Natural Gas Airships for Brazil
(250 and 440 meters)
By Jesse Blenn
Airship Design Specialist
jesseblenn@gmail.com
Teléphone: +506-8372-4113
August 2, 2012
.
Basic Program
The basic program will be to build and operate very large airships to pick up and deliver natural gas (NG) from offshore wells to offshore pipeline connections or to shore, being distances of up to 300 kilometers. The goal is to use the NG itself as both the lifting gas and propulsion fuel. This is done in the interest of saving the cost and logistics of helium filling, which would not be practical on the scale needed. Also for over-sea low altitude operations unmanned control by GPS is considered safe and practical. It is expected that for mooring and loading operations full control by Radio Control transmitters will allow visual supervision of the more complex operations. Obviously if successful in Brazil the same and similar systems could be used worldwide both over waters and over land where pipelines are not economically viable or not yet functional, or as emergency supply vehicles in the event of damaged pipelines. We expect some very important patents in these mooring and NG transfer systems that could give us a monopoly on NG airship operations worldwide.
Tentative specifications and a design approach are here given for two versions. First is a 250 meter test airship that can be assembled in the existing Zeppelin hangar at Santa Cruz, with a NG delivery capacity of near 200,000 cubic meters of NG. Second is a 440 meter airship that can deliver about one million cubic meters of NG. The 250 meter test airship would use Helium for lift to allow more NG delivery capacity and to eliminate the danger of flammable NG in the hangar during assembly. The 420 meter airship would rely 100% on the lift of NG for economy of filling, less restrictions on the fabric used, and to be independent of volatile world Helium prices. Important to note is that the carrying capacity goes up as the cube of the length, and thus a 10% increase in length gives a 33% increase in capacity, but only a 21% increase in needed power. Very large airships can be VERY energy efficient transport.
Different size airships can be made by easily scaling the basic design as needed, and as we will see the proposed outer fabric has enough strength to be used on airships at least 440 meters length. Analysis shows that the limiting condition for fabric strength is at high angles of pitch due to increased pressures from the gas lift. It is proposed that the airships be constructed in six sections for ease of handling and replacement of any damaged sections. The NG payload would be carried in the center four sections. Overall, the concept appears to be very valid.
NG Airship Specifications
Length 250 meters & 440 meters Diameter 50 meters & 88 meters
Fineness (Length/Diameter) 5.0 & 5.0
Total Volume 304,340 cubic meters & 1,659,200 cubic meters
Volume 2/3 power 4525 square meters & 14,015 square meters
Surface Area (*6.5) 29,775 square meters & 92,220 square meter
Helium fill (27 %) 82,000 cubic meters & Not/Applicable
NG fill @ 63%/90% 190,000 cubic meters & 1,493,000 cubic meters
He lift @ 1 kg/m3 82,000 kilograms & Not /Applicable
NG lift @ 400 g/m3 76,000 kilograms & 597,000 kilograms
Empty wt. 25%/33% 76,000 kilograms & 197,000 kilograms
NG lift over weight 30,400 kilograms & 400,000 kilograms
Ballast water needed 35,000 liters & 420,000 liters
NG delivery 180,000 cubic meters & 1,000,000 cubic meters
Internal sections 6 Tail fins 4 in “X”
Fins area @ 30% V2/3 1936.5 square meters & 3440 square meters
Power 2 x 1500 HP & 4 x 1500 HP 2238 kW Total & 4476 kW Total
Speed cruise 100 kph 96 kph
Design Considerations
Length to Diameter. In the interest of maximizing internal volume per surface area and also adding good resistance to bending from gusts without adding aerodynamic drag, it is highly recommended to use a length to diameter (fineness) ratio of about 5 as the best combination of low drag and low weight and cost. The fabric weight would be excessive with a higher fineness ratio. It is true that some large rigid airships used a ratio of near 8; in fact the LZ-127 Graf Zeppelin had a length of 236.5 meters and diameter of 30.5 meters, giving a fineness of 7.75. However this was to keep the diameter within an existing hangar and was not the optimum. Most non-rigid airships or blimps use a fineness of about 4 to minimize fabric weight but that increases aerodynamic resistance.
