Hot Hydrogen Lenticular Concept

Take a lenticular balloon and fill it about 70% with hydrogen. Put an engine on it and use the exhaust heat from the engine to heat the hydrogen gas. When the gas expands, the bag expands. Your lift will increase and you can take-off vertically. When cold and unloaded, the airship would be a little buoyant. With the pilot, it would be a little heavy.

Now for the details. Since the lifting gas is hydrogen, great care must be taken in the design of the heating system. The gas must be isolated from any ignition sources and should have as little proximity to air as possible. My preferred method of heating the gas is to conduct heat from engine exhaust into the envelope. This way the gas stays safely inside the envelope. 

Other heating methods were considered. One was to use a burner to heat a heat exchanger at the interface between the gas and the atmosphere on the bottom of the bag. This method uses an open flame near the envelope, so this seemed less safe than other methods. I also considered using microwaves to excite the gas, or to excite water inside the bag. This would be safe as long as the pilot is not exposed to the microwaves. The big question mark here is complexity and power efficiency. You could just use a home microwave oven and beam the microwaves into the envelope. You would need something in the envelope that adsorbs microwave energy. I know water will do that, but I am not sure about hydrogen. There is also the option of piping the hydrogen to the engine to heat it, but this seems like another opportunity to leak to me.

In order to dissipate static electricity, discharge lines would run down below the airship toward the ground. A lightning rod might also be needed on the balloon to keep sparks a few feet from the skin. 

 Seems to me ,  the  main, and lightest option for a   heat source   will be  the motor.  Anything  aircraft related that can do  two  functions  is well  worth  the  effort.  In the  copper tube  wrapped around the  exhaust idea ,  Instead of  pumping  the  hydrogen ,  use  a closed loop  water  or antifreeze ( better)  filled  system  with  an  expansion  chamber /  radiator    that  feeds up to  your  blower ,  The  blower  forces  hydrogen  through  the  radiator  inside  the  envelope for heat. If  by chance  your motor is  antifreeze cooled too   couple them  together  and  capture  all the engine heat generated. It may  require  a  secondary  liquid pump  to  get  a good flow .  No  worries  now  of  sparks   or  flame  and if any leak  in  the envelope occurs , it  will be  at  the liquid circulation lines  where they  enter  and exit  the  envelope.  as well as  any other  internal  component  that  requires  a  passage  through  the envelope.       
 The  blower  will have to  be  explosion  proof ,  and  they  are a   common found  item .   Dont know  about  CFM  and  BTU  transmition /  exchange  per  sq ft  and at  temp    though .  

honestly I dont think that the motor will be able to create enough heat to do it by it self. There is how ever no reason you cant still take that heat from the motor and use it. You can how ever now that your going to get a certain amount of mesurable heat from the engine and now exactly how much heat your going to use to suplament that.

How many  BTUs  will it take  to raise the  volumes temp  to  desired temp?  and   then  to keep it  at  temp ... In  Hydrogen?   And   then  work   the problem  for  10 degree  differences  in outside  temps.  In  the hot  of  summer   the motor  may be plenty ,  But In  the  dead of  winter  how much extra ?   
  A good  HVAC  guy  that knows his  stuff.   may be  able to  help , But wanna bet  the  Hydrogen   instead of  air   throws  him  for  a loop    .
    

To get H2 to 100 C requires about 100,000 joules. With the outside temp being 25 C it would take 2,200,000 joules to keep it at that temp, assuming 4 mil polyurethane, a spherical envelope, and 5000 ft^3 of H2.

You're talking heat transfer across a very thin wall over a very large area with a 75 degree driving force using lumped parameter analysis.

Wow  Swampie ,  your  good !  So  how  would that  relate to  what  size blower   ,what  size  radiator,  and  how much  engine heat needed  and contact area,  to transfer to a liquid  temp ,  and  its flow speed for proper  exchange  rate to  heat  the  required volume. All this  is  so way over my head it makes my  hair  hurt  . I wouldn't know  where to  start.

I am having trouble with the units here. You say you need 2,200,000 joules to keep the H2 at temp, but is that over a time period? I found that you can convert joules to horse power hours or kilowatt hours. That converts to only .61 kWh. That doesn't seem like much to me. A kWh only costs like 10 cents from the grid.

Say I have a 4 hp engine and I can use 25% of that energy to heat the gas. That is 1 hp. Is that sufficient to reach 100 C (in a timely manner) and maintain that temperature with an outside temp of 25 C?

I re-checked my integration today and found a calculus boo-boo. The heat to get the H2 hot is:

So now you're going to have to steal grandma's toaster for a whole day to get this done.

{ This is a partial correction to post #4 }

I'll re-work the heat loss part later.

When you say "lumped parameter" you're assuming instantaneous inside and outside gas heat transfers ( which aint real life), BUT if this model predicts big, then big is what you'll have to deal with. (But not as big as this is.)

"You're talking heat transfer across a very thin wall over a very large area "

This is still the problem. So doubling the thickness or adding some kind of foam insulation will make a drastic difference. But maybe not enough.

Refering to post #5: Yes IM, once you start modelling real world systems, complexity can and will go through the roof. BUT if you don't do some first order modelling, you can pay out a lot of money for something that won't work or is not safe. Look at the number of abject failures in flight before the Wright brothers did some modelling and were met with success.

Do you have this in a spreadsheet that I can play with? If I have a model to work with, I can edit the design to (hopefully) make it work.

