This week, I spoke to a “balloonist” whose hobby of hot air ballooning will hopefully inform us more on the idea of using hot air balloons, and the kinds of things we should be thinking about. Though it’s not the same thing (he balloons at a scale of hundreds of pounds) there’s tons to learn from his informative e-mail. This advice will be critical when we consider the design of our balloon. His email is pasted below.
There are several items to consider. LTA/Balloons w/ABH or hot air balloons, use a nylon rip-stop fabric that is coated (very lightly) but some are also polyester (still coated). Rip-stop has, by name, some safety build in and the coating gives it an almost airtight quality. One of the reasons for using these two materials is the temperature that is needed to deform. All the hot air balloons have a small tab at the top “tell tale” or “melt link” or “Seriloc”. These show if the temperature exceeded certain values. Any temp over 275 def F may have done damage (allowed deformation of the nylon). Mylar or other thin nylon material may deform at lower temperatures. The balloons I fly use kevlar support cables that run from the bottom to the top. They are covered to prevent UV damage. The envelop, kinda, slides up and down on the support cables. There are some balloons that use steel cables.
Hot Air Balloon burners are a well engineered device. The liquid propane comes up to the burner. The blast value allows the liquid to go into the upper coils. These coils are in the flame path. This superheats the propane. The other end of the coil is in the lower part of the burner and pointed upward to the pilot lights. Once the propane is superheated and released into the pilot lights – it will explode. This gives the hot air balloon a very large (8ft long and 8 inch diameter) light blue flame. This is important as it is not just something burning naturally but boosted to release up to 19 million btu’s. This energy release can raise the temperature of 7,000 lbs (mass) of air up 100 deg F in just a few minutes. I worked out all the math & physic’s that a hot air balloon uses to increase the energy and lower the air mass in the envelop to get to a buoyant situation.
I do tether flights for Civil Air Patrol cadets and there are issues with any type of wind when tethering. The wind, as low as 5 mph, will start to collapse the windward side of the envelope. This pushes the envelope material over the burner (you see the problem there). Once the wind gets to 10 mph the risk that the aircraft will break a tether increases. I use the Cd (coefficient of drag) of 0.3 to 0.35 when calculating the forces when tethered. Another issue is the wind will push the flame and blow out the pilot lights.
One of the other issues you will need to address it the rate of heat input. On hot air balloons, the pilot judges the amount of weight, ambient temperature and amount of heat loss. This allows the pilot to use the blast valve to add heat to maintain level flight. I can cross a 0.5 mile farm field maintaining a 6 inch height above the soybeans by adding heat at the correct times. Since temp, humidity, pressure, etc changes daily (hourly) you will need away to adjust the heat input to the envelop.
One of the balloons I fly has a 77,000 cubic foot envelope that supports a maximum lift of 1750 lbs. All aircraft use a weight and balance calculation (required by the FAA). This balloon has a basket weight of 250 lbs and envelope of 200 lbs. The three ten gallon talks support adding 30 gals or 120 lbs of fuel. As the fuel burns down the amount of heat needed (lower envelope temperature) requires less fuel to be used.
One of the resources that the US provides is information about weather. NOAA/Earth System Research Laboratory,, RUC Development Group, provides some valuable information via soundings and model analysis. We use various model results for the our location. They provide wind speed, wind direction, temperature, etc from the ground to 45,000 feet. Today
the wind a the ground is 246 degrees at 17kts. and increases to 21kts at 92ft, 31kts at 1407ft, 41kts at 4311ft and 60kts at 10,000ft. You won’t tether to 10,000 feet but the speed increase from 0 to 500ft can be 10kts more then ground. That is ok for free flight but very tough for tethered flights. http://www-frd.fsl.noaa.gov/soundings/java/
Also, when we tether a balloon we use a three point gimbaled harness. There is a rope from the top to bottom (every 120 degrees around the balloon) and then a second rope that slides/rides the top-bottom rope which will attach to an anchor. This allows the balloon to remain vertical when a wind hits it. If you just tie to the top then the bottom would get pushed out from under it and if just tie to the bottom then the top would get pushed over.
If you have other questions or just want to bounce some ideas, email me.
What I took away from this at first: we need a rigid, lightweight structure to the balloon in order to prevent collapse in windy Iceland. We should really consider using pressurized gas burners, they make a lot of heat and we need a lot of heat. Lastly, we need some seriously heatproof materials, which are also leak-proof. I hope to identify what kinds of materials we will use this week, but it may be more prudent to figure out the design of our balloon first. The design may largely be determined by the following factors: weather (wind speeds at different altitudes, precipitation) and buoyancy needs (weight of balloon, tether, burner, camera/arduino etc).