Size. See the above Specifications. Because the goal of the larger version is to depend on NG lift for the lift of the airship at all times, it is necessary to calculate carefully the lifts and weights involved. Using the recommended mathematically-defined shape, and doing the basic profile calculations we easily calculate shape, surface areas, volumes, etc. in Excel Spreadsheets. These include the two-thirds power of the volume (Volume 2/3) which is a convenient input for fabric, power,
and control calculations. The 440 meter size is preferable because it allows independence from the use of helium.
Envelope Weight. Outer envelope areas are 6.58 times the Volume 2/3 in square meters of each airship. Outer envelope weight is 1.1 kg per m2 plus a 5% addition for seams. Additional internal lightweight fabric of 200 grams/m2 for dividers and gas cells is described below, and weighs about 17% that of the outer fabric. The total envelope weight for the 440 meter airship is a little over 120,000 kilograms, as shown in the “Envelope Calculations” Excel spreadsheet.
NG Payload and Helium. Use of Helium as lifting gas in the 250 meter version will allow inflation in the Santa Cruz hangar without fire danger to the historic structure. This will also reduce the volume dedicated to lifting gas and so increase the available volume for NG delivery. However any large number of such airships could affect the limited world supply of helium and greatly increase prices. Also much more expensive fabrics are needed to retain helium. The 440 meter version uses NG as lifting gas to stay independent of world helium supplies and prices, as well as to use cheaper and more readily available fabrics.
Practical considerations/hangar. It is hoped that we can use the Zeppelin hangar at Santa Cruz for assembly of the first 250 meter airship. For the larger 440 meter size, a full-size hangar to construct the NG airship would be extremely expensive and is not absolutely necessary, especially if a site protected from winds can be used. Given the weight of the airship, it will be very necessary to build it in sections. Because six internal sections are planned and given the weight of the airship, it can be built and transported in six sections, joined together before final inflation. It could also be advantageous to use a relatively small inflated semi-circular fabric covering (cobertura inflavel) where it can be partially inflated during construction just enough to install internal components. This inflatable structure should be aligned with the prevailing winds. Once the airship is complete, the inflated covering would be removed and the airship inflated with air and NG at the same time, while attached to the mooring anchor, which will be an inflated fabric dome to avoid all danger to the envelope fabric. This dome would inflate and rise upward as the airship is inflated.
Gasbags Design. Nearly all present airships use the outer envelope as a single lifting gas reservoir with internal air bags called ballonets to adjust for lifting gas volume changes, and to adjust for pitch trim changes, These air bags are attached inside and near the bottom of the envelope, and allowed to basically pile up there when partially full or empty of air. Given the high percentage volume changes of enclosed NG in this design proposal it is recommended to use a system more like that used with past rigid airships and in fact of simpler construction. The NG delivered would be carried only in the center four sections of the envelope which do not have any internal metal framework. In this case inner gas bladders would be made with the same shape as the bottom half of the outer envelope and with divider “end caps” of semi-circle shape at the ends of each section, where there would be a full circular lateral divider also.
The division into sections is necessary to avoid NG surging in which – if free to do so- gas will naturally flow to the higher end of the airship making longitudinal pitch control and stability nearly impossible. For this reason normal airships do not have ballonet lengths of over about 20% of the airship length, and in our case 6 divisions give 16.6% each division. Being in the form of the lower 50% of the envelope, by inverting they will also allow complete evacuation of the outer envelope sections when needed during initial NG inflation and possible future maintenance. These internal gasbag and lateral dividers carry very low stresses and can likely be of 70 denier urethane-coated Nylon taffeta with a weight of about 200 grams per m2 and cost of near $8 per m2, likely less in such large quantity. Diagonal bracing webs can carry loads and prevent distortions.
