Mach Speed Club

Discussions on high/extreme altitude and mach busting rockets.

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Sumo310
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Postby Sumo310 » Tue Mar 24, 2009 10:41 am

Hi guys, can I ask a few questions about the simulations you run:

1. What software do you use to run your sims?
2. What sort of finishes (polished, bare, glossed paint etc) do you use?
3. What settings, ie weather / temperature / wind etc do you factor in?
4. What sort of launcher (rail, rod or tower) do you use?
5. What else needs to be considered for mach flights?

Cheers, Simon
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hybridhighflyer
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Postby hybridhighflyer » Tue Mar 24, 2009 11:41 am

For me Mach speed is dependant on breaking the 330m/second speed . this is dependant on a 50% humidity or less and a 20c temp.

however in australia if you go after that 350m/second you can promise.

if you need to find out for sure if you broke the speed of sound (mach1) just test the air density and work it out from there. resistance can be calculated from rocksim

Remember gravity in your calculations

g=9.86 in brisbane last cal

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Postby blackbird » Wed Mar 25, 2009 7:11 am

I'm using a combination of WRASP, RASAero, and Aerolab for simulations. All the information I have found indicates a poilshed or gloss finish as being the best (I don't know how I'm going to achieve this yet as I'm not intending on painting the rocket). I'll be launching using a rail (I use a rail even for my tiny rockets).

Things to consider (by no means a complete list):
As I mentioned before the deceleration after burnout is huge. So, nosecone seperation is likely to occur if you haven't got it secured properly.
Fin flutter - you need stiff/stubby fins that are aligned accurately.

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Postby Kryten » Wed Mar 25, 2009 8:48 am

blackbird wrote:I don't know how I'm going to achieve this yet as I'm not intending on painting the rocket.

Just sand (and sand, and sand ...) with progerrsively finer papers, finishing with 2000 wet & dry (yes, you can get this), and then polish with fibreglass polish (or brasso, or car polish)
Launch lugs or buttons will add a measurable amount of drag - it would be better to eliminate these entirely and use a pupose built tower
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Postby rocket_troy » Wed Mar 25, 2009 9:23 am

Things to consider (by no means a complete list):
As I mentioned before the deceleration after burnout is huge. So, nosecone seperation is likely to occur if you haven't got it secured properly.


I'd imagine that would be a more than an unlikely event given that most of the deceleration will stem from the dynamic pressure on the nosecone in most instances.

TP

blackbird
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Postby blackbird » Wed Mar 25, 2009 11:29 am

From the simulations and from what I'm reading most of the drag is base drag not dynamic pressure on the nosecone.

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Postby rocket_troy » Wed Mar 25, 2009 11:38 am

I'm curious as to where you read that?

TP

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Postby PK » Wed Mar 25, 2009 11:53 am

blackbird wrote:From the simulations and from what I'm reading most of the drg is base drag not dynamic pressure on the nosecone.

My understanding is that this is true for subsonic flight. Supersonc, dynamic pressure dominates.
PK

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Postby rocket_troy » Wed Mar 25, 2009 12:18 pm

Intuitively I can now see it can how it can happen near mach where base drag will max out and plateau from there on past it and maybe sub mach with no boat tail.
For the nosecone to actually separate… there’s still all that dynamic pressure on it, so I’d imagine for separation to occur the following condition must be met (assuming frictionless separation):

Separation= Mn >Mb/ (Db-Dn)

Where Mn = Mass of Nose Structure
Mb = Mass of everything else
Dn = Dynamic Drag of Nose cone
Db = accumulated other drag (fins, base, body etc) on vehicle

Does this look right?

TP

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Postby rocket_troy » Wed Mar 25, 2009 12:55 pm

BTW: the prev concept equation assumed drag to be converted to mass force.

Hummm.... The more I think about this, the more probable I can envisage a possible separation event for rockets with separation couplings lower down towards the aft end of the rocket.

I must admit, I totally neglected the magnitude of base drag in these conditions!

TP

blackbird
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Postby blackbird » Wed Mar 25, 2009 4:14 pm

I found some data from my simulations (from last year). I incorrectly stated base drag earlier but it was meant to be a combination of base drag, skin friction, fin friction, etc. Now everything I mention below is based on my rocket design which does have a slight boat-tail.

Things noted from the data:

-The nose cone drag increases sharply as you approach Mach 1 and keeps increasing until about Mach 1.2 and then drops off.
-Base drag and skin friction don't increase as much relatively speaking.
-The skin friction can be as high as the base drag depending on what finish you have.
-Deceleration after burnout is in the order of 20G.

Under plenty of thrust everything is good. The interesting thing happens in the coast phase. At speeds over Mach 1 the nose cone drag is higher than the body drag and under these conditions your nose should be OK. As you fall below Mach 1 the body drag is higher than the nose cone drag. In my case, I will have a fairly heavy nose and once you factor in forces due to gravity, I obtain a net force trying to seperate the nose from the body.

It is good this topic came up again as I found some errors in my processing. The new numbers I'm getting seem to be much much lower and almost turn it into a non-issue. Rocket_troy, as you stated if you have a coupling much lower down towards the aft end you could still have an issue. But it really depends on how much mass you have above this coupling. But in saying that, your skin friction will be much lower on the lower part of the rocket so the effect may be reduced. Body transitions will still cause a problem though.

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Postby Avachovy » Wed Mar 25, 2009 6:53 pm

or, just to solve all of the nose cone/coupler problems, just use nylon or plastic shear screws or rods. Simple
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Postby Scoop1261 » Wed Mar 25, 2009 8:36 pm

Well all I can say on the matter is..........JFTDT!
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Postby air.command » Wed Mar 25, 2009 9:09 pm

This is a very interesting discussion,

blackbird wrote:In my case, I will have a fairly heavy nose and once you factor in forces due to gravity, I obtain a net force trying to seperate the nose from the body.


I had a question regarding the above statement blackbird. Are you using the force due to gravity in your separation calculations? or are you using it only for altitude calculations? I would have thought that acceleration due to gravity is equal for both nosecone and rest of rocket and so there is no net effect due to gravity on the separation force. Both components are essentially in freefall after burnout and the only forces that need to be taken into account are aerodynamic ones.
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Postby rocket_troy » Wed Mar 25, 2009 10:06 pm

I had a question regarding the above statement blackbird. Are you using the force due to gravity in your separation calculations? or are you using it only for altitude calculations? I would have thought that acceleration due to gravity is equal for both nosecone and rest of rocket and so there is no net effect due to gravity on the separation force. Both components are essentially in freefall after burnout and the only forces that need to be taken into account are aerodynamic ones.


Yeah, you're right and he's also right, but he just used the wrong terms. I know he something like "momentum" as opposed to "gravity". There's a few terms he mixed up, but I knew what he was on about and generally agree with all his points.

TP


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