 Post   5  yea I know  LOL , and its  way over my  threshold  for  pain.   You  guys  are doing  excellent  though ,I'm of no help here,  just  gonna  sit back  and  eat popcorn on  this one. Give  her hell boys !    

{{Say I have a 4 hp engine and I can use 25% of that energy to heat the gas.}}

I'm interested in this subject too. While I was dicking with my genset and making a better muffler, I was struck by how much hot exhaust volume the little 39 cc Generac engine puts out at 5000 rpm.
   It seems a good idea to vent this hot exhaust into the lift bag chamber after sunset to compensate for negative-superheating. [Hypocooling?] Wouldn't have to drop any ballast if you wanted to [illegally] sail through the night, maybe over the Sahara or somewhere you won't get chastized  for night ops in a part 103.
   This little 800 watt genset goes 8 hours on a gallon of gas so the loss of fuel weight isn't much.

{{My preferred method of heating the gas is to conduct heat from engine exhaust into the envelope. This way the gas stays safely inside the envelope. }}

Agreed. As someone said here, exhaust gas is inert, having had the oxygen burnt out of it. It's mainly nitrogen and carbon dioxide, flame inhibitors. So the technique of dumping exhaust into the envelope can be touted as adding to the safety of using hydrogen. Of course, a spark arrestor should be in the exhaust pipe. It's just a metal screen.

   It'll take a little more thought on how to control the maximum exhaust temperature allowed to rise into contact with the melt-able plastic lift bags but it shouldn't be too hard to deal with.

   The EGT will vary with engine load and altitude and other things, so it can't be dumped directly into the envelope.  Maybe the muffler could have a thermistor-contolled fan come on and off like a car radiator to control the muffler exhaust temperature.

I'm in the mude for some altitude.

  I think I mentioned it before, not sure, but how about having having a duct behind the fan and genset directing all the heat, exhaust, and flow up into the outer envelope, between the envelope and the gas bags. This maintains shape, adds all the heat gently, carries away any H2 leaks, and puts everything into one package.
  The airflow is then exited out openings under the control ot the operator. Instead of rudders, side openings could control rotation, yaw, etc. At slow speed, rudders don't do much anyway.

Very creative Mike. !!!

I didn't think of using the outflow of the blower shroud to contain the exhaust gas and blow the both of them around the gas bags inside the envelope and up thru the top vents. !  That will add some BTUs and volume  to the superhot engine exhaust gas which, by itself, would melt the plastic lift bags.
 
   It's EZ to install a heat-proof duct on the generator that would collect both the exhaust gas and the blower air together. This blower air comes from the hot cylinder head and the very hot muffler so it contains a lot of heat and has more flow volume than does the exhaust.

   With a flapper valve in the heat duct, we now have a controllable degree of variable lift, which has big operational benefits.  Especially when this variable lift from engine heat is combined with the variable lift available with a lifting body envelope shape!  Now we are getting significant and Cheep! buoyancy control, the holy grail of airship flight. And you read about it here on the smallblimps forum. 

UP is gonna be EZ

Using the numbers from post #7, if you had 1 kW of heating power, you could do it in 4 hours. If the effect is non-linear, it would be even faster right?

The effect wont be linear. How about some form of internal mixing to increase speed of heating.

If your  gas lift  bladders  are   inside  a  non pressurized  shaped  / supported /  shroud   It  will  work.  If  this shroud is pressurized, a internal  fan  wont work ,  Even if  the shroud is open ended where the  fan is  ,   once pressure is  achieved   the  fan  will  not circulate heat  well  at all.  It  will just  hold pressure . 
  The  shroud   will need  to  free flow  to  some  extent, to  get  the  fresh  heat   inside to  do its job.    
 

Here is  a  sketch of   the  "heating pad"  situation  we  discussed
  What ever  you  figure for  needed ballast  so the  ship  can go heavy  to  land , Would be  the  water volume  in gallons  that needs  to  transform into  steam  In  a closed loop  system having its own  bladder  under  the Hydrogen .  Convection  heats  the  Hydrogen   in  a heating pad  effect  from the  steam  bladder under it.  . 
   Unknowns  are  Amount of  BTU's  needed  to  transform  the  quantity of  water into  steam ,  Dont know if  engine heat is  fast enough ,  or needs to be supplemented.  
  This  would be  a slow  process .  Not  a fast process .  But  the water ballast transfer into steam  should be  quick  enough  to  get airborne in  a  reasonable amount  of time,   , the  added  lift  from convection  into the hydrogen  would come on  slow .    
 I think it would make  a worthy  bench  test  model  to  see  how long it  takes to convect heat  through  different  bladder materials.  Hydrogen  being   in  an  Aluminized   bladder   for  rapid direct  contact  heat gain,  And  the  steam  in  a polyurethane  bladder   that  insulates   better  to  hold  steam  longer .  So  as it  heats up    It  also  stays warmer longer  while contact  with  the Hydrogen  Bladder .   
 Being  in  a  rigid  airframe / shroud  ,  the  bladders   are  free to  shape shift  to the  changing heat  and  cool  off  cycles  without  affecting  aerodynamic shape.   

If you duct tape two milk jugs together, lay them on their side with one on top of the other, and put a thermometer in the top one and steam in the lower one you'll get a quick idea of how well this idea works. You could follow this up by corking the top one and filling it with H2 to get a better model. Steam source? Buy a pressure cooker and take the weight off and pipe the steam to the lower jug.