arkie87

The space shuttle seems to launch with TWR = 1... which, since playing this game, has confused the hell out of me since it cannot be the most efficient. Do you have mathematical reasons for your conclusion or just personal experience?

Ok, so that's pretty much what i was saying. Makes sense. That is definitely true and could explain the low TWR that you reported for real-world rockets. Personally, i disable aerodynamic disassembly I don't see why it needs a tiny bit more? It will exponentially approach terminal velocity if TWR is maintained through the flight. However, this is only true in practice for stock aerodynamics, since other than blasting off to 100 m/s as fast as possible, the increase in terminal velocity with altitude is relatively mild such that it is manageable with even small TWR. In FAR, you will never reach terminal velocity with TWR=2 as it starts off too high and increases too fast to keep up with; you'll need like TWR = 10, and if so, i hope your kerbals have exoskeletons Yeah, i was thinking about body lift, so gravity losses can be managed that way, especially if lift/drag ratio is large.

Interesting response. I understand the trade-off between TWR and deltaV when it comes to selecting a different engine, and the optimum solution is dependent on the particular values that the parts in KSP have. Clearly, if they weighed the same, higher TWR would always be the most efficient choice, but for all other cases, a more complicated analysis must be conducted based on the particular numbers (i.e. we would need the mathematical relationship between TWR and total mass, so that we could differentiate it w.r.t. TWR and set it equal to zero as is done to find the optimal solution). Accordingly, this is not what i want to debate... Quite the opposite -- reduced drag means you want to get out of the atmosphere faster, since it costs little in terms of drag force/deltaV and every second you spend in your initial climb, you are wasting fuel fighting gravity. If your TWR is 1.2-1.6, you are wasting between 63-83% of your thrust fighting gravity... According to your logic, optimal TWR on the mun is lower still since there is even less drag This is true. But the alternative, for a given vehicle design which has too much TWR, is to reduce throttle or limit SRB thrust, both of which hurt your deltaV... This is the case i am interested to discuss and kind of the point i wanted to get across. That makes little sense to me. I think i am still missing something. It is always most fuel efficient to ascend at terminal velocity, and TWR will have a much harder time getting you there. What do you mean is a good approximation of hitting terminal velocity? For the reason i gave, it is impossible to go faster than terminal velocity as long as TWR = 2. By "upper bound", i think you are saying what I was saying.

I think you misunderstood what i meant by "mass in not a factor". See below. I agree, but i was talking about the mass-averaged drag coefficient (which, for the reasons you explained above, is all that matters). So all that matters is whether drag coefficient of the added part are higher or lower than mass-averaged. If added part has drag coefficient above average, then part goes less distance; if not, it goes farther. If you think about it in terms of mass average drag coefficient, then mass is not a factor...

Sorry. Title says "FAR and Stock"

I was wondering what people thought optimal TWR was for stock and FAR. It seems to me that with stock souposphere (as i've heard it called), it's very easy to reach terminal velocity (which is optimal ascent velocity). Thus, smaller/lighter engines are of great benefit since you dont need high TWR. However, with FAR, since terminal velocity is essentially unreachable (see and http://forum.kerbalspaceprogram.com/threads/32033-FAR-and-Mechjeb-terminal-velocity?p=894880&viewfull=1#post894880), it would make sense that optimal TWR is higher, since every second spent fighting gravity is ~10 m/s deltaV wasted. Now clearly, the extra weight required to increase TWR must be factored into the decision-- i.e. to choose mainsail or skipper engine, since skipper is lighter-- this makes sense. However, what doesn't make sense is if one is using boosters, what is the advantage of reducing their maximum thrust when using FAR? You do not reduce your weight, like you do when you switch from a mainsail to a skipper, so what is the advantage? What am i missing?Also, i've seen people post that having TWR=2 is terminal velocity. This is clearly silly. What they meant to say is this: having TWR =2 will ensure that your velocity will approach (exponentially) terminal velocity, but is guaranteed never to exceed it, since: m*a = T - Drag - m*g; at terminal velocity, Drag = m*g, and a=0, so T=2*m*g i.e. TWR = 2.

I am a bit confused. Are you and GoSlash27 talking about stock aerodynamics or FAR? With FAR, the loaded truck will go farther (as it will in real life) since it has more momentum at the same speed (i.e. more mass) so it will take a longer distance to stop it, despite it having a nominally larger drag. With stock, the empty truck will go farther because adding high drag parts increases total (and average) drag. Mass is not a factor since drag force is (incorrectly) proportional to mass always.

