GoSlash27

I'll put a vote in for Apollo 15. The first mission with a rover, the first mission that was primarily about science, and the most beautiful scenery. Best, -Slashy

KerikBalm, I think you can answer that question mathematically. Assuming that adding engines doesn't hurt the RAPIER's performance in atmosphere (which is not true), you're starting with the same mass and velocity in all cases. I don't think you'll find a solution where adding engine mass at the expense of fuel to get better Isp is going to pay off in higher DV... at least not in typical LKO DV budgets. It could be worth it for very long trips, but SSTOs really aren't optimal for that sort of thing anyway. IAC, that's not the sort of question these charts are designed to answer. They seek a lightest solution for a payload and DV requirement, not a maximum DV from a fixed mass. Best, -Slashy

Red Dwarf, The thrust of the RAPIER is dependant on airspeed and air density. If you're not maintaining the same speed, you're not going to generate the same thrust, so you're going to fall behind more. This is aggravated by climbing, where the air is thinner. If you're not going as fast as you should be, you have to fix it by arresting your climb until you're back on track. I don't use fixed angles for my climb, I instead shoot for airspeed/altitude benchmarks. This keeps everything consistent. Best, -Slashy

So now that I've established the upper stage, I need to repeat the same process for the lower stage with a couple adjustments because I'm working in atmosphere. Specifically, the Isp will be the average of vacuum and surface and the thrust will be surface thrust. Average Isp is (Isp (atm)+ Isp (vac))/2 Sea level thrust is T(Isp(atm)/Isp(vac)) Starting with my upper stage (4.11t), I'll add .05t for a decoupler. Payload= 4.16t Lower stage mission requirement: 4.16 tonnes payload, 1,800 m/sec DV, and minimum acceleration of 1.4G. Engine specs LV-T30: Thrust= 201KN Mass= 1.25 t, Isp= 290 sec Lift capacity is 201KN/(1.4x9.81)=14.64t Rwd is e^(1800/(290*9.81))= 1.883 Fuel percentage is (1.883-1)/1.883=.469 Fuel mass is .469x14.64t= 6.87t Tank mass is 6.87t/8= .858t Payload mass is 14.64t-1.25t-6.87t-.86t= 5.67t Again, our desired payload is less, so we'll use 1 engine N= roundup(4.16/5.67),0 = 1 Final tank mass is (1.883-1)(4.16t+1.25t)/(9-1.883) = .671t Fuel mass is 8x .671t= 5.37t Total mass= 1.25t+4.16t+5.37t+0.67t= 11.44t Throttle setting is 11.44/14.64= .78, or 78% - - - Updated - - - So that's how the process works. If you put this in a spreadsheet, it will solve the problem for all engines simultaneously, allowing you to see which engines result in the lightest, cheapest, and least-complicated stages. An example of a simple spreadsheet implementation: As you can see, the lightest lower stage would have used an aerospike and a 48-7S would also be lighter, but the Aerospike is expensive (and not available) and the 48-7S would require 10 engines, which would be awkward. Best, -Slashy

