Cunjo Carl

@xendelaar Thanks! In retrospect, the whole derivation from base principals may have been a little overkill ... Hopefully it makes for some nice food-for-thought! Thanks for checking through, by the way. I did go back and finally solve for the standard orbital parameters (like the positions of apoapsis, periapsis) in terms of initial conditions, and just editted it in after the original derrivation of the orbital equation. Though I call it "a bit spaghetti" in the post, I'm actually surprised how clean the equations wound up now that I've started going back to look over other people's derivations (like wiki). Eccentricity is a fairly complicated little convenience variable, so an equation that can catch it from conditions on any point of the orbit will necessarily be a bit complicated. In the end it was all pretty fun! Please let me know if there's any questions, and I understand if it was a bit... beyond what's directly useful . Cheers! Oh, @bewing, I'm burning with curiosity now. Do you happen to know how KSP goes from position/velocity to the orbit?

My favorite quirk from the migration is when you occasionally see people coming back after a while with, like, a thousand posts and 20 rep. It holds a nigh-mystical status for me. In contrast it seems like you were a rep grand gate crasher . . Also, it's too bad there's a 5-dot hardcap, or you and @Red Iron Crown would definitely be up to 6! Yeah, right? I'm pretty sure I'll won't be seeing that 5th dot 'till 2020, so figured I should throw a little party now while I had the chance! Your new avatar is much more.... uhh .... evocative to your alias . You're getting close on the 5% returns run- Best of luck finishing it out! That'll be a heck of a thing to make a fourth dot out of.

Hmmm.... This is a thing I've wanted to do for ages! My goal is to provide the math for @Maltman's request with high school level math/physics concepts (the kind I work best with). It'll be long, but hopefully intuitive and informative.

I'm pretty sure it's intended. Alteast, I've been using this for ages to keep my craft upright when landing a rocket on sloped ground. Often you'll land on a slope, then you want to hop out to say plant a flag, but SAS will turn off if you do and your craft will tip off balance due to gravity, and the whole thing goes pear shaped. So, you turn off SAS and set trim to hold your craft upright. Then your Kerbal can safely hop out and do their mission while the manuevering jets / reaction wheels keep the craft standing tall. You just need to be careful to not mix this with active SAS, or you'll get a funny thing I call the simon says glitch, where the craft mimics your motions, but that's another story. Yep! One thing you may not have tried yet was the content from my answer (atleast it's not clear from your posts). Did you try turning off SAS before holding alt+x to clear trim while the kerbal is still in the craft? In the present version, you can set trim with SAS on, but you can't clear it. I believe this is a new bug since v1.0.5, but it's easy enough to work around for now. If this doesn't happen to work for you, feel free to send one of your save files, and I'll give it a try on my pure-vanilla ksp and see if it reproduces on this end! I haven't been able to reproduce trouble otherwise, save the one HvP cured for me. Hopefully we'll get you going too by hook or by crook. EDIT: Trim will tend to build up on your craft if you're maneuvering while phys-time-warping. The phys-timewarp involves alt+> , and maneuvering is WASD. Together, they make alt+WASD, which sets trim. This can also happen to me a lot if I'm using rails time warp to halt rotation while doing similar silly things *cough* I mean, purely dignified things! You then need to go and clear that trim before hopping out of the craft. It's not ideal, but it's a funny result of the way the controls got laid out. Perhaps this happened for you?

A place for the occasional Dot Hype As you post, your account will accrue dots at infrequent but regular intervals. In a lot of ways they're not too exciting for their own sake- quality over quantity for posts! Still, they make for milestones that can stand for quite a lot. For everyone, there's a lot behind each dot~ For me, I'm in a place where I have free time for a single hobby: KSP + the forums! So that fourth little circle kinda represents my free vocations over nearly a year. That's a lot of weight! What went into it? ~40 challenges (a few of which I got to host) Math galore Offering occasional help in the gameplay questions board, with a habit of making posts way too long A couple hard-earned KSP firsts, and finally... More exclamation points than are healthy in a lifetime, let alone a single year! It was a good year. I'm looking forward to another one. And on that note, I can announce with great excitement that my post-count is on track to catch up with Sal Vager's by mid 2051. See you then! I realize this is a little conversational for the Network board, but... What's in your dots? For the old timers, was there one that stood out or felt momentous? For new comers, what kinds of things are you going to try to cram into the next dot?