So i found out where I can obtain terminal velocity. FAR flight display includes it. I have posted a video of how terminal velocity varies with altitude. There ain't no way you are reaching terminal velocity with FAR

Sorry. I guess i missed that. Looks fine, but for FAR it has to be gradual (assuming that's what you intended). As far as i know, this just isnt true. Optimal velocity is always terminal velocity regardless of aerodynamics model used (assuming a 1/2 rho V^2 C_d relationship for drag), even if terminal velocity causes you to light up like a Christmas tree. This can be derived as follows: Fuel Usage is proportional to Thrust*time = Thrust*Distance/Velocity Thrust is related to the other forces by conservation of momentum: sum(F) = m*a = Thrust - Drag - Weight If we assume object is already at optimal velocity, then a = 0, so: Eq. (3) Thrust = Drag + Weight So fuel usage can be expressed as: Fuel Usage ~= (Drag + Weight)*Distance/Velocity = (1/2 rho V^2 C_d + mg)*Distance / V = (1/2 rho V C_d + m*g/V)*Distance If we want to find the optimal velocity, we differentiate Fuel Usage w.r.t. V, and set it equal to zero: 1/Distance * d(Fuel Usage)/dV = 1/2 rho C_d - m*g/V^2 = 0 Re-arranging, we find the relationship: 1/2 rho V^2 C_d = m*g The LHS = Drag Force The RHS = Weight So at optimal velocity, drag equals weight, which occurs at terminal velocity. (If you already knew this, then i apologize. But perhaps it helped someone else). Again, this just isnt true. In stock aerodynamics, terminal velocity is ~110 m/s at sea level, and at about 10 km it quickly increases too infinity faster than you can accelerate (if i am not mistaken). In FAR, terminal velocity starts off much higher (>600 m/s)--due to the more realistic aerodynamics-- such that you can never reach it unless you have a huge TWR. I suspect a lot of these discrepencies in arguments occur because of stock vs. FAR aerodynamics. In stock, you have to limit your thrust on takeoff often (indeed, i used to do this when i was still using stock aerodynamics) very often, since it was easy to hit terminal velocity. In FAR, it's rarely possible. This clearly effects optimal vehicle TWR. In stock it might be lower-- like 1.4-- since you will be limited by terminal velocity. In far, i imagine it would be higher, since otherwise, it would be like taking off in stock below terminal velocity... I admitted that word was mistaken. I thought i remember hearing someone mention it's more efficient, but i was clearly wrong, and already admitted that. The remaining purpose of the thread is not to debate which is more efficient, but rather, on the other advantages and disadvantages of both methods. Once again, i will just say if you are using stock aerodynamics, then this thread doesn't really apply to you, as maintaining control during turn is easier and doesnt result in spin outs. Thanks!

Granted. But that's why i worded what i said carefully

The Oberth effect is utilized in both cases since you are doing the majority of the burning pro-grade, but i suppose what you mean is that gravity will continue suck down your velocity when going straight vertical, and therefore, you will have to use more fuel to get up to the same speed to use Oberth

I tried ascending with reduced thrust such that I didn't get re-entry effects during liftoff and everything i tried was much worse than what i had already done...so please describe to me in detail what to do. Or, make a video of you doing it better with the same craft

Thank you muchly! Maybe you want to work with GoSlash27 and put together a tutorial for making a spreadsheet? The compiled parts list is a huge help for those working to create their own spreadsheet. Now I can replace "bananas" with actual LFO tanks

What is pathfinder? I'd like to see how it works And what assumptions it makes for a "gravity turn" i.e. TWR and angle profile vs. altitude/velocity (so i can put that profile into my matlab ascent code and play around) That's not bad. Only 10% more fuel for a much easier maneuver that you cant (read: is much harder to) screw up. Optimal vertical velocity during ascent is terminal velocity, no? With FAR, terminal velocity is nearly unreachable unless you have a huge TWR (much higher than i achieve at liftoff), no? Accordingly, I assume what you meant it that i started turning sideways too early, and therefore encounter more horizontal drag than needed at too low an altitude. But you cannot possibly mean that it is more efficient if you already have a high TWR vehicle to not use full throttle during ascent (assuming you are below terminal velocity)? Regardless, will try launching with reduced thrust and see what happens. That said, what is an approximate "optimal" gravity turn i.e. should i begin turning nearly instantly, or wait until i get to X altitude? What is X altitude? What if i just burn up to 30 or 40 km, then turn 90 degrees, and launch nearly sideways? That wasn't the original question (though it was the one i posed to you in PM-- see the OP), though i initially mistakenly thought that a vertical ascent could be more efficient under optimal conditions. Speaking of which, how do i change it to say "answered"?

Sorry, i think you misunderstood me; or perhaps i misunderstood you. It's an "exact" solution, but only when additional calculations are done: to calculate deltaV for two sets of thrusters with different ISP and burn time, you have to first calculate burn times of each, and then make one deltaV calculation for when they are both burning and one deltaV calculation for when only the non-jettisioned stage is burning. That would be an exact calculation. What i thought you meant it just ignore burn times and calculate thrust-weighted average ISP and use that to get deltaV, ignoring burn times (assume you dont jettison spent engines when they run out of fuel). This is a conservative estimate, as real-world deltaV will be higher. This is not an exact calculation; it is a conservative estimate. Thank you so much! Right now, i have placeholder parts, such as "banana" and "butt" with made up stats (butt obviously has the most thrust )

Thanks! That makes sense regarding thrust-weighted average ISP. I suppose I could do the math and prove that it is the correct formula. Regarding how to treat different burn times-- it is a good approximation though it is an underestimate if you are able to jettison parts before the other runs out. Would you mind sharing the part sheet? Pretty please?

