Re: [UKHAS] First Launch - Sci1

[Anthony Stirk]

Evening Alan,

You have some very good predictions, landing just 15 miles from the launch site , I'll be tracking from West Yorkshire.

Good luck!

Anthony M0UPU

On Mon, May 13, 2013 at 5:04 PM, Alan Oakden <oakde...@gmail.com> wrote:

Hi all,

A few colleagues and I from work are looking to do a launch on the Morning of Wednesday 15th May.

Lift off will be about 10, though don't be surprised if that gets moved back.

The launch will take place at Elvington, York, North Yorkshire, UK. (53.924398,-0.964759).

 

Payload:
Sci1: 434.650MHz, 50 baud, 420hz shift, ASCII-7, 2 stop bits, no parity


The Balloon is a 1200 gram hwoyee

The payload contains a camera, our tracker board and a separate GPS tracker in case the board fails

Target ascent rate: 5.6m/s
Predicted descent rate: 4m/s

The descent rate is open to change as we're doing drop tests tomorrow and have two parachute sizes available. Both a 2ft and a 3ft. We felt the 2ft was too small for a payload that is about 600g with a 1200gram balloon attached to it (in the event of a poor burst)

Trackers would be much appreciated for the flight :)

Thanks
Alan

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[Anthony Stirk]

Hi Alan,

A 60 cms parachute with 600g will probably be quicker than 5m/s (http://ukhas.org.uk/guides:parachute_sizing_chart) its on the wrong side of the line labelled "fast". 

Anthony M0UPU

On Tue, May 14, 2013 at 4:17 PM, Alan Oakden <oakde...@gmail.com> wrote:

Thanks very much.

Unfortunately our 3ft parachute hasn't arrived so we're left with a 2ft parachute. We've done a drop test and some calculations and we think it's likely to descend at around 5m/s. We're nervous that a bad shred of the balloon may lead to a very fast descent. What does everyone think? Should we be ok? Its a payload that weighs about 550grams and the balloon is a 1200 gram hwoyee

Many thanks

[Steve Aerospace]

Hi,

What 2ft / 3ft parachute are you using and what are you using to calculate descent rate for that parachute?

Not all 2ft chutes are the same - it depends very much on manufacturer where the measure (e.g. across the top - or base) and the type (elliptical, cross form, conical). 

Also in my experience manufacture data is sometimes a bit 'hopeful'.

Steve

On 14 May 2013, at 16:17, Alan Oakden <oakde...@gmail.com> wrote:

Thanks very much.

Unfortunately our 3ft parachute hasn't arrived so we're left with a 2ft parachute. We've done a drop test and some calculations and we think it's likely to descend at around 5m/s. We're nervous that a bad shred of the balloon may lead to a very fast descent. What does everyone think? Should we be ok? Its a payload that weighs about 550grams and the balloon is a 1200 gram hwoyee

Many thanks

On Monday, May 13, 2013 10:02:48 PM UTC+1, Anthony Stirk wrote:

Steve Randall

Random Engineering Ltd

st...@randomaerospace.com

+44 7802 242135

[David Akerman]

I use Steve's graph at http://ukhas.org.uk/guides:parachute_sizing_chart.  My non-mathematical reckoning, from looking at the data from my flights using spherachutes, is that the lower solid line equates to about 5m/s with those chutes.  So a 24" spherachute gives 5m/s for a 350g payload.  550g is going to be a tad fast with that size ...

Dave

On 14 May 2013 16:54, Alan Oakden <oakde...@gmail.com> wrote:

Hmm yes what i thought. We are using the 2ft parachute from spherachutes with the weather balloon attachment sewed into it. http://spherachutes.com/items/spherachutes/24-in-spherachute-24sp-detail.htm 

That one came after a week, have no idea whats happened to the 3ft delivery...how annoying

We used this calculator http://www.calctool.org/CALC/eng/aerospace/terminal with the drag coefficient placed at 1.5 for a dome style parachute 

[Steve Aerospace]

I guess you know you could have got one of those in the UK? 

http://randomaerospace.com/Random_Aerospace/Parachutes.html

There is also a descent rate calculator for Spherachute and Rocketman parachutes at the bottom of that page.  The calculator has been designed to fit the manufacturer descent data on their websites.  Cd for Spherachutes is more like 1.0 than 1.5.

