Particularly minute minutiae of nosecones in stock aerodynamics

[Starman4308]

EDIT MODERATOR'S NOTE: This thread is cut from another, since the discussion is becoming off topic from the user's request for help.

Cheers,

~Claw

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FAR can be a bit intimidating at first. If you find it too complex you could go for NEAR. A bit less realistic but much more forgiving and still a tremendous improvement over stock.

Maybe it's because I haven't built many planes yet, but I've never really had a problem with FAR vs. NEAR, and FAR does make some things easier (for example, more gentle reentries).

The single biggest piece of advice I can give is to forget everything you learned about how to get to LKO. Using FAR/NEAR, what you do instead is design for 1.2-1.6 TWR (I strongly recommend 1.2-1.4 unless you are using short-duration SRBs), and make sure you have fins at the bottom (to pull center of lift/pressure/drag behind center of mass). You then begin a true gravity turn: at 60-100 m/s, make a 2-5 degree turn (earlier and steeper for high TWR rockets), wait for your prograde vector to catch up, and then ride that prograde vector all the way through to upper atmosphere. Once you're at 20-30 km altitude, you can begin to have your own ideas about where to point your rocket, but until then, aerodynamics will screw you if you try to deviate from prograde by more than ~5 degrees*.

*Under 5 degrees, though, aerodynamics (if your rocket has fins) will tend to pull your rocket back to prograde. Thus, if you nailed the 2-5 degree turn at exactly the right moment, you can literally go "Look Ma, no hands!" all the way up.

The primary factors behind this all.

#1: FAR removes the souposphere, and you as such have much less atmospheric drag.

#2: With proper fin placement, aerodynamic stability will return you to prograde if you stay close to prograde.

#3: Regardless of fin placement, moving far from prograde in low atmosphere is a quick way to flip out.

#4: A consequence of #1-3: you've got to start your turn early, because otherwise you will waste a lot of dV going straight up, because you'll be unable to turn horizontal until very high atmosphere.

#5: Another consequence: you can generally get away with a lot less TWR than in stock, because you're not losing nearly as much to atmo drag.

#6: Build for 1.2-1.6 TWR (probably best in the 1.2-1.4 range), have fins at the bottom, and turn 2-5 degrees at 60-100 m/s.

#7: You will probably want to use KIDS (KSP Isp Difficulty Scaler) or some RSS config, because your dV to orbit will shrink drastically and make your LKO boosters seem laughably small. I personally use 6.4x RSS and RealFuels with the stockalike config.

*From personal experience of building many 1.6 TWR rockets: not worth it. You will struggle and curse to get your velocity vector closer to horizontal.

EDIT: Also, fairings. You'll need them for anything except already-aerodynamic payloads: I tend to remove fairings around 45km altitude, but I'm really not sure where the crossover point is. There will be some point at which the reduced atmo drag is no longer worth carrying the mass of the fairings.

Edited December 14, 2014 by Claw
off topic split

[OhioBob]

Adding fairings adds weight and therefore drag.

Technically that's not true.

The only real fairing parts in the stock game are nose cones (though I suppose there are probably some mods that provide additional parts). While most parts have a drag coefficient of 0.2, nose cones have a drag coefficient of 0.1. This lowers the overall Cd of the part. For example, let's say we have a Jumbo-64 fuel tank (64 t) with a Skipper engine (3 t). Both of these parts have a Cd of 0.2. The way the game computes the Cd of an assemblage of parts is to use a weighted average based on mass. If all the parts have a Cd of 0.2 then the weighted Cd is also 0.2. Suppose we add a large nose cone (0.4 t) on top of the fuel tank. The Cd of the assembly now becomes,

Cd = (36 * 0.2 + 3 * 0.2 + 0.4 * 0.1) / (36 + 3 + 0.4) = 0.199

The amount of drag produced is inversely proportional to the ballistic coefficient. In the stock game the ballistic coefficient is simply BC = 125/Cd. Therefore, without the nose cone the ballistic coefficient is,