@Spaced Out Mun's a tricky one! There's precious little flat ground, and it's very difficult to tell from orbit how slopey your landing site will be. The good news is there's a half-dozen solutions, but none of them are magic bullets. Hopefully they'll give you a starting point for thoughts on new designs! Also, unless you have a play-style reason against, I'd say don't be afraid to test your craft ideas in a sandbox mode with cheats to get a feel for things. Making the idea-design-test-result cycle quicker can make a surprisingly big impact. No-tip lander options 1. Scout ahead. Send out comsats and rovers to scout out landing sites in advance. Finding flat-ish ground can really make all the difference 2. Broaden the leg-base of your lander. Depending on the size of your craft, you can replace taller fuel tanks with a few more stout tanks attached radially near the bottom. You can then put legs on these to make a squat, stout craft which will stay upright even on the steep slopes. These tanks will typically need aerodynamic cones for launch, but you can place your legs directly on these cones, so it's generally not a big deal during construction. 3. Increase springs on legs. If you're on a slantly slope, a higher spring constant will keep your rocket more orthogonal to the surface, rather than sagging downhill and tipping off balance. They also make you sproing off the surface like a super ball when you're trying to land, but you can't win 'em all! 4. Add tons of maneuvering. With enough reaction wheels (anywhere) or manuevering jets near your rocket top, you can just let your rocket lay flat to land and then sit up off the surface when you're ready to go. This works better on smaller, stouter craft, where the 1.25m reaction wheel really works lovely for its size, and the torque required isn't too vast. On larger craft, a couple jettisonable twitches near the top can serve a similar role. 5. Use airplane landing gear rather than legs. They naturally stick out, rather than almost straight down so it's much easier to get a broad base with them! Just make sure to account for the added weight, and pull them out of the tank like they want to be when you first place them. It looks a little silly, but it works nicely. Don't forget to apply the brakes! 6. Go to Minmus instead. *cough* It's got lots of nice flat places for easy landing, and is generally a much nicer trip. It's harder to get into its SOI, though. Reentry from Mun options 1. Multi-pass through the upper atmosphere ~35-45km. This will bleed off the speed slowly, and create less heating all at once. The downside, of course, is it'll bleed off the speed slowly. I tend to put my final pass in the 22-28km altitude ballpark. 2. Lots of empty fuel tanks can help slow you down if your trajectory isn't too steep; this works well with the multi-pass. In this case, it's best to fly 'broad side of the barn' style to catch all the wind you can to slow down. For this, make sure to bring plenty of batteries, or have a way to recharge between passes (solar panels). Especially in this case, you can use body lift from your craft to control your trajectory. Making your craft a front-slash into the wind will make it lift up. 3. Retro-boost before entering the atmosphere. By bleeding off a few hundred m/s, you can often make easy work of an otherwise tricky reentry, or save yourself a couple passes. It wastes deltaV of course! If you happen to have extra fuel in the tanks, I'd say no harm done. 3. Use heat shields. In this case, you wouldn't so-much need to use them for their ablator, but instead for their high thermal tolerance. By removing most of their ablator, you can reduce their impact on your deltaV budget. The downside of the heat shields is they're very slippy aerodynamically, so you won't get much upper-atmosphere slowing with them. This can actually turn into a downside. 4. Put your kerbal in the service bay... There, I said it! Command chairs in service bays are so effective at reentry, that many consider them cheaty. I consider it funny! To be honest, it's probably best to avoid this for your first journey, but be aware that it exists. 5. Use fins as makeshift airbrakes. While this doesn't work with basic fins, structural D wings can be used as makeshift airbrakes from Mun return velocities. It's considerably more difficult than the other options, though. 6. If you're in the lower atmosphere but lawn-darting down to Kerbin, you often have a better chance of survival out of the craft than in. Bail! This will always work over water, and works maybe half of the time over land. It's KSP, so there's of course other options for all these, too. Hopefully something sounds fun and useful for your case. Best of luck! Let us know how it goes. edit: woops, didn't notice others posted while I was typing in little bits here and there. You can post images by: 1. Press the Print Screen button (or your computer's equivalent) while looking at your craft in KSP. 2. Alt-tab out and open Paint (or your computer's equivalent). In paint, press ctrl+v to paste in the photo. 3. Save it as a jpg. 4. Go to an image hosting service (imgur is this board's favorite, but I like Postimage for its simplicity) 5. Upload your photo to this site, and it'll provide you a link. 6. Paste that link into your post, and it'll appear as a photo like magic!