Using a 2ft chute 550g then gives 5.5m/sec - but assuming that 1/2 the (1200g) balloon comes back then that gives almost 8m/sec. That said descent rate under these conditions depends very much on what happens to the balloon.

Steve

On 14 May 2013, at 16:54, Alan Oakden <oakde...@gmail.com> wrote:

Hmm yes what i thought. We are using the 2ft parachute from spherachutes with the weather balloon attachment sewed into it. http://spherachutes.com/items/spherachutes/24-in-spherachute-24sp-detail.htm 

That one came after a week, have no idea whats happened to the 3ft delivery...how annoying

We used this calculator http://www.calctool.org/CALC/eng/aerospace/terminal with the drag coefficient placed at 1.5 for a dome style parachute 

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[Ed Moore]

There may be others who want to add to this (and there is one on the list who is quite an authority...) but:

The convention is that diameters are the distance from one side of the canopy, across the canopy material, to the other. *Not* the diameter across the mouth when inflated. I.e. you can calculate it (the diameter) backwards from the surface area of the canopy. This is called the nominal diameter.

Drag coefficients are stated with respect to the nominal diameter. A drag coefficient for your kind of parachute is about 0.7. It's difficult to make a parachute with a drag coefficient much higher than that. A Cd of 1.5 is pure LaLa Land.

It's easy enough to calculate the parachute speed directly from the drag equation (which is v likely all that any web-calculators or graphs you find online are doing). The drag equation is:

Drag = 0.5 * air_density * velocity^2 * Cd * Area

(velocity^2 = velocity squared), area is calculated from nominal diameter (pi*radius^2)

The insight you need to apply this to parachutes is to realise that a falling parachute is in equilibrium, so the force from gravity acting on the falling mass is equal to the drag from the falling system (payload and parachute). So you can replace Drag in the equation about with mass*gravity (where gravity is about 9.8m/s^2 at earth's surface) and rearrange it for velocity:

V = sqrt[ (2*mass*g) / (air_density*Cd*Area) ]

where sqrt = square root. Air density is about 1.22 kg/ m^3 at sea level

Assuming 500g of payload and Cd = 0.7 I get about 6.5 m/s sea-level descent velocity.

Your shout, I probably wouldn't loose too much sleep over it provided your payoad is soft. It's worth noting this calc ignores the drag contribution from the payload too.

As for why you see inflated (>0.8) Cd values floating around, it's probably because the manufacturers are either confused or lying. Assuming the former ('never ascribe to malice...' or whatever the fella said), the confusion is usually from them not realising their parachute is gliding, and so calculating Cd from the time it takes to drop a certain height, which is only measuring the vertical component of the chute's air velocity, rather than it's actual air velocity, which can be at least the same (i.e. a 1:1 glide ratio). Gliding is usually undesirable and often a function of ropey chute design, and things like the predictor don't taker it into account, which I suspect has been a non-trivial source of prediction error over the years.

On Tue, May 14, 2013 at 4:54 PM, Alan Oakden <oakde...@gmail.com> wrote:

Hmm yes what i thought. We are using the 2ft parachute from spherachutes with the weather balloon attachment sewed into it. http://spherachutes.com/items/spherachutes/24-in-spherachute-24sp-detail.htm 

That one came after a week, have no idea whats happened to the 3ft delivery...how annoying

We used this calculator http://www.calctool.org/CALC/eng/aerospace/terminal with the drag coefficient placed at 1.5 for a dome style parachute  

[Steve Aerospace]

Thanks Ed - The Cd I calculated (of about 1.0) for Sperachutes is exactly as you say - as the size of a Spherachute is a measurement from side to side over the top of the canopy. (see http://spherachutes.com/volumeandweight.asp).

To get a Cd of 0.7 the nominal diameter would actually be larger than 24" for a 24" chute.

Its been my experience that the actual vertical descent rates are faster than the published manufactures data - hence my previous comment about "data is sometimes a bit 'hopeful'".

When you are talking about gliding - I assume you are talking about a horizontal velocity due to the parachute asymmetry - rather than due to the wind its descending in?