BC = 125 / 0.2 = 625

and with the nose cone the ballistic coefficient is,

BC = 125 / 0.199 = 628

Adding the nose cone slightly lowers the drag. However, adding the nose cone also increases the mass of your vehicle, lowers its mass ratio, and decreases the amount of delta-V the vehicle can produce. The loss in dV from carrying the greater mass may offset any gain from lowering the drag. I haven't done the math to see if there is an overall advantage or disadvantage in adding nose cones. When I have some time to spare I'll have to put it through a simulation and see if I can find a definitive answer. Nose cones also add a little cost, so that's also a factor to consider in career games.

[The_Rocketeer]

Considering how slight the difference is for very simple rocket in your example, the increase in mass is going to far outweigh the reduction in drag on larger, more complicated rockets where the drag reduction is even tinier and the mass increase is still the same.

The only application I can see for nosecones is as a fuel-saving measure for faster-than-terminal-velocity ascents. And you're still better off throttling back.

[Starman4308]

Adding the nose cone slightly lowers the drag. However, adding the nose cone also increases the mass of your vehicle, lowers its mass ratio, and decreases the amount of delta-V the vehicle can produce. The loss in dV from carrying the greater mass may offset any gain from lowering the drag. I haven't done the math to see if there is an overall advantage or disadvantage in adding nose cones. When I have some time to spare I'll have to put it through a simulation and see if I can find a definitive answer. Nose cones also add a little cost, so that's also a factor to consider in career games.

Your analysis excludes one very important factor: the drag of the nosecone. The nature of the weighted average is such that your overall drag force is proportional to the sum of (part mass * part Cd): even if Cd gets magically redistributed by a weighted average, you're still adding a part whose mass will increase overall drag.

By adding a nosecone, you add to both total mass and total drag, and definitely lose on overall efficiency.

EDIT: In terms of your example, drag without nosecone would be ((36 + 3) * 0.2) * k, for a total drag force of 7.8 k (where k is a function of velocity). Drag coefficient with the nosecone is, as you calculated, 0.19898...: as such, total drag is ((36 + 3 + 0.4) * 0.199) * k for a total of 7.84 * k

In short, you get less drag per mass, but due to the addition of mass, you have more total drag.

Edited December 13, 2014 by Starman4308

[OhioBob]

Your analysis excludes one very important factor: the drag of the nosecone. The nature of the weighted average is such that your overall drag force is proportional to the sum of (part mass * part Cd): even if Cd gets magically redistributed by a weighted average, you're still adding a part whose mass will increase overall drag.

By adding a nosecone, you add to both total mass and total drag, and definitely lose on overall efficiency.

EDIT: In terms of your example, drag without nosecone would be ((36 + 3) * 0.2) * k, for a total drag force of 7.8 k (where k is a function of velocity). Drag coefficient with the nosecone is, as you calculated, 0.19898...: as such, total drag is ((36 + 3 + 0.4) * 0.199) * k for a total of 7.84 * k

In short, you get less drag per mass, but due to the addition of mass, you have more total drag.

I see that now. Drag is proportional to C_dA, but in the game A = 0.008M, therefore F_D ∠C_dM.

[Tex_NL]

...

In short, you get less drag per mass, but due to the addition of mass, you have more total drag.

And a lower TWR.

Long story short: With stock aerodynamics you're better of NOT using nose cones and/or fairings. In some very select cases you might get a tiny benefit but in general the pros don't outweigh the cons.

[Starman4308]

And a lower TWR.

Long story short: With stock aerodynamics you're better of NOT using nose cones and/or fairings. In some very select cases you might get a tiny benefit but in general the pros don't outweigh the cons.

No, it's always worse. Nosecones always add mass, they always add drag. Therefore, unless you use FAR/NEAR, nosecones are always bad unless you are looking for aesthetics.

You can thank stock "aerodynamics" for this conclusion.