Hey! So I've actually been running this challenge again for a while now. I didn't want to spam the challenge board with updates while I'm the only entrant, so I've been making my incremental updates off in Mission Reports. That said, I've recently hit some major milestones in progress, so I'm bringing along the highlights. 1. How do you earn enough science to make a Mun launcher in 8 minutes? A: Be very picky about which science you collect. Take only the juiciest bits and rocket on! 2. How do you get 1.5M funds for your interplanetary extravaganza with a single flight? A: Figure out the World's Firsts contract system. 3. How do you gather 3000 science in 20 minutes when you only started the game 20 minutes ago? Make a very fast biome hopper (pictures link to lander video) Also, if you fly like me, crash once or twice! 4. All together now! 5. What are the limits of practical low-part/low-cost launchers and low-mass aerobraking return craft? A: Clusters of kickbacks and twinboars work a treat, and nearly-ablatorless-heatshields make for some wonderfully efficient returns. (pictures link to craft video) 6. Where to go next? The whole solar system's our oyster. I've routed it so each craft can visit 3 bodies on a trip (eg. Laythe, Bop, Eeloo), so the speedrun can be finished out by 4 of them plus a special one for Eve. Of course there's a lot of work left! The next big question is how to setup interplanetary transfers quickly? Time spent twiddling maneuver nodes adds up fast... Welp, thanks for reading. See you in a couple more months!

@Overland Will a seafloor elcano train be next up? @Kergarin I'm always dang impressed with your work. Especially the stuff on Eve! Also, I dig the new emblem. I think at the moment that this is stock KSP's fastest manned non-space-plane. But in any case, I'm the first to achieve level aerodynamic flight traveling beyond escape velocity on Kerbin. Just to put the cherry on top, I did it going West! While the shots in the slide reel show a large SAS wheel, it was replaced in later versions with a 2.5m service bay for cargo. The idea was to release probes to interplanatary space right from the comfort of Kerbin! ... but ... The probes kinda explode as soon as they're released. Yeah I shoulda predicted that. Still, the plane's neat! It features: An inflatable nose cone Pretilted wings, designed to hold the plane down 100% reuseablility Gentle aesthetic clipping A beautiful view (of fire) Slide Reel Because the upwards centrifugal force and downwards wing 'lift' both increase with speed, the plane has a lovely broad velocity range where it operates smoothly. The maneuverability also rises with speed, so the plane can be crept higher and higher (into thinner atmosphere) as it speeds up to prevent firework impersonation. Its primary limit right now is fuel!

@grafdog1138 What does it typically look like? It's funny, everyone here does things a little different, but we all get there in the end! As for how my trips look... A much-more-massive-than-necessary rocket does a vertical launch at sunrise... any 'ol sunrise! Clusters of kickbacks and drop tanks peal off the thing as it screams straight up to interplanetary space. For a Duna trip, the core rocket would be composed of a skipper followed by a set of 3 terriers, the outer two staged on radial decouplers (everything tuned to TWR ~1.5). The top can just be a standard pointy pod. The craft would push just out into interplanetary space, and then I'd make a maneuver node (in solar orbit) with just enough prograde to put the craft onto Duna's orbit. By sliding the maneuver node around, you can find the time when the burn will put you into Duna's SOI (you can find your window). Make the burn, and a couple weeks away from Duna, make a 'correction burn' manuever node to put you on course to whatever part of Duna you'd like to visit. For a first visit, I'd recommend having the periapsis at ~10km off Duna's surface. You know, the poles are nice to visit... bleak, all be. I'd normally aerobrake using body lift from the rocket to maintain a ~7km altitude, but for a first trip I'd recommend just using the Skipper stage to bleed off extra speed in the upper atmosphere (down to 1000-2000m/s), and then use it to control a pleasant descent, finally dumping the thing a little ways off the ground. The 3 terrier craft will be much more pleasant to land! My personal recommendation is to use the small landing gear over the medium landing legs, but that's just me. In any case, the outer two terriers can be put on 1.25m tanks topped with the big blue parachutes. Make sure to set them to open as high as possible in as low pressure as possible! While these parachutes won't slow you down enough on their own, a little kick from the engines just before landing will ensure you don't turn your trip into an extended impact test. Some extra reaction wheels really help at this stage. Just make sure to bring some extra batteries for them- best of luck and remember there's no shame in alt+F9 'till you can stick the landing! You can jet pack on Duna, but only barely. Pop on out, enjoy the fresh rarefied air! Plant some flags, sing some songs... sure maybe do some science. For the trip back, I am once again a fan of the vertical launch. If you just nip yourself outside Duna's SOI, you don't need to worry about what direction you're flying. Just use the same make a node and move it technique from before. For the trip back in to Kerbin... everyone has their own tricks for reentry. I'm a fan of heat shields with their ablator removed, but you'll find all sorts of ways. And that's about that! So. Brass tacks, what's it take? Traveling to Duna becomes "easily" possible once you get access to the science nodes that cost 45-science as well as the following 90-science nodes: kickback+skipper (heavy rocketry), fuel cross feed (fuel systems), the mini solar panel (electrics), and the big parachutes + medium legs (landing). I put easily in quotes because no matter what, everything is harrowing the first time you do it in KSP.... That's the fun! The mission will be easier if you have the next tech level higher in each of these, but I wouldn't consider it a necessity. For facilities, I consider the following to be a good point: Launchpad level 3, VAB level 2, Tracking station level 2 and Mission control level 2. It's totally possible with lower upgrades, but this is the point where you won't be fighting the system. I hope that's enough to get your brain tinkering! If you happen to want more specific information, it'd be handy if you let us know what kinda tech level you're at, and send us a snap shot of one of your ships. so we can get a feel for your building style. Best of luck, and let us know how it goes!

