Avoiding 'splash' down!

[sblair1290]

We have a payload weight (650g) and have roughed out some launch parameters based on using 4.5m3 of Helium with a 24" parachute. With a target ascent rate of 7.3 m/s and a decent of 7m/s we end up with a total flight time of around 240mins. Early simulations had us landing NE of our launch site (somewhere East of Aberfeldy) but more recent ones have us landing in the Ocean 9 out of 10 times! We can't really compromise of the gas volume or payload weight. Is there a time of year that we should aim for when the wind speeds will be more favourable or should we aim to reduce the flight time substantially? Perhaps we are suffering from the unpredictable weather gusting down from the north pole recently but it did seem that for a while the wind was predictably N Easterly, which allowed a far greater error margin wrt flight time.

regards,

SB

[daveake]

I can't get a flight time of anywhere near 4 hours using your figures, so something is wrong. Roughly, that would be a 3 hour ascent and 1 hour descent, and 3 hours at 7.3m/s is an altitude of 79km. You're not going to get anywhere near that with any latex balloon.

To reduce the distance travelled you primarily need to spend as little time as possible in the fast winds, which are those at higher altitudes (for example, I launched a balloon a couple of weeks ago that hit 255mph at 39km alt.). For that flight, the fast winds started at approx 33km altitude, so I could if I wanted have had it land much closer to the launch site by having it burst below 33km. Burst altitude is primarily a function of balloon size and how much gas you put in, so the first option is to use a smaller balloon.

More gas means a lower burst, and means the balloon spending less time getting there. Both of these things will tend to reduce the horizontal distance travelled. However the improvement isn't linear - more gas means the balloon being a larger diameter which increases the aero drag. So it can take a lot of gas to increase the ascent rate. So it's generally better to use a smaller balloon instead.

The descent rate really doesn't make a lot of difference to the landing position. This is because the initial descent is rapid anyway, so you can only really affect the drift at lower altitudes when the parachute is working more effectively. I don't recommend aiming for a landing speed of more than 6m/s for a payload of that sort of weight.

Try some calculations again, with a smaller balloon. Aim for an ascent rate of 6m/s and a descent rate (landing speed) of 6m/s. See where that gets you. A balloon of 500-800g should keep you below 30km, which will help with the distance travelled.

Dave

[sblair1290]

that flight time looks like an error...sorry!

Today's simulation with the HABHAB burst predictor used 7.3 m/s ascent rate using a K1200 balloon with a 650g payload. This requires approx. 4.4m3 of He and bursts after 72 min at ~31km. Putting these parameters into the landing calculator created a total flight time of about 150 min (not 240!) and we land about 85km away. If we launch from our school in Aberfeldy we land nice and safely on the NE corner of the Cairngorm National park. Lovely! the problem is that on different days the wind speeds vary greatly and thus so does the flight range. On countless occasions these same parameters have us landing in the North sea. Your comments about bursting at a lower altitude make sense. If we go for a launch earlier in the year we will perhaps go for a much lower altitude. Some analysis of UK wide Gov. wind speed data shows that Jul-Aug see's a significant dip in wind speeds (not sure at what altitude these are taken!) and the wind speeds are more predictable in these months across several years.

One last thing. Our main limiting factor at the moment besides range/time of flight is the cost of gas. BOC supply 9m3 at £260 so we have been planning for two launches under 4.5m3 each.

thanks!

SB

[DougieLawson]

One last thing. Our main limiting factor at the moment besides range/time of flight is the cost of gas. BOC supply 9m3 at £260 so we have been planning for two launches under 4.5m3 each.

I'd guess that's one reason why Dave Akerman has switched to using H2 for his launches. It seems like a waste of He which is now in very limited supply across the World.

[daveake]

Yeah H2 is cheaper, though personally I've switched to using composite cylinders rather than the steel ones (much much much easier to carry around) and that brings the price back closer to that of helium.

And yes, H2 is renewable resource so no concerns about using up helium. It also goes a bit higher, being approx half the density.

Dave

[sblair1290]

Are you aware of any teachers launching their balloons using H2? There would be significant health and safety implications for this I would imagine. Does anyone know of any techers who have done this and if so, how they managed the risk?

ta,

SB

[daveake]

In my experience, the moment that anyone with a H&S badge gets involved, it's only going one way. It's just not worth the hassle of producing risk assessments etc when the end result will be "no". So for school launches I use helium.