GoSlash27

Oh wow sorry guys... I wasn't trying to post my spreadsheet as an alternative to the OP. I was just looking for suggestions on how I can make mine visually cleaner. My spreadsheet has a lot of experimental content that's not quite ready for primetime and I wasn't planning on distributing it. Sorry for the confusion! -Slashy *edit* Mesklin has a spreadsheet similar to mine here: http://forum.kerbalspaceprogram.com/threads/122407-Optimal-engine-selector-for-v-1-0-2

Lynch, I like how you've laid out this spreadsheet visually. My spreadsheet works backwards from this, effectively automating the process. I give it the mission parameters and it designs a booster using each engine so I can see which design is lightest and/ or cheapest. I wonder if your visual layout might benefit my model... Here you see my inputs in yellow and the outputs for each engine type in green. Number of engines, mass of empty tanks, mass of fuel, and total stage mass. Best, -Slashy

mrklaw, Really it doesn't have to be that precise and it's not going to be. Your burn time isn't instantaneous, so your ejection angle isn't going to be exact. The easy way to do it is to set a maneuver node approximately correctly and just slide it around until you get an intercept solution. Once you've completed the ejection burn, you can fine-tune your prograde/ retrograde for a tighter solution. You can save the normal and radial burns for when you're halfway there (what we call the midcourse correction). Personally, I think getting the phase angle right is far more important. I do that by keeping a small probe just outside Kerbin's SoI and using it's maneuver nodes to generate an intercept. The burn timer will show how long until the window opens up. Best, -Slashy

mrmcp1, Honestly, that's more than plenty of DV for LKO ops. I'm kinda puzzled as to why you would want more. Best, -Slashy

All, I ran a new scenario using 10 engines in the single stage and vacuum Isp. Here's the crossover points by engine type and minimum acceleration: Type__|0.5G_____|1.0G_____|1.4G ______|_________|_________|_____________ LV-909|4.0km/Sec|3.5Km/Sec|3.4Km/Sec LV-T30|3.7Km/Sec|3.4Km/Sec|3.2Km/Sec Above these DV budgets, it's lighter to stage. Below, it's lighter to go single stage. Of course, this does not take into account changing acceleration requirements, changes in Isp and thrust with density, and engine advantages/ disadvantages in different flight regimes. Best, -Slashy

Yasmy, I believe this is *exactly* what I'm looking for! I'll have to review it more thoroughly. Also, I think I know why I kept getting my odd result: I was starting from a baseline where the single stage was already optimal. I was choosing the payload so that one engine was able to meet the requirement in a single stage and then trying to find a lighter staged solution using the same engine. Of course, this means that my upper stage has more engine than it needs, so r will never get under 1. *facepalm* Thanks about a million! -Slashy

Pretty much. The question is "for an arbitrary engine and uniform profile, at what point will staging yield a reduction in launch vehicle mass?". What would an equation to solve this problem look like? - - - Updated - - - As a practical matter yes. As a mathematical problem, no. I am aware that the single stage "solution" will not *actually* function on Kerbin due to changes in pressure affecting Isp and thrust, but that question is not solvable mathematically. The problem is more generic than that. If given an arbitrary engine and mission requirement, at what point does staging show a reduction in stage mass? I would have thought that it would happen when the mass of the empty tanks in the single stage match the mass of twice the added engine in the two stage design, but the model isn't supporting that assumption. It seems that the single stage design is working out lighter until the DV requirement exceeds the capability of the single stage design.

Requia, Mathematically you can't do that. Or more specifically you can't reduce that concept down to an equation that'll tell you when to stage a lifter. And in most cases in KSP 1G acceleration is overkill. That is certainly true for the early going in a Kerbin launch, but not necessary for most of it. Best, -Slashy

Ah, I bet that's what it is; you're not keeping the requirement and engine type uniform throughout the budget. Yeah, I understand that staging is worth it AFA changing requirements for t/w and changes in Isp with pressure, but that can't be solved with an equation. What my results are suggesting is that for a uniform payload, engine and constant profile demand, a single stage will be lightest so long as it is capable of doing the job. Bumping the requirements to match 1.4G, the skipper single stage can still do 64.7 tonnes, which is still lighter than the 2 stage solutions. As I said, I'll have to verify this against my work spreadsheet, which scales the number of engines automatically. It's the model I used to corroborate Meithan's work. Best, -Slashy

Starhawk, According to my spreadsheet at home, I can do a single stage using a single Skipper at 61.7 tonnes, which is lighter than either of the 2 stage solutions. My solution for the Mainsail is 80.9 tonnes. I'll have to verify this result against my work spreadsheet. What minimum t/w did you assume? Scratchin' mah head, -Slashy

Interesting... My testing so far says that (in a very generic situation with identical payload) staging never reduces total mass except where a single stage is incapable of doing the job. Example: LV-T45, vacuum Isp, 1.0G minimum accel DV Mp M1 M2 1000 12 19.2 22.8 2000 8.0 19.0 23.5 3000 4.7 17.3 22.7 5400 0.0 9.22 13.4 I'll have to think on why this would be..

