is symmetrical in only one dimension rather than two, even the slightest roll induced by connections wiggling or slight aberrations in my initial pitchover maneuver is then exaggerated by the aerodynamic forces on the side booster fins since the rocket is no longer exactly aligned. Solution: Removed side booster fins. Take Homes: Side booster fins = bad. Large side boosters = sub-optimal (especially low in atmo without two dimensional symmetry). 2.) The rocket was inherently unstable for several reasons. a.) Too short. Solution: Switched to skipper engine for more center stage thrust, added more fuel tanks in the center stage, added more payload (who doesn't need more DV eh?) b.) Payload was still very draggy. Solution: Refined the payload fairing shape further to smooth it. 3.) A word about fins. I ran many, many, many experiments with different configurations of fins. Basically more fins can somewhat stabilize an otherwise unstable rocket (usually draggy payload, COM toward the rear, or short rocket). However, they come at a cost, not just of weight and $$, but also the more fins you have the more straight the rocket wants to fly which reduces your control authority. The more ways you try to add the control authority back the more places there are for something to induce asymmetrical flight. Similar to the issue with the side fins, once the rocket had fewer issues, the difference of moving them just half a fin length to the rear to overhang the engine was noticeable in flight. When appropriately symmetrical, aerodynamic control surfaces do seem to serve better for my initial pitchover than engine gimbal or SAS. Take homes: Fins are a goldilocks problem, you want them to be just right. Solution: Four symmetrical delta winglets in the center stage positioned as far to the rear as possible. 45 degrees off from the cardinal directions simply due to the presence of the side boosters in the cardinal directions. 4.) Bobcat does seem to have more asymmetry than the other engines. This doesn't mean it can't serve well, just that my combination of a tenuously stable rocket and my stubborn insistence on not using SAS during the initial gravity turn made it not a good fit for this particular rocket. Solution: Changed to skipper for this rocket, added fuel to the center stage. Thank you everyone for your insights. What a great welcome to the community.

Hi all, Been playing for a while. I tend to design my rockets using the same general approach since I've found something that works. My designs seem to work fine with the 1.25m rockets but when I get into the bigger rockets I really have trouble. The way I like to launch is to accelerate directly upward to about 100m/s with SAS on and then do a 5-10 degree eastward pitchover. I then turn the SAS off and allow gravity to pull me through my gravity turn, adjusting throttle to keep my TWR ~1.5. Once I hit about 35,000m I turn the SAS back on and pitch over to full horizontal acceleration. On some of my 1.25m rockets this works beautifully allows me to fly a smooth gravity turn all the way to 80-100,000 meters altitude and puts me in a nice 0 degree inclination orbit. On my bigger rockets I have problems with persistent yaw and sometimes roll during the portion of the gravity turn when SAS is off. Attached is an example design I've been having trouble with. Basically it starts to yaw at about 10,000m and puts me into a highly inclined orbit unless I leave SAS on and fight it all the way to orbit. I've confirmed COT is midline and CoM is ahead of COL. I've tried more fins, less fins, fins with and without control surfaces with no apparent affect (though I did notice that when I had control surfaces they seemed to be fighting the engine's thrust vectoring and it steered better without control surfaces). I've tried every conceivable symmetry of fins. At first I was having trouble with the rocket pointing straight up in flight (basically partway up around 10k meters it swings like a pendulum and starts flying straight up). I've never figured out the cause exactly of this problem conceptually but I encounter it commonly with my rocket designs. It seems some how to be like the reverse of flipping. Adding fairings to complex payloads seems to solve it. Then I was having trouble with the rocket flipping so I changed the fuel prioritization to burn the posterior tanks first followed by the anterior tanks and this was solved. Nonetheless the northward yaw persists and I can't think of anything else to try to fix it other than scrapping my design. I don't mind starting over but this seems to come up again and again and I'd really love to understand what is happening here. Thanks in advance. Screenshot **Update** Thanks all for the many responses. I have done some more extensive testing with very little success so I figured I'd report the results of some experiments. 1.) Tried resetting trim. Trim was already zero'd (flying with keyboard not joystick so no dice). Tried trimming out the yaw. Too finicky. 2.) Tried removing the side booster fins. Tried more fins, bigger fins, w/ and w/o control surfaces. Nothing consistently made a difference. Enough fins would make it fly straight but then it couldn't be steered. 3.) Adjusted the fairing to cover the knuckle in the fuselage. This got rid of the vertical stability issues (flipping, penduluming) but didn't fix the yaw/roll problems. 4.) Got rid of the asymmetrical fuel lines. This did reduce the torque on the stages. 5.) Carefully removed one part at a time to try to figure out where the S0 and S4 torque in KER is coming from. Guess what, it is coming from the engines themselves. 6.) Tried gimble locking the bobcat and steering with control surfaces. Steers better in SAS but no change to the problems when SAS is off. 7.) Switched to a skipper (in spite of the size mismatch) thinking maybe it was the bobcat. Slight improvement? Not enough to matter. 8.) Tried stabilizing the side boosters with different configurations of struts. Current thought: could there be inherent torque from the engines and my rocket is just too short thus doesn't have enough of a lever for my fins to counteract the torque? Tried switching to a skipper and adding two additional of the tall 1.875m fuel tanks. And it's...better. Still yaws to the north and rolls a bit but it can fly all the way to 35km when I reengage my SAS while only accumulating a few degrees of inclination. Of course it is way more DV than I need for this payload. Current Added query - the reason the original was so short is that the bobcat doesn't have enough thrust to lift more central fuel tanks since it still has a full fuel loadout after the side boosters drop because of the asparagus staging. Is it just not viable as a core stage engine with asparagus staging? **Final Update** See my reply to OHara below. Here is the final design. TL;DR: - Fuel lines in radial symmetry introduced asymmetry. They were functionally unnecessary - Rocket was too short - Payload was too draggy - Side booster fins were exaggerating any asymmetric forces that developed during flight - Bobcat has slightly asymmetric thrust, consider reserving for very stable rockets and/or using SAS - 4 symmetric fins as far to the rear as possible on the center stage, with control surfaces worked best

