Nosecones...

[mstachowsky]

So I was playing around with nose cones today and I learned that they do *something*, but I'm still not sure what. Here's what I did:

I launched a small SRB with only a probe body, turned SAS off, and watched. Once it started to break the sound barrier it blew up. When I put a nose cone on, it didn't and kept on going until burn out. So somehow the nose cone was able to negate the pressure of the atmosphere.

Another very weird thing, although this may be some kind of physics I don't know, was that once the red "re-entry" animation showed up (it was going VERY fast), it started to accelerate and *increase* its speed. I would have expected it to decrease speed due to increased air resistance.

Tests with the large SRB show the same thing happening.

On the other hand, fins seem to do very little. fins are supposed to stabilize a rocket, but I found that they didn't really - the SRBs veered off course just a little bit in the same way regardless of whether or not I had fins on the rocket.

Any thoughts on why this is?

MIKE

[Jimmy Jazz]

Maybe you can do more tests... The SRB blowing up without a nosecone can be just a kraken related occurrence, without multiple test you can't really know. As for the reentry, it is normal for a body to accelerate due to gravity when going towards it's perigee, or in this case towards Kerbin, it takes time for the planet's atmosphere to catch up and for the object to start decelerating due to atmospheric drag.

As for the fins, I think they only help you maneuver the craft that has no thrust vectoring, while they can often add to general instability.

Edited February 6, 2014 by Jimmy Jazz

[TechnicalK3rbal]

The nose one shouldn't make a difference at all (without mods) because there's no reentry heating in the game yet. Meaning stuff doens heat up when you go fast. So there's no reason it should have blown up att all, unless there was some kind of glitch.

[Karretch]

Considering the physics of the game doesn't have aerodynamics, the thing that most likely happened between your tests (without knowing from multiple tries) is that the nosecone added more drag which slowed your rocket down. A slower rocket impacts the air less, thus less Gs, thus may not reach the damage threshold for blowing up. My best guess is the rocket simply went too fast for it to handle.

[mstachowsky]

This is why I was confused. The rocket was still burning and going up when the red re-entry animation showed up and the acceleration increased significantly, *upwards*.

Every time I launched the small srb without the cone it blew up before getting to the red re-entry animation. When I had the cone on, it didn't.

I'm using an unmodded version. Does anyone want to confirm or deny my results?

[capi3101]

Around 0.21, the drag coefficient of nose cones was reduced, so they should reduce drag. They don't reduce drag enough to account for their mass, IMHO, so as a rule I don't bother with them. The reduced drag is not something you should notice with the vanilla game's drag model - its based on mass instead of cross-section as in RL, which is why a flat surface with a lower mass will produce less drag than one that includes a nosecone in the vanilla game. So if you want the nosecones to count for something, install Ferram Aerospace Research.

Me, I don't play with FAR. Yet.

[mstachowsky]

This might get posted twice so sorry about that:

1) The re-entry heating animation shows up even if you're going up or down, as long as you're going fast enough. What I found was, with the small srb, the rocket would accelerate upwards once the red animation showed up with significantly more acceleration than the rocket engine normally provided. the rocket had not burned out by the time the red animation showed up.

2) Is there a stress calculation that goes on with regards to airspeed? Maybe the nose cones reduce the drag and therefore the stress?

3) Is anyone interested in replicating my results to see if it's not just my setup? I'm using the vanilla version and did not physics warp.

[Dispatcher]

My fueled nose cones and adapters of course are more massive than empty nose cones. However, I had reduced the drag on those to approach a more aerodynamic effect. This was before Squad reduced the drag of the stock nose cones. For my fueled parts, see the sig line. As for the the post above, its good info.

[Claw]

I have heard throughout the forum and in many videos that nosecones are useless because the drag coefficients simply add together. I have noticed radial drag coefficient factors in some .cfg files so I don't know if/how those are used.

That being said, I haven't experienced the random exploding SRB that related to speed (and not say, an overheat). I have seen ascent profiles affect the overheat of an engine, so there is some kind of connection.

Anyway, I think I know what I will be testing tonight.

As for fins, placement is very important. It also depends on what kind you use, so I wouldn't necessarily say they add to general instability. But they can also make a reasonably stable rocket go out of control if not done right.

[Roxette]

...the rocket would accelerate upwards once the red animation showed up with significantly more acceleration...

As the rocket ascends and the fuel is consumed, the mass reduces, but the thrust remains constant, so the accelleration will increase.

[mstachowsky]

Yeah, I agree about the concept that the acceleration should increase, but I noticed a marked increase in acceleration as soon as the red stuff showed up: the acceleration went from the green into the red on the navball display and the speed increased dramatically.

[Jimmy Jazz]

As the rocket ascends and the fuel is consumed, the mass reduces, but the thrust remains constant, so the accelleration will increase.

