GoSlash27

Okay, let's try this: Let's back up and pretend the term "specific impulse" doesn't exist. A chemical rocket works by combusting it's propellant and throwing it out the back as fast as possible. Some propellants will be able to accelerate to higher velocities than others due to "chemistry" or "technology". Ideally you want an extremely light molecule that undergoes a very violent chemical reaction, so you can throw it out the back at a higher speed, because moar speed = moar thrust, right? Well, this means we can define the efficiency of a rocket engine another way; thrust (kN)/ propellant mass flow rate (tonnes/ second). If we put them in like terms, 200 kN of thrust becomes 20.4 tonnes thrust/ 0.06375 tonnes per second mass flow rate = 320 seconds. We've just invented the "specific impulse" figure. Or we could define the efficiency another way; the velocity at which it kicks the exhaust out the back. An LV-T45 accelerates the exhaust to 3,139 m/sec velocity. If you divide that by g0, you get... 320 seconds. Isp is really pretty unavoidable. If you were to use a more efficient fuel or method, the resultant specific impulse would increase as a result. See wikipedia for examples of how changes in fuel affect an otherwise identical engine's stats. HTHs, -Slashy So at the very basic level, let's "invent" a rocket engine together. We hook it up to a test stand that records the thrust as tonnes of force. We supply it with a fixed mass of propellant. We fire it up, and time how long it runs. You pick the values. How much thrust, how much fuel, and how long did it run?

As I said before, what's important is that you know how the fuel mass you *are* using relates to the fixed value of 20.4 tonnes. If it's half, you'll run for half the time. If it's double, you'll run twice as long. And I think you're still confuzzled. You can run an infinite mass of propellant through the nozzle. All you need is an infinite stretch of time. You see... Propellant mass flow rate (stated in tonnes per second) is a fixed, finite value. At least until you go monkeying with the throttle. Yeah. And? Trying to figure out the hangup here, so please bear with me

Not according to Newtonian physics. You're just taking a mass and chucking it out the back. It doesn't care if you're throwing hydrolox, or solid propellants, or powdered doughnuts. The changes in efficiency from using different fuel types are reflected in the (wait for it) specific impulse itself

Because the value assumes equality between the thrust and the fuel supply. You can express a ratio between how much fuel you give it and the thrust, and your resultant burn time will hold the same ratio to Isp. Likewise, you can change the throttle setting and have the inverse apply to the burn time. Example: The LV-T45 produces 200 kN of thrust and an Isp of 320s. 200 kN is equivalent to 20.4 tonnes. If I were to feed this engine 20.4 tonnes of fuel, it would run for 320 seconds. But if I instead gave it 6 tonnes of fuel, it would run for (6 tonnes/ 20.4 tonnes) x 320 seconds = 94.1 seconds. Likewise, if I were to set the throttle to half and keep the same 6 tonne fuel supply, it would run twice as long; 188.2 seconds.

Everything you do in physics has a unit, unless it's a conversion factor or is dimensionless. It is important to keep track of the units and state what they are. Specific impulse is a measure of efficiency and it's unit is "seconds". If you convert the thrust of a rocket engine to the equivalent mass in fuel, then the engine will fire for "Isp" seconds. This handy fact allows you to directly compare one engine's efficiency to another's and calculate all sorts of other helpful things. Not 'efficiency per second', just efficiency. But it can be used to calculate burn times, changes in thrust with atmospheric density changes, the efficiency of ganged engines of varying types, exhaust velocity for the Tsiolkovsky rocket equation... Most of your math in KSP will involve this figure. Best, -Slashy

Weird... even the HTML used on Wikipedia equations fails to parse in the KSP wiki.

