OhioBob

Thanks, I missed seeing that reply. FERRAM gave a good explanation of his reasons for doing it that way.

I don't know anything about FAR or KSP modding, but it seems to me it would be easy to compute via iteration using a loop.

Your example confused me because your meaning of the term "drag" was not clear. In my way of thinking, max_drag = drag coefficient, while "drag" is proportional to Cd * mass. Since you used a mass of 1 for both objects, then the values of max_drag and Cd * mass were the same. Although the max_drag values are not cumulative, max_drag * mass is. You happened to use an example where it was impossible to discern your meaning.

Did I also read in this thread that the terminal velocity is calculated using the rocket's instantaneous drag coefficient, rather than what it's drag coefficient would be at the terminal velocity?

Mathematically it is simply the thrust of the engine divided by the weight of propellant ejected per second. In other words, an engine with a high specific impulse will produce more thrust per unit of propellant burned.

If at anytime I've referred to the mass-weighted drag coefficient as "drag" then I misspoke. I think I have been fairly consistent in referring to that as "drag coefficient", or at least that has been my intent. When I have used the term "drag", or more specifically "drag force", I have meant it to be the force calculated by the formula Fd = ÃÂv2CdA/2.

I just performed a series of experimental launches to test the effect of launching with and without a nose cone. I used a rocket with a Mainsail first stage, a Skipper second stage, and a 14 t dummy payload. Everything was identical except for the addition of a nose cone in the one case. The nose cone was jettisoned along with the first stage at an altitude of about 12 km. After a couple practice launches to optimize my turn, I performed three launches each with and without a nose cone. That's not a big enough sample size to cancel out all the variables, but is still gives us some data to work with. Here's what I got. No Nose Come: Launch vehicle ÃŽâ€V = 4748 m/s, ÃŽâ€V remaining after orbit insertion = 238 m/s, ÃŽâ€V required to reach orbit = 4510 m/s. With Nose Come: Launch vehicle ÃŽâ€V = 4734 m/s, ÃŽâ€V remaining after orbit insertion = 224 m/s, ÃŽâ€V required to reach orbit = 4510 m/s. I think these results are consistent with what most of us expected. Adding a nose cone reduces the ÃŽâ€V of the launch vehicle and leaves us with less ÃŽâ€V remaining at orbit insertion. However, we can see that the ÃŽâ€V required to reach orbit is unchanged. In theory I would have expected the required ÃŽâ€V to be slightly less with the nose cone, however the mass of the nose cone is so small in proportion to the rest of the rocket that the decrease in drag coefficent it provides is almost negligible. Perhaps with a larger sample size we would start to see a small effect. In conclusion: Nose cones provide negligible aerodynamic advantage and are just needless added mass that harm the overall efficiency of a launch vehicle. I don't think this comes as a big surprise to anyone.

If it's based on the real world atmosphere, scale height is not constant. I don't know what model FAR uses, but the following tells you about all you need to know about the real world model: http://www.braeunig.us/space/atmmodel.htm Table 4 sums it up pretty good.

That's an incorrect restatement of my argument. I agree that if you attach Objects A and B the resulting Object C will have 2.0 mass and 0.3 drag. My argument is that Object B has 0.2 drag per unit mass, while Object C has 0.15 drag per unit mass. Since the drag per unit mass of Object C is lower than Object B, Object C will have a higher terminal velocity even though the drag force is higher.

I just performed an experiment that settles this. I launched a vehicle with two probes that I could detach and allow to free fall. Both probes included a remote control unit and a battery bank. The only difference was that one probe had an aerodynamic nose cone and the other didn't. I think the following images speak for themselves. The probe without the nose cone actually started out with a very small lead, likely due to the ejection force of the decoupler, however the probe with the nose cone quickly overtook it and pulled away. Clearly the probe with the nose cone has the higher terminal velocity. This is because of that probe's higher ballistic coefficient, despite the fact that the drag force acting on it is greater.

Are you talking about the real world Space Shuttle? The real Space Shuttle had a liftoff TWR of about 1.7.

I found that in stock the sweet spot for first stage TWR is about 1.6-1.7. One of the reasons for not using a higher TWR is to save mass on smaller engines. However, if for some reason you are forced to use a larger engine, I've found that throttling back to the 1.6-1.7 range doesn't necessarily achieve the best result. I didn't do extensive research on this, but I did find for one example that throttling back to about 1.85 gave the best result. I guess the moral is, use smaller engines when you can, but when you can't, you don't necessarily want to force a big engine behave like a little one. You might have to throttle back, but it might not be necessary to throttle back as far as you think.

Sounds good to me.