Let's not forget that atmosphere gets thiner as you ascend, reducing drag and allowing greater acceleration.

[PlonioFludrasco]

Applying nose cones reduces drag coefficient a little because, unlike many (stock) parts they have drag cohefficient of 0.1 instead of 0.2.

The total drag coefficient of the craft is not simply the sum of the various drag cohefficient, but their ponderal average relatively to mass.

If you apply a small nose cone (mass=0.03t; drag=0.1) on a large SRB (mass=7.875t; drag=0.3), the result is that you have a craft of mass=7.905t and drag=(0.03x0.1+7.875x0.3)/7.905 = 0.299.

Now consider total drag force, and remember that at sea level that is proportional to mass, drag coefficent and (speed)^2.

Then drag on the SRB without nose cones is proportional to 7.875x0.3xv^2 = 2.3625v^2

With nose cones: proportional (with the same constant as before) to 7.905x0.299v^2 = 2.3636v^2

In this case is a little worst to put a nose cone in front.

In general:

Drag(w/o nose cones) -proprotional to-> M(=mass of the craft)D(=drag coefficient)

Drag(with nose cones) -proportional to-> (M+m(=mass of nose cone)) x [MD + m x d(=drag c. of nose cone)]/(M+m) = MD + md That is ever > of what written above as md is >0

[Pecan]

Tried it and can confirm it.

Okto2 probe core (the little one) on top of a small SRB blows up "due to overheating" at ~4km when the initial (white) atmospheric effects start to show up. NB: the booster doesn't but the probe core does.

With an aerodynamic nosecone on top of the okto2 the atmospheric effects proceed to plasma (red) and the whole thing goes higher until the SRB burns-out.

Edit: Some of the probe cores have the lowest heat-tolerance. Further tests show putting just about anything on top (in front) of them will 'absorb' the heat and prevent them exploding. Cubic Othagonal Strut works instead of nosecone, for instance.

Edited February 6, 2014 by Pecan

[Jimmy Jazz]

Hm, I didn't know that there is overheating due to atmospheric friction in stock game. Is it thus possible to blow up on reentry if you go in too fast?

[Greenspan]

Yeah, I don't think the "red stuff" causes more or less drag in this game, it's a result of going a certain speed through an atmosphere certain density.

My understanding from goofing off with probe bodies, sepatrons, and unlimited fuel cheats is that the nose cone doesn't really prevent your ship from blowing up due to less drag/heating, its simply protects the probe body from "windshear". You might try repeating the same experiment with a structural plate or some non-areodynamic "shield'" and see what happens, I think the result would be the same as with the nosecone.

Edited February 6, 2014 by Greenspan
spelling

[Pecan]

There's no readout of current temperature as far as I know so no way to tell. As far as I can see the probe cores with max 1,200 degrees tolerance are the only ones that explode so they'd be the only ones that might do so on re-entry.

To check, make sure they are the front-most item. As noted above anything else appears to shield them.

Edit: More testing. Appears to be an artefact of sitting low-temp tolerance probe core on overheating booster. Just driving the core through the air at plasma speeds doesn't make it overheat/explode. In addition anything between the core and booster prevents explosion. Not sure how this fits with things the other side of the core also preventing it. Anyway - no stock re-entry overheating explosions.

Edited February 6, 2014 by Pecan

[Jimmy Jazz]

So, does the core explode do to overheating, or due to atmospheric drag - like the solar panels do?

[Pecan]

Flight report (F3) says overheating - all I have to go on. If it were 'torn off by drag' there'd be no reason something behind it (between it and the booster) would change anything. Still, how often does anyone need to sit a probe core on an SRB with nothing else attached?

[Jouni]

The probe core overheats due to the heat from the SRB. Attaching just about any component to the probe core will enable enough heat transfer to prevent that. If you have just the SRB and the probe core, the booster will also eventually overheat due to not being able to transfer the heat anymore.

I think that drag is applied individually to each of the components. If you have a Mk1 Cockpit (drag 0.1) attached to a fuel tank and an engine (drag 0.2), drag keeps the nose pointing forward at high speeds. On the other hand, if you replace the cockpit with a Command Pod Mk1 (drag 0.2), the rocket does not favor one orientation over another. This means that if we had heavy enough nosecones (aerodynamic fuel tanks, for example), they could be useful in balancing the rocket under some circumstances. The stock nosecones are unfortunately too light to be useful in the drag model used by the game.

[Geschosskopf]

So, does the core explode do to overheating, or due to atmospheric drag - like the solar panels do?

AFAIK, in stock there is 1 and only 1 source of heat: running engines. So not air resistance. And there is 1 and only 1 method of heat transfer: conduction. No convected heat, and (very significantly unrealistic) no radiated heat. Thus, what's happening is that heat from the SRB is conducting into the probe core, raising the probe's temp above its tolerance until it blows up.