Falkenherz, Apologies, my internet was out last night and I wasn't able to post. my example. I'll post it here in sections and update as I go. It'll be a wall of text, but hopefully helpful. So for my example, I'm building an early career manned rocket to collect science in orbit. It needs to stay within the 18 tonne limit since none of the facilities are upgraded. I decide to go with a 2 stage design, upper using an LV-909 and lower using the LV-T30. The payload including the decoupler is 1.7 tonnes, so we start there. Upper stage mission requirement: 1.7 tonnes payload, 1,800 m/sec DV, and minimum acceleration of 1G. Engine specs LV-909: Thrust= 60KN Mass= .5 t, Isp= 345 sec Process overview: 1)Design a single engine example 1a) Determine maximum stage mass for defined minimum acceleration 1b) Determine fuel percentage for defined Isp 1c) Determine payload mass of single engine example 2) Determine number of engines needed 3) Determine fuel & tankage mass 4) Set throttle limiter 1a) Based on our engine's thrust and acceleration, determine the maximum mass of a single engine stage. T/(9.81*Glim) 60/(9.81*1)= 6.12 tonnes 1b) Determine the fuel percentage of our single engine example (this is the part I had screwed up) Our wet/ dry ratio (Rwd) is defined by our Isp and DV requirement. e^(DV/9.81Isp). 2.718^(1800/(9.81x345))= 1.702 This is the ratio of our mass when fueled to our mass when empty, but we need to know what percentage of our fueled mass is fuel, so it works like this: (Rwd-1)/Rwd (1.702-1)/1.702= .423. 42.3% of our mass is fuel. 1c) Determine payload mass of single engine example Since our tanks weigh 1/8 of our fuel, our tanks are .423/8 of the total mass; .0529 fuel percentage* total mass = fuel mass and tank percentage* total mass = tank mass. .423*6.12t=2.59t fuel mass .0529*6.12t=0.32t tank mass and of course engine mass is .5t and total mass is 6.12t. therefore... total-engine-fuel-tank=payload 6.12-.5-2.59-.32= 2.71t Our single engine example could handle up to 2.71 tonnes and still meet the need. 2) Determine number of engines needed Since our payload is less than what a single engine can do, we're simply going to use 1 engine. In the event it's not enough, we have to scale the design up. (Our payload/ max payload) rounded up to the nearest whole number of engines is how many we need. N= roundup(Mp/Mp1),0 3) Determine final fuel & tankage mass The mass of tankage will be (Rwd-1)(Mp+NMe)/(9-Rwd) where Rwd is the wet/dry ratio (step 1b) Mp is our payload mass N is the number of engines and Me is the mass of a single engine. For our upper stage, it would be (1.702-1)(1.7+(1*.5))/(9-1.702) =.212t and since our fuel is 8x heavier than our tanks, Mf= 8*.212=1.69t So there's our design: 1.7t payload, a single LV-909, 1.69t of fuel, and .212t of tankage. It weighs 4.11t total. last step for this stage 4) Set throttle limiter total mass/(N*max mass)= throttle 4.11/(1*6.12)= .67, or 67%.

Falkenherz, there's an error in the process I outlined upstream. I'll correct it when I get home. I'll also walk you through a simple example to show you how it works. Best, -Slashy

Ninja'd by RIC That would definitely be the easy way to go, and yeah... computing the DV of a stage isn't something that can be eyeballed. Unfortunately, you've got to do some math. No, it's part of the stage. Payload mass+fuel mass+tank mass+engine mass is your total stage mass. It really wouldn't work that way. For a rocket to generate a DV and minimum thrust/ weight, the payload can only be a fixed percentage of the stage. Exactly what percentage depends on the engine you're using and it varies a lot with engine specs and the amount of DV you build into it. It has to be computed. You can use KER and mix/ match parts in the VAB and eventually hit on something that'll work, but you never know for sure if you've found the best (lightest and/ or cheapest) solution. Finding the best solution requires doing the math. Luckily, you only have to do it once if you set up a spreadsheet. Best, -Slashy

Falkenherz, The theoretical maximum DV for LF&O rockets is limited by the mass efficiency of the tanks. For an infinitely large tank, the rocket will weigh 9 times more when full. The DV would therefore be Isp*9.81*ln(9), or roughly 21.6*Isp. Rounding it off to make it easy, about 20 times the engine's Isp. *edit* Ninja'd! As a practical matter, this isn't a useful way to design a stage since you need to keep acceleration above a minimum and you must move payload. Instead of asking "how much more DV can I get", you need to be asking "what do I need to do to get the DV and acceleration I need for the payload". You define the requirement by DV, payload mass, and minimum acceleration, then design the stage to match it. I have linked to the procedure upstream, but here's the quick version. Start with your selected engine. How much mass can one of these push at my required acceleration? thrust/10 for 1G, or thrust/5 for .5G. Now how much of your mass needs to be fuel to meet the DV budget? [e^(DV/9.81Isp)-1]*mass And how much needs to be tanks to hold the fuel? fuel mass/8 So now you have a single engine rocket that can move a payload the needed DV and acceleration. How much payload can it move? total mass-engine mass-fuel mass-tank mass. Now it's just a matter of scaling it to work for your payload. If your payload is 10 times what a single engine rocket can lift, then you'd scale the entire rocket up 10 times. Best, -Slashy

Interplanetary SSTOs are definitely doable again. I just burned my payload fuel during a test using the RAPIER and got a periapsis inside Eve. It was more of a radial in than a retrograde, so lots of spare DV there. My sweet spot so far seems to be around 2 engines, 800 fuel (in wet wings and strakes), and 900 fuel/ 1100 o2 oxidizer. This yields a 32 tonne vehicle with 8 tonnes of payload. Best, -Slashy

Holy schnikeys! I just dabbled in 1.04 for the first time tonight, and the RAPIER is hitting nearly 1500 m/sec on open cycle! This is going to make SSTO spaceplanes a fair amount easier than 1.02, even with the Isp nerf. Best, -Slashy