I was gonna try to fix it this morning, but the legal popup interferes with the login process. *edit* Trying an experiment... <math>I_{sp} = \frac{(F_1 + F_2 + \dots)}{\frac{F_1}{I_{sp1}} + \frac{F_2}{I_{sp2}} + \dots}</math> Best, -Slashy

COD, The division sign represents "per". 1.31 lb/sec is 1.31 pounds per second. Every second the engine is running, it consumes 1.31 pounds of fuel. HTHs, -Slashy

Thanks, bewing. I went ahead and completed the run without this info, but I'm sure it'll come in handy next time. Best, -Slashy

The Dunatian, I believe I just get Apatite for that. It was a normal difficulty career. Best, -Slashy

Finally finished with my 1.4.0 Caveman run. Final elapsed time 2 days, 5 hours, 40 minutes. I was hoping to finish it before getting my probes home, just missed it by 20 minutes or so. https://imgur.com/a/l9RBD I think I may be able to do it quicker next time... Best, -Slashy

Kobymaru, Clearly, it does boggle you. That's not what's important, though. What's important is that there are others who (for whatever reason) *do* find it fun. Their preferences must be respected just as much as yours. Best, -Slashy

If you really like the deprecated parts, be sure to set a copy of their part files aside for later use. Best, -Slashy

I'm wrapping up a Caveman attempt, and would really like to complete my final node before my Munar flyby probes return. I only need about 13 more science, but have exhausted all the biomes I can find. I'm lookin' for just one more biome. I've found splashed biomes for shore, grasslands, desert, and badlands. Are there any more splashed biomes for highlands, mountains, tundra, etc? Thanks, -Slashy

JamesL86, There are, actually. I've linked one upstream for mathematically designing stages. There are others for determining your stage's DV, and others for determining the DV of "mixed bag" stages. *EDIT* I'm sorry... or do you mean an in game tutorial? If so, then you're right and I agree. There ought to be an in- game tutorial or reference for this. Best, -Slashy

^ 3) take the science and transfer it to an experiment storage unit. This doesn't need a kerbal to EVA. Best, -Slashy

^This. If we're speculating about craft that can cross interstellar distances and then proceed to maneuver around with no thermal signature, then "outside the bounds of known physics" is mandatory. Best, -Slashy

I'd agree with that. Best, -Slashy

Kobymaru, Your original question was "how do we deal with building rockets without using KER/MJ"? My post was in response to that question. That's how *I* do it. Others here have gotten good at guesstimating DV by sight. Even others never have a clear idea of their DV and TWR and they prefer it that way. I understand that you don't share our preferences and that's cool... That's why they make mods. Now the question is "How is a newbie expected to deal". Well... space is hard, so newbies are expected to fail a lot and then learn from their experience. Some will trial and error and develop rules of thumb, some will get into the math and develop their own tools, some will install KER or MJ. There's no "wrong way" to do it. Keep in mind that proposing changes to the stock game affects all players, not just yourself. A lot of us prefer not having this information handed to us. It's easy for you to install a mod to provide this info. Not so easy for us to install a mod to remove it. AFA displaying Ap/Pe as a heads- up thing, I absolutely agree with that. I think the player should be able to see the data provided in the map view while watching the launch. 1st level tracking shows only Ap/Pe, 2nd level adds time to both. Best, -Slashy

A spreadsheet that is designed in the conventional manner (how much twr and DV will this stage produce) is definitely going to be tedious. A spreadsheet that is designed *backwards* (what stage do I need to build to achieve this twr and DV goal), OTOH... is not only *not* tedious, but actually more useful and user- friendly than the mods. Just plug in what you need, build what it recommends, and *kaphoomph*... profit. Your rocket does exactly what you need it to do. This is how I operate in stock, whether or not KER and MechJeb have been updated with the new release. I simply don't need the DV and TWR readouts. DV and TWR are not the answers, they are the questions. The true answers are which engine to use, how many engines, and how many fuel tanks. Best, -Slashy

You know you play too much KSP when you can "read between the lines" and recognize the failures that each of these examples obliquely references Best, -Slashy

@Norcalplanner is a man after my own heart. I build my rockets to be cheap, simple, and easy to fly. Best, -Slashy

adsii1970, Not particularly. I have a big monitor and don't really need to use the whole thing for KSP. Best, -Slashy

Nah, it's never too late. Spend enough time playing KSP, and you'll know more about this stuff than your teachers ever did. Best, -Slashy

That's "g0", gravitational acceleration at sea level. 9.81 m/sec2 It's also the conversion factor between mass and the force the mass exerts when sitting at rest. g0 is like RedHot; we use that $*&^ in everything Best, -Slashy