@HvP Nice! Well sussed, you got it for me. It was ladder drive! Between this and the "turn off SAS before holding alt+x" advice hopefully this will get it for our OP as well.

@Npreadr You can also add your own flags by adding a .png picture file to the following folder: Kerbal Space Program / Game Data / Squad / Flags / The preferred size is 256x160 pixels, but it'll stretch things to fit. Also, you need to reload ksp for them to appear. Now you can spread NyanCat to the cosmos! Or, whatever other picture may be near and dear to your heart. Have fun!

Both good suggestions! Sadly, I've also been having the same trouble with fully-stock KSP v1.3, on a keyboard with alt+x liberally applied. I'm planning on doing a work-around hunt pretty soon, so I'll let you know if I find anything. Edit: @bartekkru99 Hm... It seems to be caused directly by the Kerbal exiting the craft. So the first finding is alt+x doesn't clear trim while SAS is on anymore, which I believe is new since 1.0.5. You need to first turn off SAS to alt+x clear trim. Even if you do this for both the craft and the Kerbal, the problem persists. Also, the problem isn't affected by recent user changes to trim. I can't find any permutation of changing/clearing trims while hopping the Kerbal in and out or quicksaving/loading that effected the magnitude or direction of rotation. In my case, the effect also doesn't seem to occur when exiting a command chair, just the pod I'd been piloting from. I wonder if different kerbals would spin the craft differently, or if it's tied to the pod.... I'm not so sure this is entirely trim based for me...Turning off all wheels and batteries doesn't even effect it! Well then. Does turning off the batteries and reaction wheels make a difference for you, @bartekkru99? Anyways, bottom line, I found a work around! Turn off SAS and clear trim, then exit the craft, let go of the ladder and rails-time-warp. At this point the craft behaves normally (not spinning like a wacky perpetual motion gyro.) It will only fix the problem until you exit the craft again, but it atleast works temporarily. I hope that works for now!

In my small but jam-packed lab, Prof. Honeydew and his luckless assistant stand as a constant reminder of the promise and dangers of science... Release the fumes!!

@Loren Pechtel Depending on the size of your craft, the tiny octagonal (and 'cubic octagonal') struts are surprisingly stout, and can prevent things from rolling around 'log style' . Whether they'll survive your reentry is another question! Just coming from LKO shouldn't be an issue, though. Best of luck!

Video update! It covers the advancements in design for the new general-use launcher. My hope is to use 3 or maybe 4 of them for everything except Eve and Gilly. I think I'll do a small photo/text update rather than a big video for the next one, I had to stare at this mysterious error a few too many times for my liking. Jeez, youtube... Here's the synopsis though: Q: For a vertical launcher what's better: fuel or TWR? A: Strangely, fuel came out ahead. Still scratching my head on that one. Q: Since v1.2 is extra draggy for flat-top tanks, should I bother with nose cones? A: Nah! Q: What's the fastest we can reenter Kerbin straight down? A: 6.5km/s is no big deal! So long as you use this one weird trick (a nearly Ablator-less heat shield backpack) .... I've realized in retrospect this is a bit overkill. Even the most reckless Moho-return only has 5km/s at reentry. Q: Can you reenter with explodey engines still attached? A: Yep. Q: How floppy is rocket? A: Too floppy .... Q: What's the best possible rocket? A: Just consult the MATH! (I mean, it's actually still super nuanced, but the math definitely helps with guiding some decision making) Q: Wanna go to Tylo? A: Pshya! Well, that was enlightening. Thanks for joining me once again, and for next time please look forward to pictures of craft going all kinds of funny places, and likely my first attempts at making "direct from surface" interplanetary burns. Alright, that gives me 2 weeks to come up with a flashier term for it. See you then!