Internal Suspensions. Given the reduced lift of NG compared to helium the envelope distortion due to lift will also be less relative to the airship size (about 40% as much), but still be high due to the great size of the airships. This tendency to distort will also vary greatly with the percentage of fill with and without payload, and it will be absolutely necessary to maintain shape and distribute lift via internal suspensions that maintain a nearly circular cross-section independent of gas lift. The variations in cross-sections can be calculated graphically, simulated by computer, and/or modeled accurately with a water model of about 1/75th scale (3.33 or 5.87 meters length). This is made of the actual envelope materials to be used, hung inverted and filled with water to the same percentages as NG to simulate distortions, and can be excellent hands-on practice for engineers and workers. Based on these investigations simple internal top suspension curtains (catenaries) running between the dividers in the top one-half of the airship will minimize changes in cross-section. The number (likely three to six) and location of suspensions will be chosen based on the simulations. At their lowest points they connect to internal framing or water ballast loads which they carry up to the lifting gas.
Fabric Stresses. Fabric stresses are only in the form of tensions (including sometimes diagonal tensions) because fabric gives under compression. Tensions can be from internal pressures and gas lifts, or from aerodynamic or static loads. Hoop (circumferential) stress is normally twice that of plug (longitudinal) stress except in the much curved sections near the bow and to a lesser extent the stern. Aerodynamic forces are mostly in the form of bending from wind gusts which will increase the stress or tension on one side of the airship and reduce it on the other. Thus plug stress tensions can vary during aerodynamic loading much more than hoop stress tensions. Normally sufficient internal pressure (around 40 mm water column) is maintained to assure that tension never reaches zero.
Fabric stresses are found by multiplying the internal pressure by the local radius to find hoop stress, then dividing by two to get normal (no gusts) plug stress. A safety factor of four is normally used for passenger airships. One kg per square meter of pressure gives 1 mm water column pressure, and rarely if ever do airships use over 50 mm water column pressure = 50 kgs/m2. We have fabric tension strength of 900 kgs per 5 cm width or 18,000 kgs per meter. For our larger airship maximum radius of 44meters maximum radius we then have 18,000 kgs / 44 meters = 409 mm water column pressure at rupture, giving a very good Safety Factor for horizontal flight.
We will see on the Excel sheet that the limiting condition is local high gas pressure at high angles of pitch, but even then a Safety Factor of over 3 is maintained during a strong gust at maximum speed of 100 kph and 30 degrees inclination, an extremely rare occurrence. The gas pressure component is independent of the air inflation pressure, so is added to it. The necessary internal pressure is calculated in Excel based on Transport Airship Requirements certification rules. However, based on millions of kilometers experience in Goodyear airships, a pressure of about 70% of the TAR result should be adequate, which would be 0.7 x 47 = 33 mm water column, which again increases the fabric Safety Factor. In reality a large part of this increased pressure with inclined flight will be carried and thus reduced by the lateral dividers, probably enough to keep a Safety Factor of 4, but not enough to allow a much larger airship. This effect will need to be investigated, which will be easy with the mentioned inverted water model. However it is expected that an airship design of near 1 million cubic meters NG delivery capacity is near the maximum safely built with the mentioned fabric.
Water Ballast and NG Transfer. While filling with NG payload the airship will want to rise and tanks will be filled with water ballast by surface-mounted water intakes and air-powered pumps. These fabric tanks should be in the form of modified cones at each of the five lateral divisions. Their top points would be attached to the lower points of the internal suspensions at each division. Multiple cones would be fused laterally across each of the lateral divisions.
With the extreme size of the airship, standard ducts if used to move the NG and air would have to be extremely large because volume increases as the cube of dimensions but cross-section of ducts increasing only as the square. Thus ducts are not practical and we propose simply opening large valves in the upper half of the lateral dividers to allow the upper half of the airship to act as a duct. These transfer valves are then closed for flight. Air-powered fans and valves in the lower envelope sides – probably 2, 4, 6, 6, 4, and 2 in each of the sections 1 to 6 – will move air in and out as necessary. The filling and emptying of each section will be adjusted by varied operation of the different air valves and fans. The time to transfer will be calculated but twenty minutes would be a reasonable time. Logically during this load or unload time a corresponding quantity of sea water will be pumped into or released from the ballast tanks.