So I figured out #1: Making a sheet for parts, and stages seems most elegant to me, and you can dynamically call all part specs by looking them up in the first worksheet easily by using "Match" and "Index" functions in excel

A few questions: (1) How exactly does your spreadsheet work? The best way i can think is to have one worksheet for all the part names and specs (wet and dry mass, thrust, ISP etc..). And each subsequent worksheet are stages, starting from uppermost. In each worksheet, do you type in a part name and it looks up specs from first worksheet, or do you have to find them yourself (a HUGE time killer)? (Maybe a screenshot of your spreadsheet word help?) After that, each sheet can sum up wet/dry mass and calculate deltaV, TWR etc... (2) What is the formula for calculating deltaV for two engines with differing ISP's and burn times? Is there a simple equation, or do you have to make simplifications/assumptions to arrive at a formula in the first place?

Well, i dont think i had the biggest SRB's unlocked... I usually gun for solar panels and science first, so aerodynamics is second

That's what i thought. Can you elaborate on "doing it right" and "know how the rocket climbs. I dont see how you can use that information to inform your flying? Because controlling fins arent unlocked yet in my career mode save but big SRB's are, as mentioned in original post....

I think you misunderstood what i was saying. I didnt actually land on the Mun in either case. What I meant is that with the first craft, i had to tap into upper stage just to get up to the Mun's altitude, whereas with the second craft, i left Kerbin SoI before even tapping into upper stage. Thus, second craft has more deltaV (this is clear from KER anyway so it's pretty obvious). Did you limit thrust of SRB for both cases? Clearly, the vertical approach will perform better without the thrust being limited, but to get to LKO, you might have to limit thrust. Also, using FAR is very important, since spaceships will not spin around violently with stock aerodynamics if you perform too-aggressive a gravity turn. Once again, the main point of the thread isnt to claim vertical ascent is more fuel efficient, but rather, in career mode, before you unlock reaction wheels and winglets/fins with control surfaces, it is very difficult to perform gravity turn using torque from command pod alone (i think maybe you have yet to experience this since you use stock aerodynamics rather than FAR). You said before that you "design your rockets with this [ability to turn] in mind". I would like to ask how? If you use LV-T45, the it quickly becomes more expensive in terms of Kerbucks than simply strapping MOAR BOOSTERS and using vertical ascent. And you are not allowed to use control surfaces or reaction wheels... Do me a favor: remove the winglets from your ship and try to get it into LKO efficiently using FAR (you may reduce thrust) Then launch vertically with no thrust limit and see which has more fuel left. I think that is a good test.

Craft #1: Cost: 9972 Kerbucks; deltaV: 5412 m/s Craft #2: Cost: 9842 Kerbucks; deltaV: 6685 m/s I launched craft #1, into LKO then to mun, and arrived there with enough fuel to land, but not sure about come back, since i already used some of my upper stage fuel I launched craft #2 vertically upwards and escaped Kerbin SoI before SRB's burnt out..., thus, still had full fuel in upper stage Craft #2 is cheaper than Craft #1... so i dont know where you are getting your rules from...I also think it would be very difficult to fly Craft #2 into LKO first. But you are more than welcome to try!

Maybe I should built two craft that cost roughly the same amount of Kerbucks, and have the same upper stage lander, but the booster stages will be different: (1) Booster stage made from LV-T45 engines for thrust vectoring-assisted gravity-turn into LKO and then horizontal transfer to the Mun (2) Booster stage made from SRB's using vertical take off And we can see which one can get farther

That is true, but we are comparing the performance of the same rocket being flow differently (one with high TWR, in this case). While it's true that high TWR is a deltaV killer, it might be cheaper in terms of Kerbucks, since SRB's are much cheaper and can be used to increase TWR at launch. The career definitely tries to push SRB's at present, since they are about the cost of a liquid engine (not including fuel), but its been hard for me to steer them up into LKO efficiently, since they dont stop burning and dont have thrust vectoring, so this entire post has been about that observation and one potential way to deal with it

(1) From my video, the increase in deltaV was 70 m/s on a 3000+ m/s burn, which is negligible... I agree though in theory -- 20% is too much, but from my video, its less than 2%. And I'm sure you would agree that 2% is negligible... (2) I only care about cost. And SRB's are CHEAP! (3) How do you turn a rocket with only a command pod to provide torque (no control surfaces or separate reaction wheels)? You could replace SRB's with gimboling rocket engine (LV-T45) but its more than double the price... It's a parachute, mk1 command pod, decoupler, medium size fuel tank, LV-T30 (i think), decoupler, adapter, orange fuel tank, mainsail with 5264 m/s deltaV at launch Let me know how it goes!