You phrased it very politely, but this is more than semantics, this is knowledge. Thank you, this was exactly the sort of information I was looking for. That makes sense. Sometimes I have some trouble with roll and yaw during the unassisted gravity turn. This comment made me wonder if I could use the advanced tweakables to disable pitch control from the SAS when I start my gravity turn to let gravity take over but still allow it to maintain yaw and roll constant. Nonetheless, there is something beautiful about a rocket than can ride a smooth gravity turn all the way to orbit without any controls inputs required (other than throttle). Makes you feel like you got the design just right. I read through your drag post. Very thorough and interesting. Some of it is a bit beyond my ken but I did notice one interesting component of your analysis that may be relevant to this problem. If I understood your post correctly, it looks like the drag forces on trailing parts decline significantly at Mach 1. Looking at my screenshot, it looks like my "flip out" happens right as I approach the sound barrier. The fairing solving the problem does suggest that the underlying problem was drag at the front of the spacecraft. Perhaps at subsonic speeds the trailing part drag is sufficiently balanced with the leading part drag that the lift from the fins and the force of gravity on the COM are able to maintain straight flight, but when it passes through Mach 1, the relative increase in drag at the front of the craft as the rear drag decreases is enough to throw the forces out of balance and push it out of straight flight. Thanks! I'll give that a try with my next post. I have few more nagging challenges I may pick your collective rocket scientist brains on.

Hi all, I feel I've started to get the hang of my rocket designs from reading some good posts on the forums as well as from lots and lots of trial and error (feels like the scene from October Sky). I still have some challenges with the in-atmosphere performance of some of my rockets that have been pernicious and I was hoping some of you experienced designers could enlighten me on some more theory. For reference, I am playing vanilla KSP current release career mode and have all the 90 science parts unlocked except the aircraft tree with a few 180 science parts. With some help from the forums I've basically solved my "flippy rocket" problem. What has helped me as far as design principles are: Rockets fly in atmo like a badminton shuttlecock. You want mass in the front, drag in the back and for stability you want some distance between them. So for solving flippy rockets you can: 1.) Move your center of mass forward. This seems to be difficult as generally your payload is your payload and the basic layout of the booster rocket is often driven by the TWR of the engines you have access to. The main helpful tip here for some rockets has been turning off fuel consumption in my most anterior second stage fuel tank and releasing the fuel only after the after tanks have been nearly exhausted. 2.) Don't fly too fast. Throttling my LF engines to keep TWR around 1.5 has worked well for me. Knowing that I can throttle down SRBs in the vehicle assembly has also been helpful when working with lighter payloads. 3.) Add drag to the back. For me this has been the most helpful (though still feels like the least elegant solution). Sometimes you just need MOAR fins at the back of the rocket. I would love any other tips to help with this problem that are more elegant than MOAR fins, but in general, my problem with rockets taking a nosedive mid gravity turn have disappeared. Unfortunately another trickier problem has cropped up. I've managed to solve it eventually for my various rockets, but I haven't been able to figure out the principles behind why it is happening and how to prevent/fix it. The basic probably is that mid gravity turn, the rocket suddenly swings like a pendulum and points straight up. I feel like this may be related to the "flippy rocket" problem, the two solutions I've found on specific rockets have been adding MOAR fins on one rocket and using a fairing to cover a very draggy payload on another rocket (to much drag on the anterior part of the craft??). Like I said, I would love some help understanding the "why" this happens so I can avoid it in my designs and fix it without so much trial and error (I admit, I play hardcore once I get my flights into space, but I do use the revert flight option if I have problems in atmo and consider it the roleplay equivalent of the simulations and wind tunnel testing one would be able to do in real life). Here is an example of the rocket where I solved the problem by adding the fairing. (Can't figure out how to upload images, hopefully can access through the links). Original design without fairing Original Design penduluming during gravity turn Successful design with fairing For reference if needed, my typical flight profile is launching 90 degrees eastward with SAS enabled. I make a 5-10 degree gravity turn once velocity exceeds 50m/s and turn off SAS once it stabilizes (usually by 100 m/s if I fly well). I then adjust my throttle to maintain TWR ~1.5 until I reach 35km and reengage SAS, adjust course as needed to circularize as quickly as possible w/ apoapsis between 80-100 km