GoSlash27

My first Mun landing was done with the trial version using RCS stages. It went without a hitch. My first Mun landing using the stock components was done with liquid engines and also went fine. It was either my 4th or 5th munar landing that went south.

Currently 113.2 tons of fuel in Kerbin orbit. Half of it will be expended on the way to Eve and the other half will be expended landing and leaving there.

My official position: Any argument that begins with "it is scientifically impossible for something to be scientifically impossible" is bound to contain a logical fallacy somewhere since it is fallacious to argue the impossibility of the basis of your own argument. It's the logical equivalent of dividing by zero. By declaring scientific impossibility impossible, you have just argued that your own argument cannot possibly be true. FWIW, I agree

Not if you hold it upside down *EDIT* Ninja'd!

Oh, I'm definitely a nerd. I love the math/ science/ engineering and Jeb is hilarious. I'm just not the kind of nerd who would dress up like a Kerbal, learn to speak Kerbin, or get into details about trivia or which aspects of Kerbin history are "canon". Not looking down on people who are into that sort of thing, but it's just not me and in that moment it appeared like I was.

So I'm standing in the middle of the Riverside, IA Star Trek museum ("The future birthplace of Capt. James T. Kirk!" ®) musing over the absurdity of the whole "trekkie" thing and suddenly I hear a beeping noise and a familiar voice. "Sir, we have an incoming transmission." And yeah, it's Lt. Uhura. And yeah, everybody can hear it. And yeah, it's coming from my phone... Aaand the lady behind the counter is grinning at me... I couldn't even begin to explain that I was only there because I thought it'd be fun to gawk at the trekkies and I just had the ringtone because it was funny. The damage had been done...

I had worked out the same thing a couple weeks ago using a spreadsheet. While staging does generate better mass ratios, there is a hard limit to how many staging events in a single phase of a launch will yield positive results. Usually 2-3 is optimal for a single phase and I've never found an instance where 4 was an improvement. The most efficient method I've found is 7-5-3-1 asparagus staging, followed by 4-2-1 layer cake staging (which yields over 90% the efficiency of asparagus staging). The least efficient method is a single stage, which yields approximately 55% the DV for the same mass. To answer your question, the asparagus pancake design you see used is actually very efficient, although a poorly- executed asparagus setup is a lot less efficient than other methods when properly executed. Where this analysis leads you astray is you're not fully simulating each stage working lifting a payload. The mass ratios get worse with each stage you add, which would lead you to believe that less is more, *but*the mass ratios of early stages have no negative effect on the mass ratio of the entire assembly other than the additional decouplers. The DV is additive, not the mass ratio. So while the additional stages contribute less to the total DV of the vehicle the more you add, your total DV is still improved. A couple different ways to look at it are to: -compare a simple 3 stage rocket to an SSTO. While the SSTO has superior mass ratio, the 3 stager needs a tiny fraction of the mass. -Mathematically, doubling a DV requirement requires 7.4 times the mass ratio. (e^2) Therefore splitting up the DV requirement into 2 stages will yield a mass roughly 27% the original. Likewise, 3/e^3 for 3 stages yields 15%; 56% the mass of a 2 stager. (N+1)/e^(N+1)/ N/e^N shows diminishing returns until the added mass of decouplers finally exceeds the gains. HTHs, -Slashy

I'm thinkin' you should define the precise circumstances that apply to your chart. Are we talking a burn from LKO for a direct intercept using add-ons? Will we be using a vanilla installation and shooting for an intercept from Kerbal escape at an ascending or descending node instead of a full plane correction at apoapsis/ periapsis? Will we be assuming a missed intercept with a 1 orbit correction? These scenarios create wildly divergent DV budgets and they can't be factored for. You can't simply take a baseline DV budget and alter it by a certain percentage to cover a different set of assumptions, so step 1 would be to lay out what the assumptions are. Best, -Slashy

I always dock with my SAS on. I've never found a reason not to. Bringing a load in straight really isn't very difficult and I don't use any mods. ^ This is personal preference rather than actual advice and is subject to change upon my first affliction with Kessler syndrome Best, -Slashy

An example of what this approach can yield: A stack of "TRAC" utility rovers bound for orbit aboard a Shotput/ Courier launch vehicle. That little booster is capable of taking that big chunk of hardware all the way to orbit at 80KM, execute an intercept/ rendezvous/docking and still have enough DV left to execute deorbit... A "courier" delivering a stack of TRAC rovers to a Startrader shuttle outbound for Eve. I also launched that Startrader aboard a Shotput booster, although I did away with the insertion stage in that case since the Startrader already had all the required hardware aboard to complete it's own orbital insertion.