Semi-Rigid Framing. A large percentage of historic airships (with over 70 built in Italy, others in France, Russia, and the USA) were fitted with simple frames along or inside only the bottom of the envelope to reduce distortions and distribute loads. The lifting gas itself pressurizes the upper envelope portion so it tends to keep its shape much better. Semi-rigid framing allows distribution of concentrated mooring, landing, propulsion, and control loads into large areas of the airship envelope. However, in the interest of ease of manufacture, reduced cost, and increased safety it is recommended that only minimal framing be used to distribute these loads and only near the bow and stern, in sections 1 and 6. The tail fins structures in section 5 can be all external to the envelope to not interfere with gas capacity. As a rough estimate of framing strength and weight, we can assume a longitudinal compressive force of about 10,000 kgs for each side frame, as explained below.
Inflated Strengthening Tubes. Longitudinal tubes at 45 degrees both sides of the bottom centerline can provide the necessary rigidity over much of the length (from lateral dividers 1 to 5) to handle occasional compression loads and to mount systems to conduct ballast water and control connections throughout the ship. These air-filled tubes also will allow safe internal access to large areas of the airship during construction and maintenance. Similar inflated tubes at the upper centerline will also be desirable for access to valves, lightning protection, emergency rip systems, and the external envelope.
Because it is important to judge if they will really work, we include some preliminary calculations. As an estimate, we might choose to have the pressurized tubes impart a bending resistance to the lower part of the airship nearly equal to that provided by the Natural Gas pressure in the upper part, so that the airship is equally resistant to bending in all directions even if internal air pressure is lost. According to the Excel file “Cargas de Vuelo y Presion Operativo Dirigible Gas Natural”, Momentos y Presiones sheet, cell E32, the gas pressure moment is 770,800 kilogram meters (to be modified for different sizes). If internal pressure is lost, a gust hitting near the nose or tail at 45 degrees to either side of the top of the airship would be resisted mainly by the pressurized tubes opposite the gust, as illustrated in the accompanying drawing “Typical Cross Section NG Airship”. Purposely ignoring the helpful effect of a portion of the gas pressure, we simplify to say that the moment of 770,800 kgm would be divided by the 80 (50 or 88) meter diameter and would become a compressive force of 9635 kilograms, divided among the three tubes shown with a total cross section area of about 5.63 square meters. (The lesser true compressive force can be calculated also but is more complex.) Dividing 9635 kg by the 5.63 square meters we see that a pressure of 1711 kg/m2 or 1711 mm water column would be necessary to maintain shape. To see the safety factor of the fabric at that pressure, we first find the maximum pressure that the fabric can withstand at 1.5 meters radius, which is 18,000 kg/m / 1.5 m = 12,000 kg/m2. The 1711 mm pressure would induce a tension of 1711 kg/m x 1.5 meters or 2567 kilograms per meter length. Dividing the 12,000 kgs above by the 2567 kg we get a safety factor in circumferential tension of about 4.67, which is acceptable.
We see from the Excel calculations that the gas pressure moment can absorb about 14% of the maximum gust bending at high speed by itself. With the addition of the pressurized tubes this would become about 28%. The gust bending moment increases nearly as the square of the speed, thus with only the gas pressure moment and the pressurized tubes the airship could easily maintain shape at up to near half speed or 50 kph, sufficient for emergency flight in case of loss of envelope pressurization.
We can appreciate that the pressurized tubes could also withstand a longitudinal force, for example by a tail wind into the mooring mast, of about 10,000 kg each side or 20,000 kgs. Given that the propeller thrust at 96 kph is about 13,000 kgs, the pressurized tubes could withstand any likely tail wind effects pushing the airship into the mast, even with the reduction in diameter of the tubes towards the bow and stern, where the forces are fed into the metal framing. Also, in case of higher loads, the tubes and envelope are free to simply distort with no damage, creating a very damage-resistant structure.