Steve G8KHW

Steve Randall

st...@randomaerospace.com

+44 7802 242135

[John Underwood]

Ed's description is just about spot on.

For a parachute to glide, you don't need any asymmetry - just low porosity (like the solid cloth parachutes used for HABing - especially those with imporous material). A simple parachute such as one manufactured from a solid, imporous disk of material can easily glide at 45 degrees to the vertical. Note that the glide angle is relative to the air (i.e. over and above any sideways motion due to the wind).

If you want it to glide in a particular direction, asymmetry is useful, otherwise it just glides whichever way it feels like. Over a long descent, the glide direction will tend to average out so the predictor tends to be quite accurate.

I would guess that the spherachutes do glide - particularly at low level. Gliding isn't necessarily a bad thing. You can reduce descent velocity significantly for a given parachute size by making it glide. The downside, as Ed says, is horizontal velocity - you reduce vertical velocity but not impact energy. You would, however, have a glancing blow rather than a direct impact so it may be more survivable.

An added complication is that a parachute which glides at low altitude on Earth is likely to be quite unstable at high altitude. This is the effect we are currently investigating with the Vortex series of flights.

On the subject of descent rates, 5 m/s doesn't sound too scary providing your payload isn't too solid. A higher rate of descent also reduces the chances (slightly) of ending up in a tree or caught in power lines!

John, M0JCU

[Steve Aerospace]

Thanks John - Very Interesting stuff.

I'm just wondering if gliding might be contributing to the predicted landing spot oscillating about a mean that I have noticed on the latter phases of some of flights.  Or maybe its due to winds or descent rate differing from the model in an oscillating way.

I've witnessed 'coneing' occurring on model rocketry descents (typically parasheets) - where one side of the chute seems to lift and sets up a spiralling descent - I have not seen any evidence of that in the few HAB payload descents I've seen.

Steve G8KHW

 

[Ed Moore]

On Tue, May 14, 2013 at 7:32 PM, Steve Aerospace <st...@randomaerospace.com> wrote:

Thanks Ed - The Cd I calculated (of about 1.0) for Sperachutes is exactly as you say - as the size of a Spherachute is a measurement from side to side over the top of the canopy. (see http://spherachutes.com/volumeandweight.asp).

To get a Cd of 0.7 the nominal diameter would actually be larger than 24" for a 24" chute.

Its been my experience that the actual vertical descent rates are faster than the published manufactures data - hence my previous comment about "data is sometimes a bit 'hopeful'".

When you are talking about gliding - I assume you are talking about a horizontal velocity due to the parachute asymmetry - rather than due to the wind its descending in?

Yes, i.e. its speed relative to the surrounding air, which I think is also called pitot speed or indicated air speed. You mention calculating a Cd of 1.0 - how did you do this? It seems high, though as I say this might be explained if it's calculated using descent time.

[Ed Moore]

On Tue, May 14, 2013 at 9:29 PM, Steve Aerospace <st...@randomaerospace.com> wrote:

Thanks John - Very Interesting stuff.

I'm just wondering if gliding might be contributing to the predicted landing spot oscillating about a mean that I have noticed on the latter phases of some of flights.  Or maybe its due to winds or descent rate differing from the model in an oscillating way.

I've witnessed 'coneing' occurring on model rocketry descents (typically parasheets) - where one side of the chute seems to lift and sets up a spiralling descent - I have not seen any evidence of that in the few HAB payload descents I've seen.

Take a look at the final few hundred meters of descent from HAPS-1 in 2008, whose KML you can find on http://ukhas.org.uk/projects:launch_list

There is definitely some coning there - I remember being pleasantly surprised as it was an early example of the improved dynamic performance of the ublox chipsets over the various thing we were using before. Here is a screenshot I just took of it, looking approximately down the path for the final few tens of meters:

http://i.imgur.com/kkihZNy.jpg

[John Underwood]

Hi Steve,

It's certainly possible that the gliding is causing the predicted landing spot to move around. We always attempt to predict landing positions, both before and during the flight. Our best results have usually been where we have non-gliding parachutes. The parachute we used for Vortex flights 3, 4 and 5 glides at about 45 degrees at low level and it's been very difficult to get the chase teams close.

