By Robert Brand
Two weeks ago I was a guest lecturer in Aerospace at a Sydney University spoke about the current space projects I am involved in. It was good to see interest from some students to take part in some of the activities. I stayed on to listen to the second guest lecturer talk about high energy particles and there effects on astronauts and also equipment.
Following the lectures I was invited to talk about a difficult project of taking a science experiment to the stratosphere and holding it there for 3 hours. Now this creates a real challenge. It can either be done by a seriously expensive Zero Pressure Balloon (ZPB), shown in the picture at right, or it needs some way of holding a weather balloon below it burst point, both are not ways i would like to think about. Either big dollars or big problems.
My solution is to turn the Weather Balloon into a ZPB before the envelope pops and the lot comes down. I have designed a Weather Balloon to Zero Pressure Balloon Converter. Due to the commercial ramifications, I cannot give the fine details of the specific way we will do this or the materials used, but as you will obviously realise, it means opening up the balloon canopy so the helium (or Hydrogen) stops expanding the balloon fabric thus the balloon should then find a floating point, provided that the canopy is not too elastic.
Since this will require additional weight and we need to get extreme height to be in the stratosphere at a required altitude and we don’t want the risk of the canopy bursting early, I expect that it will require 2-3Kg weather balloons. Not cheap, but way less than ZPB that probably start at a price of US$7,000 or so.
Now for the hard part. We will need to test to see exactly what happens and how to control the eventual height based on gas fill, elasticity of the balloon, the balloon size / weight and the payload size.
The balloon should survive until next morning in the stratosphere when the sun’s UV will cause the envelope to deteriorate along with the punishment it has received during the night. Given that it is not fully stretched, it may in fact last much longer. This will the subject of more testing.
The next problem is that the stratospheric winds run east to west – the opposite of the jet stream – yes there is balance in the world! the difference is (from our experience over Australia) that the wind starts out light and then increases with strength at height. Several times we have seen stratospheric winds reaching 100kph at altitudes of 33.333Km (/3 the way to space). As that is our highest record and we have been involved in flights by others to that height, it seems a fairly linear increase over height and it may get faster at higher altitude. Only tests will tell, but 3 hours at 33.33Km is a long drive. It means carefully planning our launch points and recovery points. In fact we may need to launch on days when the jet stream is fast to drag the payload a long way to the east (say 150km and then allow the winds in the stratosphere to pull it 300Km to the west and allow it to fall back through the jet stream with a big parachute, allowing time to pull the payload back to near the launch point.
With radio cutdown an option, we need to be close enough for that to work on UHF frequencies of to create a HF cutdown on much lower radio frequencies that will travel further.
As APRS is not an option on commercial flights, We will use SPOT3 units in gimbals for the commercial tracking. For non commercial flights I have toyed with the idea of using the HAM radio based APRS to upLink commands for cutdown. As a final cutdown I am looking at a time based mechanism to terminate the envelope or cut down the payload. More testing!
As a teaser, the photo to the right is some of the “plumbing” without the servo and other systems. It is intentionally difficult to see, but the point is that it is off the shelf technology that is very light weight. In fact not all of this mechanism flies – some is only there for the “fill” and another device (not shown) makes the final configuration. When the cutdown occurs, we will lose the servo and the plumbing – a total cost of about US$15. Total weight of equipment lost will be in the order of 200 grams and the balloon envelope will also be able to fall to earth but since it will not be blown to pieces, it will flutter to a soft landing. I expect to have a number of the mechanisms ready and off the shelf to provide services to customers that wish to have low cost long times in the stratosphere. Note, that if we can keep the payload closer to the lower parts of the stratosphere, the drift is negligible from our general experience for a three hour duration in the stratosphere.
Other benefits here are a new easy fill system that requires no more struggling with cable ties at the last moment while holding on to a big balloon. I expect that we will use a smaller version for smaller balloons. The weight is likely to be an additional 50 grams that we can factor in, but the benefits will be great in securing the payload and ensuring an easy and safe tie off in the final moments. Once we test that I will publish the arrangements. More on the Zero Pressure Balloon Converter in future posts.