Building an efficient rocket made easy: Break up your launch into 3 phases for planning. Each phase can be covered by whatever staging scheme you prefer (or even just a single stage), but the engine requirements are different for each. The 3 phases are boost, transstage, and insertion. The boost phase gets you from the pad to your gravity turn kick downrange. It's straight up and lifting the whole rocket, so it needs good thrust. Weight isn't an issue for this phase. You want at least 2:1 thrust to weight. Your DV in this phase is 1,500. The transstage takes you from the gravity turn to your apoapsis. It requires good Isp in atmosphere and should be as light as possible while maintaining at least 1:1 thrust to weight. Your DV is 2,000 The insertion stage circularizes your orbit and carries out any intercept, rendezvous, and docking. Thrust isn't important here, but light weight and high Isp in vacuum are critical. 1:2 thrust to weight or even less will work just fine. DV is 1,500. The process starts with the payload. See if you can lighten it up to give your launch vehicle an easy time. Send it up empty and fuel it in orbit/ make it modular and bring up the pieces separately, etc. Once you've got your payload set, you're ready to start. Warning, math ahead... The 3 equations you will be working with: A) e^(ÃŽâ€V/9.81Isp) = Rwd Given a DV you need to achieve and the Isp of the engine you intend to use, this will spit out the required mass ratio (Rwd)for your stage. "e" is the natural log base, approx. 2.718 Now plug that mass ratio in here: 9(Rwd-1)(Me+Mp) ________________ = Mft (9-Rwd) Me is the mass of your engine Mp is the mass of your payload This gives you the mass of your tanks when fully fueled. But is your engine powerful enough to lift it? Note* This equation only works with small and large radius tanks. Other tank sizes require more C) 9.81Rtw*Mt=T Given a desired thrust to weight ratio (Rtw) and total mass (Mt), this spits out how many kilonewtons of thrust you need (applies only to launches from Kerbin). Find the number of engines you need and repeat the process. You can experiment with different engines and staging schemes to find the lightest stage that will do the job. You do this first for the injection stage carrying the payload. Then you apply that stage+ payload to the transstage. Finally you apply both of these stages + the payload to the boost stage. Once you've got a firm grasp on what each stage needs, go and build your rocket. That's really all there is to it. Best, -Slashy

Looking at this post, I'm thinking you can get by with the efficiency you already have. You just need to get cute with how you use it. If your lander lacks DV to get back to Kerbin, the easy solution is to send another ship to Munar orbit that doesn't have to waste fuel on landing/ launching from Mun. You can hook up with that one after you take off and ride it back to Kerbin with plenty of DV to spare. Plus your return ship can use engines that are optimal for that trip instead of the radial engines you used to get off of Mun.

If I understand your question, you would like to know how to design an efficient rocket. When to use which engine, etc. The problem is it's impossible to do that without math. Since the process of designing a lifter is pretty much all math, I won't walk you through it, but I can explain a couple things that might be of help to you: Your launch is composed of 3 phases and each phase requires different types of engines: *Boost phase- This is where you lift off the pad and lift a big heavy rocket straight up out of the thickest part of the atmosphere. *Transstage- this is the phase that carries a lighter load from the initial tipover up to space. *Injection phase- This is the stage that accelerates the payload into orbit. Each phase has unique needs from it's engines. The boost phase is all about brute power. You want something that can lift a heavy load quickly, so use engines that have high thrust numbers. You want this phase to generate about 2 Gs of acceleration. The transstage is working with a lighter load and, since it is tipped over on it's side, it doesn't have to push as hard. Look for an engine that has high Isp numbers in atmosphere and medium- low thrust. It needs to push just hard enough to keep the show rolling. The lighter you can build this stage the better, since the boost stage has to haul it up there from the pad. It only needs to generate about 1 to 1 1/2 Gs of acceleration. The injection stage will be working in space for a long time and thrust really isn't an issue with it. You need to use an engine with high Isp in vacuum and don't worry so much about the thrust. Getting this stage light and efficient is critical, since it's weight has a cascading effect down the stack. If it's heavy, then your transstage will have to be a lot bigger to move it and your boost stage has to be gimongous to lift both. 1/2 g acceleration is fine here, and if you fly it right it can still do the job with less than that. A good design starts with the injection stage and works backwards, but since that's pretty much 100% math, that's where I'll have to leave it. When working with the upper stages, always be thinking "how can I make this lighter and more efficient while still making it work?" Try to get creative with your payload by doing things like breaking it into smaller pieces and assembling them in orbit, or sending them up empty and fueling them in orbit. Good luck! -Slashy

This is the sort of situation that people stress over when designing a spacecraft and why most of the DV maps are of limited value. Nobody wonders "how long would it take to get to Jool if everything went perfectly" when designing a ship. If I'm designing something with life support, I want to assume that we miss our rendezvous and have to correct our orbit to intercept on the next pass. I would say the time for transfer plus one orbit of the target body plus whatever fudge factor makes you comfortable. Best, -Slashy

Thanks! These numbers seem like a pretty good ballpark from what I've seen.