EDIT: Unless you're looking to, for some reason, have a ballistic collision with terrain at the highest possible velocity. Nosecones will reduce drag/mass ratio, so they'd be good for that. I have no idea why you would want to do this, but it would be a use for nosecones.

Edited December 13, 2014 by Starman4308

[OhioBob]

Wait a minute! I let you guys talk me out of my own analysis. I return to my original claim, however I did misspeak. Adding a nose cone does not reduce the drag force, it reduces the drag loss.

The loss of delta-v from drag results from the acceleration produced by the drag force, which is F_D/M. We've already established that F_D ∠C_dM, therefore mass cancels out and we find that drag loss ∠C_d.

Going back to the numerical example, Starman4308 showed that the drag force with and without the nose cone is 7.84k and 7.8k respectively. To find the acceleration produced by this force we divide by mass, obtaining

With nose cone: a = 7.84k / 39.4 = 0.199k

Without nose cone: a = 7.8k / 39 = 0.200k

The rocket is slowed less with the nose cone than without.

[OhioBob]

Considering how slight the difference is for very simple rocket in your example, the increase in mass is going to far outweigh the reduction in drag

My gut feeling is that you are almost certainly correct. However since drag is so ridiculously high in the stock game, I would want to test it to be 100% certain.

[Kerbart]

Doesn't the reduced drag factor of the nose cones (providing they're pointing upward) somehow help in making the rocket more stable? I vaguely remember that there was an effect along those lines.

[Starman4308]

Wait a minute! I let you guys talk me out of my own analysis. I return to my original claim, however I did misspeak. Adding a nose cone does not reduce the drag force, it reduces the drag loss.

The loss of delta-v from drag results from the acceleration produced by the drag force, which is F_D/M. We've already established that F_D ∠C_dM, therefore mass cancels out and we find that drag loss ∠C_d.

Going back to the numerical example, Starman4308 showed that the drag force with and without the nose cone is 7.84k and 7.8k respectively. To find the acceleration produced by this force we divide by mass, obtaining

With nose cone: a = 7.84k / 39.4 = 0.199k

Without nose cone: a = 7.8k / 39 = 0.200k

The rocket is slowed less with the nose cone than without.

That's true under the assumption of zero thrust. Under the assumption of "actually trying to get to space", increased total drag force will mean more overall force resisting your rocket at a given speed. In addition, the added mass means your thrust counts for less.

So, under thrust (let's assume 200 kN)

With nose cone: a = (200 - 7.84k) / 39.4 = 5.076 - 0.199k

Without nose cone: a = (200 - 7.8k) / 39 = 5.128 - 0.2k

At k = 0 (stationary), no nose cone is better. The crossover point is at k = 51.282.

Now let's see what k means: Fd = 0.5 p*v^2*Cd*A = 0.5*p*v^2*0.008*Cd. Since I had Fd = k*Cd, k = Fd/Cd = 0.5*p*v^2*0.008 = 0.004 * p * v^2

This means that the crossover for acceleration occurs when p*v^2 = 51.282 / 0.004 = 12820.5. At p = 1 atm, that would require a velocity of 113.23 m/s. I haven't quite worked out the math yet, but I suspect that terminal velocity (most efficient velocity of ascent) will always lag behind the k-crossover. It just doesn't make sense to me that a rocket would ascend with greater efficiency when it has more drag and gravity force working against it.

Edited December 13, 2014 by Starman4308
Edit no longer relevant

[Claw]

Off topic posts split into a new thread. Carry on, with civility please.

Cheers,

~Claw

[GoSlash27]

Wait a minute! I let you guys talk me out of my own analysis. I return to my original claim, however I did misspeak. Adding a nose cone does not reduce the drag force, it reduces the drag loss.

The loss of delta-v from drag results from the acceleration produced by the drag force, which is F_D/M. We've already established that F_D ∠C_dM, therefore mass cancels out and we find that drag loss ∠C_d.

Going back to the numerical example, Starman4308 showed that the drag force with and without the nose cone is 7.84k and 7.8k respectively. To find the acceleration produced by this force we divide by mass, obtaining

With nose cone: a = 7.84k / 39.4 = 0.199k

Without nose cone: a = 7.8k / 39 = 0.200k

The rocket is slowed less with the nose cone than without.