Tail Fins. A major source of drag in past and present non-rigid airships has been the tail fins. Besides being suspended by dozens of high-drag cables they also use antiquated NACA 00 airfoils and no attention to root streamlining to reduce drag and extend their area of influence on the envelope surface and thus their effectiveness. Rigid airships were usually better but still could be much improved. Also undesirable was their mounting in cruciform “+” configuration which made the lower tail fin very easy to hit on the ground and damage, and did nothing to control roll when moored. The last airships used by the US Navy after World War II had 45 degree or “X” tail fins or three fins in an inverted “Y” which proved much superior for control and damage avoidance. We would use fins of the “X” configuration with 90 degrees between the lower fins, thus aligning them with the internal inflated tubes.
Tail fins area (per one side) should be near 30% of Volume 2/3 for the best control, as given in the above specifications. “Dorsals” or extensions of the tail fin roots forward of the main fin improve resistance to stalls at high angles of attack and also spread loads over a larger area of envelope. The forward points of the dorsals reach the 4th internal divider, and the main cantilever loads are taken into the framing and envelope at the 5th (last) internal divider.
The tail fins will be full cantilever with no external cables, yet with a simple pneumatically operated folding system of the lower fins that allows easy ground (water) handling and maintenance. Detailed loads analysis will be done to estimate maximum tail fin loads for the design of internal structures and accurate weight estimates. The simple internal structures that locate the fins also serve to distribute the rear propulsion loads (near 13,000 kgs) forward to the large fin bases and from there into the envelope.
Rudders and Elevators. Because of the concentration of weight near the bottom of the airship and the cyclic control of the main propellers it is practical to have the main control surfaces on the two lower tail fins only, eliminating most maintenance on the smaller upper fin. In this case the lower fins and their control surfaces are divided at the fold line. Emergency or trim ruddervators can be installed on the upper fins. The inner halves near the envelope include rudders which are moved opposite to their mate on the other fin and thus give lateral or yaw control. By a simple common actuation these can at the same time serve as elevator trim. In contrast, the control surfaces on the outer (folding) half of each fin work in unison to give up and down pitch control, and can be trimmed against each other for roll control, for example to counteract torque reactions from the large diameter rear propeller(s).
Propulsion. For simplicity and to give the greatest possible control, two helicopter rotors will be used, rotated to a horizontal shaft axis. These will mount on each end of a lateral frame integral with the internal framing at the very stern of the airship, with one rotor on each side. This system will mean a minimum of modifications to the helicopter system (rotating the gearbox 90 degrees and adapting a partially reversible rotor) and still isolate the envelope from possible damage from engine heat or fires. Maintenance access will be through the internal framing. Cyclically controlled (helicopter) rotors give excellent control plus reverse, which was tested successfully on the Goodyear airship Mayflower (Project Silent Joe, almost 50 years ago).
Gas turbine helicopter engines are available rebuilt (“zero timed”) and for example those for the common twin-turbine US Army UH-1 puts out up to 1500 HP each continuous and can cost under $100,000 each because they have been in use for several decades. For example see page 47 of http://www.bellhelicopter.com/MungoBlobs/87/728/EN_UH-1Y_PocketGuide.pdf.
These engines would need to be converted to NG use, including with high-volume NG pumps. Fuel gas would be taken from the upper fin bases in Gas Cell 5 and fed down tubes inside the tail fin to the two engines. Fuel use would be unimportant during trips of a few hundred kms, as there will be enough NG in the ship to fly around the world multiple times nonstop. Cooled compressor bleed air can operate valves, water pumps, the bow thruster propeller, mooring cable feeds, or other power needs with minimum weight and complexity and no danger of oil spills or fire. There should be two small back-up compressors APUs for emergency and maintenance use.