@53miner53 I think... I think there might be two of them. If the logic with the reentry rocket staging doesn't make sense, let me know, and I'll give it a look. Good luck! We'd be happy to have you onboard, @Bubbadevlin! What are you interested in?

Suggestion: Hammers and ions?

Somehow this posted before it was ready. Avert thine eyes! Edit: The topic's fairly different, so doublepost! I did some thinking and worked together an equation exclusively for sizing drop tanks. The idea is we have a stage that starts at TWRmin, burns for a while, drops a tank and then finishes its burns. In this way, the drymass of the tank doesn't hold you back, and you can coax a tiny bit more tasty deltaV out of whatever mass you've got at your disposal. The question is, how large should this droptank be relative to the stage it fuels. If it's too small, it's just not worthwhile, but if it's too big then it drops just before the next stage and there's not enough time to enjoy it being gone! The answer should depend on the mass of the engines, which I happen to handle as the min TWR requirement of the craft divided by the TWR of the engine (like from KSP wiki). The easiest way to handle it is to consider mainstage and droptank as two separate stages: first the drop tank, then the main stage. The drop tank stage gets to include its engine into its 'payload' mass fraction, because you get to keep on using it. Then the follow-up mainstage needs to size its engines to be large enough to start the droptank stage with enough TWRmin. It works out to: p = mmainStage / mfull = payload fraction of droptank 'stage' q = mpayload / mmainStage = payload fraction of main 'stage' ΔVcraft ln(1/9 + 8/9 * p) + ln(1/9 + 8/9*(1/p)*TWRmin/TWReng + 8/9 * q) = g0 * Isp * ln(mfull/mpayload) ln(p*q) (or its equivalent in e-scale, which I've been using!) So I discovered on paper what I'd been finding in practice, that you only really get like 3-5% boost to your deltaV when using drop tanks like this at 1g-ish TWR, and that droptanks are best sized to be larger (heavier) than the mainstage when TWRmin/TWReng is greater than about .11, and smaller when less. The optimum is quite broad, so there's no real reason to get picky about sizing them precisely. I'm happy to send derivations or spreadsheets if folk would find them useful. Have fun!

@GoSlash27 Ooh, it's cool to see your approach rederived from the base principals! I'm plugging through it at the moment. I like that you're using the term "goodness" for your figure of merit, that term has always made me happy somehow. One thing I'll suggest is to use is a goodness factor in terms of deltaV times 1/ln(1/payloadFraction) rather than times normal linear payloadFraction. The place it becomes important is when saying your results for a stage should also apply to that stage in a rocket. I hand-waved right across the distinction in my analysis for the original question, but for a few days before posting I struggled with the concept, having gotten some very promising looking answers that didn't stand up to reality because I also used a linear payload fraction in the goodness factor. So what's different between the two options? Why would we want to use one vs the other? I think it's easiest to describe through case-and-point. ... Let's say we have a stage with a given goodness factor (from its given TWR, engine, payload fraction, and deltaV/stage) . If we build a rocket using two stages of this configuration, that rocket should have the same goodness factor as the individual stages. This is what lets us say that the goodness of a stage will make goodness for our rocket, which is the end goal. Otherwise we can't compare the two. So let's look at our options for goodness and see how they fit this description. And just for visual simplicity, let's rewrite the logarithmic goodness factor: deltaV*1/ln(1/pyldFrac) = -deltaV/ln(pyldFrac) characteristic 1 stage 2 stage rocket deltaV X 2X pyldFrac Y Y2 deltaV*pyldFrac X*Y 2X*Y2 ... The goodness changes when using the same stage twice, that can't be right. -deltaV/ln(pyldFrac) -X/ln(Y) -2X/ln(Y2) = -X/ln(Y) Success! The goodness stays the same. So the realworld interpretation of this logarythmic goodness factor is that it's a number that when multiplied by ln(RocketWetMass/FinalPayloadMass) will give you the rocket's deltaV... I've been calling it DeltaV per e-scale. Given the logarythmic nature of the rocket equation, it kinda feels intuitive that the masses should be tucked in a log... well, intuitive after a bit of hand waving perhaps . I hope that makes sense and doesn't muddy things. I'm happy to talk further, and I'm looking forward to seeing what you come up with next! Also, I like that you're comparing LV-N, Dawn and LV-909. It's the classic feud! (Well, the classic feud after they nerfed the 48-7S spark at any rate ) I've been stuck at the same point for about 2 weeks! I have a couple heuristic restrictions I've been tinkering with, and I'll pass them along if they come to anything.