Our experience suggests that gliding and coning are different modes of motion of a parachute which is not stable at zero degrees angle of attack. Whether a parachute cones or glides depends on the mass of the parachute and payload and the atmospheric density. There may also be a link to system geometry. On Vortex flight 4, the parachute coned at more than 45 degrees to the vertical for the first part of the descent and then transitioned to a glide at lower level. We are currently working to quantify the processes causing the transition.

It's quite possible that your HAB parachutes cone at high altitude and then transition to a nice stable glide before you see them. You can sometimes spot the phenomenon by looking at the rate of descent of the payload (having allowed for the change in density with altitude). Unfortunately, balloon debris tends to complicate matters which is why we are flying a two payload system. For a parachute to cone at low level would usually require a particularly heavy payload (and thus high rate of descent) or an interaction between the payload and parachute

John, M0JCU

[Steve Aerospace]

I have a spreadsheet with the data from the manufactures site - then I re-calculate it using adjustable Cd and tweak the Cd to get the best match to the descent rates across the range.  Here is a typical example (but its a very close match across the range - more than explainable by the rounding in the manufacturer data).

30" Spherachute - 21oz @ 15fps or 37oz @ 20fps (sea level) - see http://spherachutes.com/chart.asp

Bear in mind 30" is an across the top measurement - http://spherachutes.com/chart.asp

converting to gods own units:

0.762m (24") diameter , 0.595Kg (21oz) @ 4.572m/sec (15fps) or 1.049Kg (37oz) @ 6.096m/sec (20fps)

Re-arranging the terminal velocity equation:

Cd = M * G * 2 / (rho * A * V^2) 

where A = Pi * D^2 / 4

for 21oz @15fps

Cd = 0.595 * 9.81 * 2 / (1.21 * (3.142 * 0.762^2 / 4) * 4.572^2) = 1.012

for 37oz at 20fps

Cd = 1.049 * 9.81 * 2 / (1.21 * (3.142 * 0.762^2 / 4) * 6.096^2) = 1.004

I was a little suspicious about how close a match it was - and especially since the value worked out to a nice round 1.0.

Steve G8KHW

Steve Randall

[Steve Aerospace]

Sorry 

converting to gods own units:

0.762m (24") diameter , 

should read

converting to gods own units:

0.762m (30") diameter , 

[Ed Moore]

They're cheeky buggers!

[John Underwood]

I think your parachute is definitely gliding. I'm guessing your "Velocity" is actually rate of descent which is a proportion of the overall velocity (which also includes a horizontal component).

If you neglect the horizontal component, you can still use your "drag coefficient" to calculate the rate of descent with different payload masses and scaled parachutes. It just isn't a drag coefficient in the strict definition of the term.

Any parachute with a measured drag coefficient of more than 0.7 is almost certainly gliding. Unfortunately, it's almost impossible to determine whether this is the case without a carefully set-up experiment (i.e. non-gliding wind control vehicles to work out what the wind is doing and allow for it).

John

[Steve Aerospace]

The Rocketman data is similarly cheeky - just a different round number.

Steve

[John Graham-Cumming]

On Tue, May 14, 2013 at 7:57 PM, Alan Oakden <oakde...@gmail.com> wrote:

Ok thanks all, very informative and helpful. Looks like we'll have to cancel the flight for now. Such a pain, we realised early on that the 2 ft parachute was too small based on the charts. I realised there is steve's site as i got the balloons from there, but after getting the first parachute from spherachutes and having 3 weeks time i decided to go with them as they were a tad cheaper (although a negligible price so a bit of a dumb decision really).

I have the 36" Spherachute that was used for GAGA-1 which I'd be happy to post to you (http://blog.jgc.org/2011/02/gaga-1-parachute.html) if it would help.

John.

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[Anthony Stirk]

Hi Alan,

Did you get my mail last night ?

Cheers,

Anthony

On Wed, May 15, 2013 at 10:23 AM, Alan Oakden <oakde...@gmail.com> wrote:

That would be fantastic, how long would it take to get to us?

Alan 

I have the 36" Spherachute that was used for GAGA-1 which I'd be happy to post to you (http://blog.jgc.org/2011/02/gaga-1-parachute.html) if it would help.

John.

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[Anthony Stirk]

Evening Alan,

Was the payload recovered ? We are all quite interested to know.