Performance. Power needed is a function of the aerodynamic and propulsion efficiency of the airship and its Volume 2/3. Power needed also goes up and down nearly as the cube of the airspeed, but with some variation due to scale or Reynolds Number effect. This permits a small speed reduction to save a lot of power and fuel. The famous Hindenburg (LZ-129) was not the most efficient airship, being beaten by the USS Akron and even more so the Macon. With the more efficient stern propulsion and better shape we can expect about a 50% improvement over the LZ-129. Its top speed was 135 kph with four 1200 hp engines, total 4800 hp. With a 50% higher efficiency they would have only needed 3200 horsepower to go 135 kph, and to go 100 kph they would only need 40% of that or 1280 hp. This times 1.32 or 3.35 for our 250 or 440 meter sizes shows we would need about 1690 hp or 5188 hp to maintain a 100 kph cruise.
The recommended main goal is to use the larger 440 meter size as the main delivery vehicle, so it would be very practical to use two UH-1 engines for the 250 meter ship. This would give the practical experience of converting the engines to NG and converting the UH-1 rotor system to horizontal, and allow very easy and confident experience with the engines. Then the same but four engines would be used for the larger 440 meter ship. Thus we see that the 440 meter airship could deliver over 5 times the NG with only twice the power. It is easy to see the absolutely immense increase in efficiency of the huge airship over the four helicopters with the same engines!
Mooring. Mooring has always been a problematic area for airships normally requiring 3 to 5 times as many people on the ground as in the air. The Hindenburg used a landing crew of about 200-500 men, and its volume was 200,000 cubic meters, with a mass near 200,000 kilograms. The proposed 250 and 440 meter designs are 1.5 and 8.3 times this mass. The inertia forces on the NG airship are thus 1.5 or 8.3 times as much as those of the Hindenburg, but the actual wind forces involved are relative to the our Volume 2/3, so are 1.3 or 4.1 times those of the Hindenburg at 3420 square meters. A reliable automated mooring system is a must.
A large airship of this size requires a mechanical mooring system, and a mechanical mooring system requires a minimum of three hard points on the airships, which standard airships have no provision for. Thus, they have never solved the mooring situation. Our three hard points will be provided by the front “V” section of semi-rigid framing within the envelope section 1. Besides the necessary three-point mooring system using cables controlled by constant- tension air motors, it will be advisable to have a bow thruster system to give up-down and right-left control at the nose. The basic system has been designed and could use fixed propellers operated by bleed air from the main engine compressors and thus eliminate any fire danger. The expelled air adds to thrust. With a combination of excellent control and automated mooring attachment to a wide-base mooring cone, we should have an operable system. Once attached, NG transfer valves will allow flow of NG in and out of the ship via pressurizing fans. Due to the low pressures involved, a simple and inexpensive water seal can be used to allow airship yaw rotation with wind changes while transfering NG.
Proof of Concept. It would not be advisable to make a 250 or 440 meter airship that plans to solve major problems of past airships without a flying proof-of-concept airship, as well as ground-based testing of NG transfer and other important systems. Due to the limited lifting power of NG, a small and useful airship would be difficult to build using only NG. We are working on an independent project with the aim of building a very modern and advanced solar-powered airship to fly non-stop around the world, hopefully in 2014. We are near starting preparations to build a half-scale demonstrator of 32 meter length and 900 cubic meters volume.
We will be testing the mentioned improved mooring systems, stern electric propulsion, and other relevant features mentioned in this analysis on this small and relatively inexpensive airship. I can recommend a joint effort as a good way to save time, money, and risk for your project, as well as train your personnel in airship design and operation.
Project Costs
It is not easy to accurately estimate costs and time involved in such a program, as historical airship data can only be used as a starting point. Most of the available data is for the large and complex rigid airships, which included complex aluminum frames with millions of hand-pressed rivets and many kilometers of cables, besides hand-tightened and hand-painted fabrics. .
Modern non-rigid airships are very high priced. For example the envelope for a 60 meter airship uses near 2000 square meters of external fabric at under $40 per square meter, or $80,000. This helium-retention fabric is very expensive and is not needed for Natural Gas. Internal fabrics are less than half of that area and less expensive. So perhaps the total materials cost is $120,000 including some special fittings. The envelope is sold for near $1 million, or 8 times the material costs and near $500 per square meter outside area. The envelope design and construction can be simplified and done in-house by XXXX in Brazil at a greatly reduced cost.