Actually, you're thinking the same way I am about it. In a finite case, the added engine mass and decoupler are balanced by the dead tank mass. Maybe it's just that simple; the mass of the empty tank is twice the mass of the second stage engine and decoupler? *scratchin' mah head...* -Slashy

Starhawk, I have the ability to generate the data in his charts. I'm just wondering if there's a formula that will predict the crossover point where staging saves mass. Best, -Slashy

Is there some way to mathematically determine whether or not to stage or how many stages to use for a given engine, payload, DV budget, etc? Best, -Slashy

hsp, 3 RAPIERs 1 TJ and 1 nuke seems a bit much considering I can orbit a 54 tonne spaceplane with just 4 RAPIERs. This "Texaco" design is 107 tonnes and makes orbit with 6 RAPIERs. They represent 22% and 26% payload fraction, respectively. Best, -Slashy

The generic recommendations here work well for me. Not much to add except that RAPIER designs can build speed at around 20 km altitude while TJ hybrids like to build speed around 15-17 km. Good luck! -Slashy

KerbMav, The main advantage of using a reverse rocket equation spreadsheet instead of KER is that you can optimize your designs without having to build them. You just plug in the mission requirements (minimum t/w ratio, DV, etc) and it will tell you how many engines you need, how much fuel/ tankage, and total cost for the stage. It does this for all engines simultaneously, so you can see at a glance which design will best-suit your needs. This takes all of the trial and error out of the process and allows you to design missions even when you're not at home and playing KSP at the time. Example: Here you can see that for a 7 tonne payload with a minimum accel of 1G at Kerbin in vacuum, the best engine to use is the LV-909. I will need 2 engines, 3.83 tonnes of fuel and tanks, and the cost will be approx. $780. But perhaps 2 LV-909s would make for an awkward stage, so I limit myself to stages that only need one engine. Now the Aerospike is best for mass, but it's expensive and I can't easily stack a preceding stage underneath, so I reject that one and go with either the Poodle or LV-T30 depending on whether I want the wider diameter. Whichever way I go, I can see how much fuel I need for the stage and how much it will cost. Best, -Slashy

I calculate the DV on my own. It's not as difficult as it appears, but the math isn't for everyone. Step #1: refer to the DV maps for basic mission planning. For all else, there's Basics of spaceflight Step #2: Plan the mission from end to beginning. Each stage is payload for the preceding stage Step #3: Apply the rocket equation backwards Step #5: ??? Step #6: Profit! Best, -Slashy

Engines now lose thrust with altitude as well as airspeed, and it's much lower and slower than before. No more "turbojets until apoapsis, then a little puff to circularize". And as stated above, airhogging doesn't work anymore. You also have transsonic drag, so you need to punch through Mach 1 at around 10km before continuing. This makes SSTO spaceplanes harder to do and lowers their payload fraction, which is a good thing. You might as well throw out your old spaceplanes and start over. There's lots of threads around regarding spaceplane design in the new system. Good luck! -Slashy

No funding = ain't happening. Best, -Slashy

Rune, Wow! I'll have to try that! Thanks for the tip Best, -Slashy

Rune, No, that one's just the runabout. It's not loaded to full capacity and has lots of extra fuel for orbital maneuvers. I think I've only got that one loaded to 2 tonnes or so. My cargo variant (which I don't have pics of right now) carries 3 tonnes. Yeah, the RAPIER is anemic around Mach 1 and that sets the limit for those engines. Multiengine designs can exceed 18 tonnes per engine and still break Mach 1. The difference is that while getting supersonic isn't a big deal for turbojets, getting hypersonic is a problem. That's the end where all the fuel gets wasted in closed cycle. Perhaps I'm flying them incorrectly, but my TJ lifters are barely exceeding Mach 2 at 20km up. My RAPIER designs are hitting nearly mach 4 at 23 km up and that has a huge impact on the closed cycle DV budget. Best, -Slashy

Rune, I thought I had an example posted upstream... I'll have to take some pics after work. No DV readout though; I don't use KER. Mine weighs 13.4 tonnes on the ramp and hauls 3 tonnes of payload. *edit* Oh, I do have a pic of the "runabout" variant here: http://forum.kerbalspaceprogram.com/threads/123114-Single-Turbojet-SSTO-Spaceplane?p=1981756&viewfull=1#post1981756 Best, -Slashy

Rune, I have no doubt that a TJ can outperform a RAPIER in an airframe designed for a TJ and likewise the opposite is equally true... but that doesn't really mean anything. An optimized RAPIER will still outperform an optimized TJ in payload fraction and operating cost and that's the important part. 18% payload fraction isn't awful (in fact it's pretty darn good), but I can get 22% with a single RAPIER. This exercise has shown me that TJ SSTOs are still useful and I'll be making use of them in my career, so that's pretty neat. It's also very convenient that the TJ has an alternator. Best, -Slashy *edit* Oh, no dice so far on the fuel-only SSTO. The nukes keep blowing my wings off from the heat Still trying, though...