(This is a next-level follow-up to a previous posting.)
Over the break I found my sliderule and got more serious about estimating this thing’s size and performance.
I dropped the idea of using methane (CH4) as a lifting gas and instead went with just hydrogen (H2). Hydrogen can be made from diesel fuel (vital for military markets… sigh), lifts more per cubic meter and has a lower fuel energy density (about 75% less), which is exactly what we want in this case.
See, the backbone of this whole scheme is that instead of having to vent lifting gas upon putting down something heavy, it allows you to productively recover its fuel value over the return trip (if it’s a certain distance or longer).
It uses deployable hanging parafoils to pull down on the blimp after putting down something heavy. Its propulsion engines switch from a lift/weight equilibrium of hydrogen and diesel to hydrogen only. Ergo, the blimp’s static lift is contantly decreasing until the downward-pulling parafoils are no longer needed. Once enough hydrogen has been burned the parafoils are retracted back up against the envelope (and out of the airstream), and the engines switch back to hydrogen and diesel for the rest of the return trip.
Okay. So far so good.
What I learned is that the parafoils-deployed hydrogen-burning first part of the return trip is still 800-1000 miles long. To set down an interesting-size payload (a 40-ton container or a 80-ton M1 tank), that’s still a lot of hydrogen gas to burn up in the engines, and it takes a while.
And then I found that most interesting off-road shipping routes (Kuwait<->Badhad, ‘Stans<->Kandahar, Yellowknife<->the diamond mines in northern Canada) are less than 1000 miles each way.
That’s a bummer, because the blimps need a return trip 800-1000 miles long to profitably burn up all that hydrogen.
Cheat #1: Just vent the rest. Doing so torpedoes the scheme’s whole raison d’etre, though. If you’re usually going to vent ~half of it then why not vent all of it and skip out on the deployable parafoils complication entirely?
Cheat #2: Gain weight along the way back by scooping up water from a lake or ocean.
The bummer about water-scooping is that it’s dangerous. The snorkel could clobber a person, boat, fishing net, sandbar or iceberg. Ergo, it’s much more expensive to do as a robotic UAV (unmanned aerial vehicle). A UAV can go from point A to point B with no problem as long as both points are up in the air, but to buzz the surface takes skill.
As long as it works, though, water-scooping is always a possibility when delivering cargo straight from a ship or barge. There are probably a number of instances where this ability to operate “port-less” would be a big winner.
OTOH, if the route is entirely over land, with no bodies of water handy, one would instead have to dig long water-filled trenches somewhere handy for the blimps to scoop from.
So yeah. I’m a little bummed that the water-scooping complication is turning out to be necessary, but I don’t think it’s a show-stopper yet. More later.
Longshot City 
January 3, 2009 at 7:05 am
[…] Hanging-Parafoil Dynamic-Ballast Blimp! By craigrmeyer (This posting was followed up in a more analytical way here.) […]
January 4, 2009 at 3:41 pm
What if you worked on liquifying the lifting gas during the return trip? If not cryogenically, maybe you could do it chemically (reform H2 + N2 into ammonia?) Liquifying might be time consuming, but you ought to have plenty of time even on a 100 mile return trip.
Not being more fuel efficient than an airplane is a killer though. You can land airplanes on ice, and probably build runways more easily that “water scoop ponds”.
January 5, 2009 at 10:34 pm
Well, having a fuel efficiency just comparable to, if not any better than, an airplane is surely still a winner in many situations that benefit from its other nify properties:
o vertical take-off and landing
o (the aircraft itself being probably cheaper on a per-payload basis… but only in production)
o being much easier to automate into a UAV since it doesn’t take off and land on a runway.
Now as for the liquifying lifting gas idea as another way to eliminate lift, many have tried to figure out how to pull that off in a low-enough-weight manner.
The combination of H2 with N2 from the air to make ammonia is the Haber-Bosch process, which involves catalysts and insanely high pressures, so I’m wiling to guess that that’s a non-starter, but there are other possibilities.
Ammonia itself is a lifting gas, and can be compressed (but also necessarily cooled) to make a liquid at propane-tank pressures. Ammonia can also be dissolved into roughly equal weight of water at ambient pressure too.
The stinker with ammonia is that either of these phase changes (liquifaction or dissolving into water) involve lots and lots of heat, and exchanging that heat with the abient air involves a lot of metal surface area, which means weight. So it’s not a slam-dunk. At least with the parafoil scheme, as you pointed out, one has some time over which to do it instead of WHAM all at once.
Speaking of propane, its heat of vaporization is… oh… 70% less than ammonia’s, so it’s easier to liquify, but it’s also heavier than air as well and has to be considered part of the payload.
I think the next thing would be to look into the ammonia scheme, and whether something can possibly work there by adding some cleverness.
For instance, a pound of ammonia gas will happily dissolve into a pound of liquid water so long as a bunch of heat can get into the liquid and keep it from freezing solid along the way. Maybe there’s some sort of way to “queue up” a bunch of heat in the form of super-hot water that can be quickly “injected” into the ammonia+water solution without heat exchangers. That’s what I’ve been wondering about, anyway.
January 13, 2009 at 11:29 pm
Why do you have to finish burning the H2 on the way back? Why can’t you just burn H2 as long as you’re en route, but then just have some kind of anchoring system once you get back to base? It sounds like it’s OK to have complicated stuff at the base, just not at the other end of the round-trip. You already need some kind of infrastructure to refuel and pick up new cargo, etc.
January 13, 2009 at 11:40 pm
That’s interesting. So instead of re-ballasting on the way back, instead one could just use the parafoils to control altitude and “snag” something to hold the blimp down at the base. ‘Not unlike how a Navy jet flighter uses its tailhook to catch a wire and stop.
Far out.
I guess my main beef with this is that a tailhook-type thing would be pretty violent and involve some big forces, though perhaps those forces could be reduced if the braking were spread out over a longer distance, like if oodles of bungee cord were involved.
Also, if the parafoils are out the whole way back then it impacts the theoretical fuel efficiency, though who knows how big a deal that is in reality compared to all the other qualitative effects it’d have. Hm.
January 26, 2009 at 7:38 am
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