Cheers,

Anthony

On Mon, May 13, 2013 at 5:04 PM, Alan Oakden <oakde...@gmail.com> wrote:

Hi all,

A few colleagues and I from work are looking to do a launch on the Morning of Wednesday 15th May.

Lift off will be about 10, though don't be surprised if that gets moved back.

The launch will take place at Elvington, York, North Yorkshire, UK. (53.924398,-0.964759).

 

Payload:
Sci1: 434.650MHz, 50 baud, 420hz shift, ASCII-7, 2 stop bits, no parity


The Balloon is a 1200 gram hwoyee

The payload contains a camera, our tracker board and a separate GPS tracker in case the board fails

Target ascent rate: 5.6m/s
Predicted descent rate: 4m/s

The descent rate is open to change as we're doing drop tests tomorrow and have two parachute sizes available. Both a 2ft and a 3ft. We felt the 2ft was too small for a payload that is about 600g with a 1200gram balloon attached to it (in the event of a poor burst)

Trackers would be much appreciated for the flight :)

Thanks
Alan

--

[Matt Holmes]

Morning all,

The payload was recovered successfully last night!

After the launch at around 10:20am yesterday we tracked the payload from
Elvington for a short while before heading back to base in Heslington,
York. A chase car with Alan and Tom set off in pursuit of the balloon
whilst myself and others remained in York to help track and coordinate.
The plan was to use the onboard GSM tracker to locate the payload's final
resting place. Balloon burst happened at 12:23 and the payload was last
heard at 13:07 at an altitude of 687m. Predictibly the payload landed
somewhere without GSM coverage and the last reported position we had from
the GSM tracker was also at 13:07 in roughly the same spot (54.8598,
-2.0605)! The chase team were not far behind the payload and arrived at
the landing site to start a search or the area between the last known
location and the predicted landing spot. At around 16:00 things were not
looking hopefull and a second chase car armed with a laptop, radio and
other tools set off north. After a valiant attempt chase car one called no
joy and headed home. Chase car two continued to the last known coordinates
and at about 18:30, 1.5km away, we were greeted with the comforting tones
of RTTY. A few successful decodes later and we had the coordinates entered
into a Garmin eTrex and were homing in. The payload's final resting place
was 54.8565, -2.0630, behind a dry stone wall about 70m off the side of a
small road, a very easy recovery with the right tools! After spending
quite a while sampling the Ales and Food in the Duke of York on the A68 we
arrived back in York just before midnight - a fantastic day was had by
all.

http://mjholmes.org/IMG_3750_small.JPG


Alan and Tom will be writing this up more fully over the next week or so
and including images/videos that we captured during the day and the
flight. We did suffer some freezing/fogging on the lens which has impacted
on our high alitutude photos and an issue with the camera settings meant
the camera didnt capture the whole flight but fingers crossed there will
be some interesting ones amongst the mix. Here is one taken by the payload
of Elvington airfield shortly after launch
http://mjholmes.org/IMG_5001_small.JPG

On behalf of all of us involved in this project i'd like to say a big
thanks to everyone that helped track our flight, especially M0DTS who
tracked the payload almost to the ground. I'd also like to say a special
thanks to Steve Randall, John Graham-Cumming and Anthony Stirk who
helped/offered to help Alan with our parachute problem. We've certainly
learned a lot whilst planning this flight, especially over the last 72
hours!


Matt

[Rick Hewett]

On Sat 18 May Matt Holmes wrote:
> At around 16:00 things were not looking hopefull and a second chase
> car armed with a laptop, radio and other tools set off north. After a
> valiant attempt chase car one called no joy and headed home. Chase car
> two continued to the last known coordinates and at about 18:30, 1.5km
> away, we were greeted with the comforting tones of RTTY. A few
> successful decodes later and we had the coordinates entered into a
> Garmin eTrex and were homing in. The payload's final resting place was
> 54.8565, -2.0630, behind a dry stone wall about 70m off the side of a
> small road, a very easy recovery with the right tools!

Heh! Not the first time a payload's been hard to locate because the
chase crew didn't have a live radio to receive and decode the payload on
the ground. Glad you found it in the end. :)

--
... 73, Rick, M0LEP (aka lazyleopard on Freenode IRC)