I'm not familiar with FAR so I can't comment about that. In stock, however, I've actually done some research/simulations to try to pinpoint the optimum TWR. There's no way I could test every possible configuration, so I just concentrated on a simple two-stage launch vehicle. My simulations revealed that the ideal TWRs are 1.64 for the lower stage and 1.31 for the upper stage. I've used these same TWRs for more complex vehicles (strap-on SRBs, etc.) and have found good results there as well. I've had no problem obtaining payload fractions of about 0.16 using these numbers. (ETA) These are, of course, the TWR at initial stage ignition.

The moderator split this off from another thread because he believed we were getting off topic. He apparently thought we were starting a FAR vs. Stock debate, though that wasn't the case. I agree we may have wandered a bit OT, but the discussion has always been about stock aero.

Incorrect. Drag force is proportional to mass x drag coefficient. If mass goes up while the drag coefficient goes down, then both quantities go up but not by the same proportion. In stock the difference is very small but it is there. That is the misconception that I've been arguing against. It is not the magnitude of the drag force that determines how fast the vehicle slows down - it is the acceleration that the drag force produces, i.e. a = F/m. While adding parts increases the drag, it also increases the mass. Both together must be taken into consideration. It is not valid to make the blanket statement that increasing drag force increases drag losses. Drag force and drag loss are not the same thing.

Let's try this one. Say we have a truck traveling down the road and we throw it into neutral and let the it coast to a stop. (Let's ignore rolling resistance say it is only drag that slows it down.) We now fill the bed with a load of stone. Let's say the aerodynamics of the air flow around the load increases the drag 10%. However, the weight of the load increases the total mass of the vehicle by 30%. If we now repeat the previous test and throw the truck into neutral on the same road at the same initial speed, what will happen? The truck will decelerate more slowly and travel further before coming to a stop. This is because we have 1.1 times as much force trying to slow down 1.3 times as much mass. The loaded truck, with more mass behind it, will cut trough the air more easily.

Losses are expressed in delta-v. Velocity is a function of acceleration, v = at. Acceleration is a function of force and mass, F = ma. In stock KSP drag is proportional to the product of mass and drag coefficient, F ∠mCd. F = ma ---> a = F/m F ∠mCd. ---> F/m ∠Cd Therefore, by substitution a ∠Cd Likewise, v ∠Cd When the drag coefficient decreases, the resulting velocity loss also decreases. It's true that adding a part increases the drag force, but that drag force is trying to slow down a more massive object. When you add a part that does not change the drag coefficient, the drag force and the mass both increase by an equal fraction. The acceleration produced by the drag force does not change and, hence, the drag loss does not change. However, when you add a part that decreases the drag coefficient, the drag force increases proportionally less than the mass increases. We have, F1 ∠m1Cd1 ---> F1/m1 ∠Cd1 F2 ∠m2Cd2 ---> F2/m2 ∠Cd2 If Cd2 < Cd1, then F2/m2 < F1/m1. F/m is, or course, acceleration. If the acceleration is lower than clearly so is the resulting velocity. In this case the velocity is negative and is the drag loss.

To illustrate this point I submit the following: Here we have a simplified scenario in which we assume a constant acceleration and neglect gravity and drag. In all three cases the rocket produces 3000 m/s delta-v, represented by the black arrows. In example A the rocket makes a 90-degree turn halfway through the burn. In example B the rocket make a 45-degree turn 1/3 through the burn and another 45-degree turn 2/3 through the burn. In example C the rocket makes a smooth constantly bending 90-degree turn. The red arrow represents the resultant final velocity vector. You can see that the more gradual the turn, the higher the magnitude of the final velocity vector. This illustrates why frequent small turns is far better than sudden large turns. The common technique of going straight up to 10 km and then pitching over 45-degrees is not particularly efficient.

Fair enough. I'm not out to cause trouble.

I agree with all of that.

Exactly. That's my point. The ballistic coefficient is greater on the part with the nose cone. At a given velocity the acceleration produced by the drag force is less on the part with higher ballistic coefficient. Therefore the part with the nose cone will fall faster and have a higher terminal velocity. (ETA) Sorry, I miss read your comment to say the part with the nose cone would have the higher terminal velocity. So apparently we disagree on this point.

Yes. I acknowledged in my original post that adding the mass of the nose cone was going to decrease the launch vehicle delta-v. I never claimed that adding a nose cone was going to increase the performance of a launch vehicle. I claimed that adding a nose cone would decrease the drag loss, but this gain would likely be negated by the decrease in delta-v resulting from the added mass of the nose cone.

Yes, drag force is increased, I agree with that. However drag force is not the same thing as drag loss. Adding a nose cone increases the drag force, but the increase in drag force is proportionally less than the resulting increase in mass. This means that the negative acceleration produced by the drag force is less.

With all due respect, my posts were not about FAR vs. Stock. They were about whether it better to launch with a nose cone or not launch with a nose cone. Since the OP asked if fairing parts help during a launch, the discussion seemed to me to be exactly on topic. For the record, I've never used FAR and I know almost nothing about it, so I don't see how I could possibly be having a FAR vs. Stock debate.