AXCN, Sorry, I'm trying to follow, but this question involves mutually- exclusive concepts. You can't orbit a body *and* be completely still. Orbiting requires motion. Also, you have to define what "being still" means. Zero velocity in reference to what? Best, -Slashy

Thanks for plugging that! I'm surprised it wasn't mentioned already. I'm a big fan of Meithan's work in this area. I just sent him the corrections this morning for the LF&O engines, so hopefully it won't take long to update. His approach is excellent for identifying broad trends, while the spreadsheet excels at designing for a specific requirement. They fulfill slightly different roles, so I don't really argue for the use of one over another. I find that Meithan's charts are best for mission planning, a spreadsheet is best for vehicle design, and KER is best for live on-the-fly info. Best, -Slashy

Meithan, I took the liberty of reviewing the config files for changes in the LF&O engines since the 1.03/1.04 updates. I haven't looked at the other engine types yet. Unchanged: *24-77 Twitch *48-7S Spark *LV-1R Spider *LV-909 Terrier *LV-T30 Reliant *LV-T45 Swivel *RE-M3 Mainsail *Mk-55 Thud *RE-I5 Skipper *KR-1x2 Twin-Boar *KS-25x4 Mammoth *LV-N Nerv Changed: T-1 Aerospike: Mass reduced to 1t (formerly 1.5) LV-1 Ant: Sea level Isp reduced to 80sec (formerly 85) RE-L10 Poodle: Sea level Isp increased to 90sec (formerly 85) CR-7 RAPIER: Max thrust increased to 180KN (formerly 140) KR-2L Rhino: Cost increased to $25,000 (formerly 21,000) Mass increased to 9t (formerly 8.5) Sea level Isp increased to 255sec (formerly 170) Atm curve extended to .001 @ 5 Atm (formerly 4) I haven't looked at the rest of 'em yet. Best, -Slashy

RIC, Oh, that's just to do one-off design problems that I'm not liable to encounter again. For anything I do regularly, I set up a spreadsheet. Laziness is the mother of invention and I'm *very* lazy! It's enough for me to know that I could work it out on paper again if I ever needed to. Although... Everything that I do with a spreadsheet was first worked out on paper. I'm not sharp enough to figure out the math on the fly, so I kinda have to. And plus algebra... Best, -Slashy

A discussion that's relevant here: http://forum.kerbalspaceprogram.com/threads/125893-Kerbal-Rocketeering-101-By-Professor-Lynch-1-0-2?p=2038270&viewfull=1#post2038270 In this post I'm explaining how to set up a spreadsheet to mathematically design stages based on DV requirement, payload, and minimum acceleration. Starting from these criteria it is possible to derive the exact number of engines, mass of fuel and oxidizer, and mass of tanks required. From that you can calculate total stage mass including payload, payload fraction, fuel fraction, fuel cost, stage cost, etc. If this is done simultaneously for every engine type, it allows you to find the ideal design for a stage immediately. This is an incredibly powerful tool and I find it indispensable. KER doesn't give you this capability. Best, -Slashy

Lynch, Sure thing. I have limited time this morning, so I may need to come back this afternoon to fill in the blanks. The idea is to start with a mission requirement and end up with a design derived from it. -Mission requirement: Payload mass, required DV, minimum acceleration -Design result: Number of engines, mass of fuel, mass of empty tanks. We set up the math for one engine and then copy & paste for all others so that we can see the result for all engines simultaneously. This allows us to compare designs so we can pick the best design. Step #1: How much total mass can a single engine of this type accelerate at our minimum rate? I define the acceleration in Gs and reference it to the local gravity, such as .5 Gs on Kerbin We take the thrust in KN and divide it by the local gravity and Gs acceleration. Step #2: What fraction of this total mass must be fuel in order to achieve the desired DV? This uses the rocket equation in reverse; e^(DV/9.81Isp)-1 Step #3: From step 2, derive the mass of fuel and tanks for our single engine reference. Multiply the fuel fraction by total stage mass to yield the mass of fuel. Multiply the fuel mass by 1/8 to yield the tank mass. Step #4: Find the payload mass of our single engine reference. Total stage mass-engine mass-fuel mass-tank mass=payload mass. Step #5: Scale it up to fit our need. Payload mass required/ reference payload mass= number of engines needed for the full scale stage. Use the roundup(x,0) function to yield an integer value. At this point, we could simply use the scaled numbers and call it good, but because we have to round up to the nearest whole number of engines, we have a little more mass than the model expects and this will hurt our total DV. We have to reiterate with our total engine mass and payload in order to find our fuel load and tankage. Step #6: Compute tankage mass for final vehicle. 6a: e^(DV/9.81Isp)= Rwd 6b: (Rwd-1)(NMe+Mp) _______________ = Mt The mass of your required fuel tank in tonnes when empty. (9-Rwd) where N= number of engines Me= mass of an individual engine Mp= mass of payload 6c: Mt*8= Mf; the mass of your fuel and oxidizer. So now you have everything you need. Number of engines, amount of fuel, and tankage. From here you can figure out total vehicle mass and cost so that you can directly compare results from different engines. I design my missions backwards this way with later stages+couplers entered as payload for previous stages. For a first stage lifter I'll use the sea level thrust and an Isp mid-way between Vac and Isp. For all others I use the vacuum numbers. It only takes a few minutes to design a multistage rocket this way, and then I build it IAW the numbers. Takes all the guesswork out of the process and assures that my design is cheap, efficient, and will do exactly what I need it to do. Best, -Slashy