Except for the communications and remote control systems, the proposed airship is NOT high technology, and the speeds, stresses, and materials are much more comparable to small piston airplanes than to jet airliners. As a general example, based on experience in the US, a small 4-seat airplane like a Cessna 172 can cost about $300,000 new and weighs empty about 800 kilograms, about $275 per kilogram. About half of that price is materials and half is labor and company overhead costs. That is much more reasonable than present airships! The complex components such as engine, instrumentation, fasteners and systems are much more expensive than the raw materials such as aluminum sheet, and cost near $300 per kilogram, whereas the aluminum might be $10 per kilo. The average could be near $200 per kilogram in materials and $200 per kilogram in labor, all other fixed and variable costs, and profits.
Given the outer envelope surface area for the 250 meter airship of near 30,000 square meters (3 hectares) we have a cost at perhaps $20 per square meter or $600,000 for the outer envelope. With internal fabrics that total fabric cost could be near $1 million. It would seem reasonable that the XXXX company experience with inflatable structures can be used to estimate the time and cost of the outer envelope much better than using the market price for airship envelopes, as explained, or our estimations. But to include an estimate, we expect that the labor cost will be approximately two to four times the material costs, for example not over 4 million dollars, but XXXX can estimate this best.
An estimate of the weight and cost of the framing can be made based on a rough estimation of about 10,000 kgs compressive loads per side. Simple “drum” girders of coiled aluminum corrugations were shown to be equally or more efficient and much cheaper and less work to build than the Zeppelin type triangulated girders of complex aluminum stampings. With an efficient framing system such aluminum girders should be able to carry at least 20,000 pounds per square inch of cross-section, thus near one square inch cross-section per side. Using a safety factor of 6, we would have 6 square inches per side, about .08 square feet. At 165 lbs per cubic foot this gives 13.2 pounds per foot or 19.6 kilograms per meter. The total length of aluminum structures including the fins main spars will be something near 600 meters, giving us about 12,000 kilos of aluminum. Aluminum sheet (for example the corrosion resistant 6061 T6) is inexpensive, and the main material cost would be less than $10 per kilo. Thus about $120,000 of aluminum would be needed for the main framing structures. If labor costs were 5 times the material costs then the framing would cost $600,000 dollars.
The tail fins themselves are near 1940 square meters with a weight of near 10,000 kgs. At a cost including engines and labor of $50 per kilogram their total would be about $500,000 dollars. Other propulsion and systems components including ground equipment might cost $1 million. Ground support equipment might cost an added $500,000. Helium might cost $1 million. If engineering costs are $1 million (equivalent to 10 engineers/assistants 1 ½ years), and prototype and testing costs $1 million, the total comes to $10,100,000. Even if cost overruns add another 50% to $15 million for the 250 meter airship, that is very reasonable for the largest flying object in history, and for what it will do.
COSTS FOR 250 METER NATURAL GAS DELIVERY AIRSHIP
Envelope $4,000,000
Framing $600,000
Fins $500,000
Propulsion $1,000,000
Ground support $500,000
Helium $1,000,000
Engineering $1,000,000
Prototype/tests $1,000,000
Infrastructure $500,000
SUBTOTAL $10,100,000
Plus +*50%* $4,900,000
TOTAL $15,000,000
What will it do? If we assume that it can make three trips per day, 250 days per year, then the first smaller 250 meter NG airship could deliver about 135 million cubic meters of Natural Gas per airship per year. If this NG is worth $50 per thousand cubic meters (below world price) at the pipeline, then the delivery would be $6.75 million dollars per airship per year. The larger 440 meter version could deliver about 750 million cubic meters worth $37.5 million dollars. This would be near the cost of the larger airship, delivered yearly. Clearly the system could pay for itself in a relatively short time. All told, with a pure NG airship, XXXX has a great idea, and I congratulate you. We will be happy to work with you to make it a reality.
Jesse Blenn
Airship Design Specialist
Da Vinci Costa Rica