I agree with all of this. The variations of eccentricity and inclination could be included by simply projecting the worst case scenarios. For example, a DV map estimate from Kerbin to Eve would assume a Hohmann transfer from Kerbin's apoapsis to Eve's periapsis and the steepest possible inclination change, which would be the sum of their inclination.

Yeah, I gotcha. Whereas I would prefer a chart that shows "realistically feasible" numbers instead of "technically possible". YMMV

Rendezvous about 3/4 of the way through your intercept orbit, you should see your green prograde marker get close to the target prograde (pink). At this point, you should... 1: burn to the opposite side of the pink prograde in order to "pull" the green prograde onto it. 2: Burn toward the target keeping the symbols overlapped until your closure rate is 10 times the range to target. 3: flip and begin burning retrograde, keeping your retrograde (green) on the target's pink retrograde, but this time by "pushing" it instead of pulling. You want to burn just enough to keep your closure rate 10 times the range to target. 4: At 30m range, you should reduce your closure rate to zero. You should now be in close formation with the target.

Intercept Step 1: Get in the same orbit as your target. Step 2: If the target is behind you... 2a: wait until you've just passed the CPA markers (red arrows) 2b: burn prograde until the 2 arrows are touching. If the target is ahead of you... 2a: wait until you've just passed the CPA markers (red arrows) 2b: burn retrograde until the 2 arrows are touching *OR* until your periapsis is at minimum. In either case, you don't have to complete the intercept in a single orbit if you're short on fuel or pushing minimums. Note that when I say prograde and retrograde in this case, I'm referring to the direction of the orbit itself, and *not* the symbols on your nav ball. I'll post the "rendezvous" part of it after work...

#1 thing I can recommend is this: Balance the center of lift to the center of gravity for the fuel tanks first so it doesn't go all hinky as the fuel drains off. Everything else can be balanced after that. -Slashy

Most of us *try* not to do that. Myself included. But in reality unless you're planning on everything going right, the maps are too optimistic. You can get most places that way with less DV than the maps suggest, but you don't build a rocket based on the assumption that everything will go right. That's the point of DV maps.

I'm not knocking the map itself. AFAIK it's just about exactly right. But in my experience so far, it represents a best scenario rather than a worst or average, so I look at it as a rough guide rather than a number to hang my hat on. A more realistic number IMO would be one where it assumes an average phase angle, average alignment of apoapsis, and a worst case ascending node so you're doing a Hohmann transfer orbit and need to correct at periapsis say 50% for an intercept on the next orbit.

After your initial "gravity turn" kick at 7km, you bring your nose toward the horizon at a slow and steady rate so that you're pointing 22* off vertical at 15kM and 45* at 25km. Keep the pitch rate steady, and you will see your prograde marker switch from surface to orbit. At that point, switch to your orbit map view. if you've kept your slow rate of pitch going, your apoapsis should be getting close to LKO altitude. Now... the idea is to raise the periapsis without raising the apoapsis, so you steer it by what the apoapsis is doing. If you're pointed to the opposite side of the horizon far enough, your apoapsis won't change. If you're down even further, your apoapsis will decrease. Less, and your apoapsis will increase. No matter what, your periapsis will continue to increase. The time to apoapsis will also be affected by what you're doing and vice- versa. the closer you get to apoapsis, the less effect your pitch angle will have on it's altitude, and the more effect it will have on it's time. You want to try to keep it 30 seconds ahead throughout the circularization. So you juggle it simply by pitch to keep your apoapsis constant while raising your periapsis. Once it becomes impossible to keep your apoapsis marker steady at 30 seconds ahead, you're there, so cut the engine. I've never used FAR, so I don't know how or if that will affect the process... but it shouldn't since the circularization happens outside the atmosphere. Best, -Slashy

1) Not kill or strand any Kerbals 2) Place an inhabited outpost on every body in the system (with the exception of Kerbol and Jool) with a space station in orbit and a launch vehicle in attendance. 3) Never land, crash, or ignite a nuclear engine while in Kerbin atmosphere. That should be doable...

I don't look at the ratio of thrust from one stage to the next, but rather the thrust that I need from each stage to do it's mission. For a Kerbin payload lift to orbit, I break it down into 3 phases (terminology may be wrong) in reverse order: 3) Injection. Stage + payload to establish uniform orbit, intercept a target, rendezvous and dock. 1,500 M/sec DV at no less than .75 G. 2) Transstage. Stage + injection stage + payload from gravity turn (roughly 7kM) to establishing apoapsis at 80 Km. 2,000 DV at no less than 1G. 1) Boost stage. Stage + subsequent stages + payload from the pad to gravity turn. 1,500 DV at no less than 2G. I may (and usually do) break these stages down into sub- stages to lighten the upper stages. Upshot is: The thrust ratio ends up being whatever it happens to be. Best, -Slashy