No, Sir. You were correct the second time. (*counts toes*) Yeah. The second time

Your drag losses are still increased by adding the nose cone in stock. While the drag for that part is reduced, it's drag is still a positive finite number. Since the drag is cumulative for all parts, your total drag is still increased, just by a smaller amount than it otherwise would've been.

*edit for clarification*

Drag in KSP is calculated as .004ÃÂv^2dM and is accumulated for every part used.

As an apples-to-apples comparison, we're just interested in the dM part

If we have a part with (say) a mass of 1kg and a Cd of .2, then our drag from that is .2. If we add to that another 1kg part with a Cd of .1, then our drag from that becomes .3.

The drag coefficient of the entire vehicle collectively becomes lower, but the total drag produced is still increased. */edit*

The only way to actually reduce the drag of an assembly in stock soupodynamics is to add a part that exhibits negative drag, such as a control surface. Under the advice of my counsel, I respectfully invoke my fifth amendment rights...

Best,

-Slashy

Edited December 13, 2014 by GoSlash27

[Starman4308]

Your drag losses are still increased by adding the nose cone in stock. While the drag for that part is reduced, it's drag is still a positive finite number. Since the drag is cumulative for all parts, your total drag is still increased, just by a smaller amount than it otherwise would've been.

I do see one way in which nosecones could maybe reduce drag losses. Nosecones will increase terminal velocity, thus decreasing time spent in low atmosphere. Whether it is possible for this effect to outweigh the increased mass/drag of a nosecone, I am uncertain, and I suspect the answer is "never". I just haven't got a mathematical proof of that yet.

[Tex_NL]

No, stock aerodynamics nosecones increase overall drag and decrease terminal velocity. You'll spent more time in the souposphere wasting fuel.

[OhioBob]

While the drag for that part is reduced, it's drag is still a positive finite number. Since the drag is cumulative for all parts, your total drag is still increased, just by a smaller amount than it otherwise would've been.

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.

[Starman4308]

No, stock aerodynamics nosecones increase overall drag and decrease terminal velocity. You'll spent more time in the souposphere wasting fuel.

On that one, you're incorrect. Nosecones will increase the absolute amount of force, but terminal velocity in stock aero is a function of drag force normalized to mass. Since a nosecone will reduce the drag/mass ratio by having Cd < 0.2, a nosecone will increase terminal velocity.

If you need convincing: build a small 1.25m rocket, and build a big rocket with multiple 3.75m stacks. Same terminal velocity, because they both have about the same weighted-average Cd (0.2).

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.

You did see the thing I posted about what happens when you add thrust to the analysis, right? Your acceleration to drag is less, but your acceleration to thrust is also less for the added mass (which reflects less dV due to more mass).

Edited December 13, 2014 by Starman4308

[GoSlash27]

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.

No, Sir. I can assure you that this is not the case. The negative acceleration produced by the drag force is still higher than it otherwise would have been.

Applying a drag force of .3Kn to a 2 Kg mass as opposed to applying a .2Kn force to a 1Kg mass may *seem* like less, but the mass itself is misleading.

You were pushing the the mass with a (comparatively high) thrust, and you have negated more of that thrust than you otherwise would have. You took away .1Kn more thrust than what you had previously.

You also added a Kg of mass, but that's a whole other issue.

As an experiment to confirm this for yourself, you could try dropping a pair of assemblies, one with a nosecone attached and one without from a sounder rocket.

The object without the nose cone will actually have a higher terminal velocity.

It's soupodynamics.

Best,

-Slashy

Edited December 13, 2014 by GoSlash27

[Red Iron Crown]

Doesn't the reduced drag factor of the nose cones (providing they're pointing upward) somehow help in making the rocket more stable? I vaguely remember that there was an effect along those lines.