@Galileo Oh no, is that an Arizona bark scoropion? <checks wiki> . There's apparently a million varieties, but it looks like something from the genus. Those stings sound awful, sorry! Er... apparently don't be bashful about calling your advice nurse if you feel terrible today, there's apparently treatable lingering symptoms. If that is some kind of bark scorpion I mean. Huge props for catching a scorpion at 4am with nasty stings distracting you, that can't have been easy. Edit: looks a bit more like the striped bark scorpion, with the less gangly tail. Could still be all kinds of things of course- just reading. Sorry again, that's a rough way to start the day.

Welcome to the forums, @aerodis! Nice detailing on the Jag', I especially like the classic enormous spotlamps. Let us know if you have any questions- good luck in your challenges to come! I'm impressed with how well you got that design to work on the fly, @tsgaerospace. Fine work! The bump at 12:55 had me biting my nails . Also, that craft had a mind of its own, you did a good job finding the balance between guiding it and letting it drive as it wanted. @Ragingdonut Hey, a tricycle! I have to admit, I wasn't expecting to see any in this challenge. They're notoriously difficult to balance for high speed driving, which makes the vehicle more impressive to me! Am I noticing you've splayed the rear wheels a touch for enhanced friction control? I know it's a little late for your run, but to answer your questions in case others have them: The car needs to be able to stop, though parachutes are a fine approach . I'd rather not have folk worry about droppable markers- just nip a bit off the distance if you take a mile to slow down! You only need to save the 1 kerbal, though I'll make mention of it if you have multiple kerbals hitch a ride. Again, great job balancing the trike!

It was a good try, but I happen to know the next will be @Ultimate Steve.

@ARS I'm a connoisseur of science minutia, so it's nice to know I can still call 'em! That is the coolest looking iso for your design mockup, did you draft that? I had to look up the name- cute reference Railguns are notoriously resistant to simple theoretical analysis. It's frustrating, because the elements that go into them are all pleasantly fundamental and politely linear, but when you get them all together it turns into a jumbly mess unless you make some.... not-entirely-justifiable assumptions. Especially in regards to heating. But They are double happy funtime for finite element analysis spreadsheets! We can Googlesheets a model together if it sounds like fun? For fair warning from the start, a system like the one you're proposing won't do much for boosting velocity, but it will be fun . It'd require about 20 hours of work, most of which would of course be up to you. I'd be happy to help get you started, though. Not trying to butt in, just an offer In any case, best of luck on your thesis!

@Ragingdonut, Sounds neat!

@ARS Should all work great! The first bit with the acceleration after the barrel can also be a little easier when calculated with energy rather than momentum. In this case, we can see the projectile as having a kinetic energy (speed) which gets added to by the force as it travels over that distance. The energy that gets added is conveniently force*distance no matter how fast the projectile is moving! Initial KE + Energy Added = Final KE 1/2* m * v12 + F * d = 1/2* m * v22 How do you know when to use energy (W=f*d), and when to use momentum (f=m*a)? It's tricky, sometimes you can use one, the other, both... sometimes you need both! In general, if something is happening over a certain distance it'll be easiest with energy, while if something is happening over a certain time it's easiest with momentum. Rocket stuff (like engines) are almost always firing for a certain time, so momentum reigns king around here. Finally, how is it accelerating after it leaves the barrel? Especially at 30g, it could be a rail gun . Railguns have a hard time getting the projectile started, so one concept has been to start the projectile moving with another means, and then finish accelerating it with the rail gun (in this case a snubby little 1m one). Of course the practical cons heavily outweigh this in general, but it's a fun rationalization! Where is the right place for homework questions, @Shpaget? Edit: If there happens to not really be a place, would you mind if I made an offshoot thread for them?