I concur with the earlier responses. SSTO spaceplanes have a narrow window of success after KSP 1.0. You have to learn how to make a functional spaceplane before you start trading off practicality for cool looks. Good luck! -Slashy

The RAPIER is still the basis of the most mass- efficient spaceplanes. You've just got to have a properly balanced design. Spaceplanes had to be nerfed because they were absurdly dominant. Now they're far from "pointless", but still very useful for LKO logistics. Best, -Slashy

Xyphos, Shameless plug: http://forum.kerbalspaceprogram.com/threads/118728-Slashy-s-1-02-Spaceplane-recipe The state-of-the-art has advanced a bit since then, but this will get you something functional. Best, -Slashy

Pawelk, I don't understand what you mean by 'unstable'. It looked okay to me. Best, -Slashy

I think Matt Damon is a fine choice for the role. You really don't want somebody who would be typecast or it'll distract the audience. I also think the portrayal of NASA staff looks good. It isn't the same environment as it was back in the '60s. Best, -Slashy

That's because it doesn't. He was reading about the so- called "bullet train" in California and came to the conclusion that it was going to be a colossal waste. That led him to think about better solutions and this is what he came up with. Best, -Slashy

Yeah, I suck If you'd like, I can walk you through the math and you can make your own version. It ain't hard. As mine currently stands, it's got too much content that's in an unfinished state.

Sorry, I didn't understand what you meant by "ELI5". Now I get it Any mass that you add that is not fuel is going to hurt your performance in terms of DV. Likewise, any mass that you add that is not engine is going to hurt your acceleration. Since engines aren't fuel and vice-versa, adding mass will always hurt something, which requires adding more mass to fix it. The only way to avoid the pitfall is with massless engines or infinite fuel. The trick is to figure out exactly what you need before you build and *then* build it. Or as the KER guys do it, keep track of what the rocket will do as you build. Trying to add rocket to your payload on a trial/ error basis is a highly frustrating and inefficient way to go about it. By the time you've stumbled across something that works, it'll be a lot bigger than it needed to be. The idea of "moar boosters" is a running joke around here. It doesn't actually give you an effective launch vehicle, but it is a hilariously 'kerbal' way of going about it. What you actually need isn't "moar", but rather "enough". Designing light gets you more results than designing powerful. Best, -Slashy - - - Updated - - - Hmm... simplifying the concept further... You have a car, a fat guy driving, and a fuel tank. Adding an engine will make the car accelerate better, but it won't help fuel economy at all. Adding a fuel tank will help the car go farther, but it won't help the car accelerate. In both cases, the gains won't quite double due to the added mass of parts. If you bring along the fat guy's fat friend, then add another engine and gas tank, you essentially have two cars. You won't get twice the acceleration or twice the range, just twice the number of fat guys getting to their destination.

SlabGizor, The problem of diminishing returns sets in when you start with a payload and strap moar boosters and fuel on it until it performs as needed. This is an inefficient design approach. In vacuum it's really just a matter of linear scaling once you've got your proportions correct. All the rocket really cares about is 1) how much of your vehicle is fuel and 2) how efficient your engines are. This is all easy to sort out mathematically using the rocket equation. If you do the math and then build in accordance with the results, your rocket will do what you need it to do. If you'd like, I can walk you through the process to show you how it works. Best, -Slashy