That is about the only practical use for nosecones in stock aero (they are great aesthetically). Parts with low Cd, like nosecones, increase stability when placed above the CoM on a rocket or ahead of the CoM on a plane. Conversely, high Cd parts like intakes increase stability when placed below the CoM of a rocket and behind the CoM of a plane. Other than that, they are just more mass and drag for the craft, and you're usually better served (by the numbers anyway) finding better ways to make the craft stable.

[OhioBob]

You did see the thing I posted about what happens when you add thrust to the analysis, right? Your acceleration to drag is less, but your acceleration to thrust is also less for the added mass (which reflects less dV due to more mass).

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.

[GoSlash27]

That is about the only practical use for nosecones in stock aero (they are great aesthetically). Parts with low Cd, like nosecones, increase stability when placed above the CoM on a rocket or ahead of the CoM on a plane. Conversely, high Cd parts like intakes increase stability when placed below the CoM of a rocket and behind the CoM of a plane. Other than that, they are just more mass and drag for the craft, and you're usually better served (by the numbers anyway) finding better ways to make the craft stable.

I find it disturbing that when I quote RIC, it's almost always to say "^ What he said"... and here I'm doing it again. ^What he said.

Placing parts with low drag coefficients in the front of a vehicle and parts with high drag coefficients is a good way to make them fly pointy- end first at very high speeds.

Failure to compensate for this leads to a lot of spaceplanes that prefer to fly tail- first on reentry.

I only figured this out myself within the last couple months, and I've been doing spaceplanes for a while.

Best,

-Slashy

[OhioBob]

The object without the nose cone will actually have a higher terminal velocity.

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.

Edited December 13, 2014 by OhioBob

[Starman4308]

The object without the nose cone will actually have a higher terminal velocity.

Incorrect. Adding a part with a lower Cd than the average will reduce the average Cd, which, in stock, means a higher terminal velocity. This is, as mentioned, because terminal velocity is inversely proportional to average Cd, and is independent of actual mass. Gravitational force goes up proportional to mass, while drag force is proportional to (average Cd * mass), so if you reduce average Cd, you have less drag at any given velocity.

You can test it out yourself. I took an OKTO2, and added either a 0.1t nosecone, or two 0.05t batteries. The one with the nosecone reached about 20 m/s faster peak velocity when dropped from 5 km (using Hyperedit).

Now, this doesn't mean you should slap a nosecone on stuff in stock: I'm still not convinced you will ever find a scenario where a rocket can lift off more efficiently, because while a nosecone increases terminal velocity, it also increases mass and absolute drag. This means less dV available to your rocket, and I strongly suspect it won't do you much good for net atmo/gravity drag losses.

[OhioBob]

Now, this doesn't mean you should slap a nosecone on stuff in stock: I'm still not convinced you will ever find a scenario where a rocket can lift off more efficiently, because while a nosecone increases terminal velocity, it also increases mass and absolute drag. This means less dV available to your rocket, and I strongly suspect it won't do you much good for net atmo/gravity drag losses.

I agree with all of that.

[GoSlash27]

I'm racking my brains trying to figure out a way to explain this in a way that won't get muddled...

If you add a low- drag part to an assembly in KSP, you have still added drag. It does not matter that it's less drag than what would've been added with a normal part, it is still more drag than you had without it.

And that's really all that matters. You cannot add drag and then expect lower losses from it. It simply doesn't work that way, at least not in stock soupodynamics.

If you add drag, you increase losses from drag.

Really bad analogy:

Imagine you have a truck dragging a boulder down the street. That sucks, so you say "Let's strap another, smaller boulder onto the truck. It's more boulder overall, but the overall drag per pound of boulder is less."

Does the workload of the truck increase, or decrease?

It increases. And the fuel economy of the truck decreases. So the drag and the losses from the drag get worse. It does not matter that the little boulder was less- draggy than the big boulder. All that matters is that it's another boulder.

I'm probably gonna have to let someone more eloquent have a go...

Sorry,

-Slashy

Edited December 14, 2014 by GoSlash27