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Airborne 155mm Self Propelled Howitzer (spring driven shooting, RC)
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2 studs smoothbore L35 barrel, upward folding breech, spring driven propellant cartridges and projectiles, RC-controlled elevation+traverse and 6×6 chassis, independent suspension retractable for airlift, retractable spades and stands, 2 Bionicle/Technic figures, in scale 1:10
About this creation
1 Introduction and inspiration

Our Airborne 155mm Self Propelled Howitzer (in short ASPH) is an attempt to design shooting + realistic looking + compact gun MOC. To achieve the 3 goals together is a difficult issue in Lego:
- TLG does not support modeling actual firearms (despite offering many parts for historic or sci-fi guns), therefore there is a complete lack of strong larger diameter, long barrels.
- We cannot use chemical propellant for projectiles, because ABS material would melt. Rubber-band driven shooting mechanisms are pretty effective, but they are non-compact, and pretty dissimilar in shape compared to modern breech-loaded artillery pieces. Therefore one has to conceal rubber band mechanism in pretty sizeable model with large material requirement to get shooting AND realistic look together.
- Moreover, when rubber bands are tensioned manually, it will disrupt exact aiming of the artillery piece. Alternatively, they require bulky RC-controlled electric tensioning and triggering mechanism.

In my former Shooting Towed Howitzer MOC - which is based on BAE Systems M-777 Howitzer - I encountered the very same modeling problems. I solved them creating detachable barrel from trunnion to prevent tensioning disrupt aiming. But this way I sacrified realistic look and compactness of breech for shooting performance.


Figure 1: Airborne 155mm Self Propelled Howitzer (ASPH) functions overview
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See model in LDD

In this current MOC I accept reduced shooting performance for realistic look, developing self-contained, spring driven, base primed propellant cartridge, which is very similar in shape, size and usage to real one. This way, the gun itself can be a relatively simple barrel with lockable breech. The price I have to pay is that spring-driven self-contained cartridge can store considerably less energy than long rubber bands.

To test whether I could design really compact shooting gun mechanism, I did what nobody did before:
- I placed the gun on 6×6 electric RC chassis together with 2 seat cockpit to create 155mm (6in) Self Propelled Howitzer in scale 1:10.
- Then I squeezed it in the confined cargo bay of my forthcoming SkyTank Heavy Battlefield Helicopter MOC making it airborne.
Airborne artillery pieces traditionally are transported to the battlefield as underslung cargo of heavy helicopters. But any helicopter with underslung cargo can be an easy target of even a single shoulder launched AA-missile. I tried facilitate rapid airlift to battlefield in force to make ASPH transportable internally.

This required to build the most compact 6×6 chassis with good cross-country mobility. Therefore, I omitted traditional 6×6 transmission: gear shift, differentials, transmission shafts, synchronized Ackerman-steering, because they eat incredible amount of space from TLG parts. I solved both propulsion and steering with 6 PF XL wheel hub motors – using their high torque - together with independent suspension, which is retractable for air transport.

I got the inspiration for excellent cross-country mobility chassis from SpGH M-77 DANA 152mm L52 self-propelled howitzer, Czech Republic , which has genuine 8×8 chassis made by Tatra.


Figure 2: SpGH M-77 DANA from Czech Republic

2 Action screenshots of ASPH

*For considerable part of rendering and artwork, very special thanks to C BigBoy99899


Figure 3: ASPH folded for airlift
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ASPH is waiting for backing onto the cargo ramp of a heavy helicopter. Independent suspension is retractable to allow the chassis to “sit down”. Driver’s cabin is split into two halves to accommodate gun barrel retracted. Rearview mirrors are foldable.


Figure 4: Loading ASPH into heavy helicopter
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Figure 5: Prepare for roll out ASPH from heavy helicopter
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At the drop zone, load master of the heavy helicopter directs rolling out of ASPH on opened cargo ramp. Rear ammo magazine of the helicopter is not removed yet.


Figure 6: Rollout path of ASPH from heavy helicopter
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To facilitate rolling out oversized cargo on ramp, back door of cargo bay lifts together with opening ramp. Main landing gear bay is left void when landing gear is opened. Moreover, ammo magazine of the rear gun turret can be lowered by on-board winch and rolled aside on its castor wheels.


Figure 7: Roll out ASPH from heavy helicopter
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After ASPH rolled out, rear magazine is lifted back to its position. Suspension of ASPH is extracted, rearview mirrors, and UHF-aerial are opened transforming it into road mode.


Figure 8: ASPH in road mode
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Figure 9: Backing ASPH to dig in spades
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Reaching the appointed position of the battery, spades are opened, locked by locking pins and pressed to the ground by their linear actuator. Then ASPH backs to dig in spades safely.


Figure 10: ASPH in battery for indirect fire
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20 projectiles and propellants plus the panoramic targeting telescope is transported by a separate airborne amphibious ammo van for the battery. 155mm howitzer of ASPH can be elevated maximum 20 degrees, enabling long range indirect fire.


Figure 11: ASPH battery fires long range indirect salvo
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Figure 12: ASPH in battery for direct fire
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Alternatively, ASPH can be used as field gun for direct fire, disassembling the cockpit. Backward opening armored cockpit doors serve as shield for the crew against small arms fire and fragments.


Figure 13: ASPH Open breech view
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With breech opened 62 degrees upward, we can clearly look through 2-studs smooth bore of the L35 barrel. The red trigger lever on breech is pulled 1 stud left from center as safety, to prevent its accidental usage. Breech cannot be opened without setting trigger safe first. Breech is locked both in opened and closed position pressing/pulling the right/left ‘Hammer’ parts on it.


Figure 14: ASPH battery in war game
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Battery commander sometimes gets headache from rookie privates during war game.

3 Technical details of ASPH

*This part is technical and for MOCers with at least some basic artillery knowledge. If you do not understand how modern guns work, you can find an excellent summary at Wikipedia

**In the forthcoming technical description, functional parts of ASPH are referenced by numbers which can be found on technical drawings attached

***Parts of ASPH are color-coded by their function:
- Yellow: Manual handles of working functions
- Gray/Black: Static parts
- White: Dynamic parts
- Blue: Seats of pilots
- Red: Firing trigger, Propellant cartridge, Fire extinguisher
- Dark green: Projectile


Figure 15: Left cutaway view of ASPH
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3.1 Projectile and propellant cartridge


Figure 16: Propellant cartridge and projectile of ASPH
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I designed the 2×6.5 studs self-contained, spring driven, base primed propellant cartridge to emulate size, shape and usage of real propellant cartridges. This allows to make gun barrel and breech more simple and realistic looking, but the price of it is limited muzzle velocity. Eight 1×2 studs steel springs extracted from TLG part ‘Shock absorber extra hard’ compressed 1:4 can store less energy than a long rubber band. Usage of single 10×128mm steel spring would make propellant cartridge more effective, as short separate springs “kick aside” each other during shooting. But there is no such a spring in TLG parts, so we omitted this option.

2 studs diameter in scale 1:10 roughly equals 155mm (6in) bore. This is the heaviest projectile in the reality, which can be loaded manually with reasonable speed. Therefore real 6 in rounds usually do not use relatively heavy metal-coated propellant cartridge, but gun powder is rammed behind projectile in silk bags (silk burns without leaving solid residues in barrel), which can be cut in different sizes by crew to adjust propellant force to actual range. Metal cartridges are used at smaller bores: they are easier to handle and can be loaded automatically, but less flexible and heavier. Moreover, they have to be extracted from barrel after firing. My propellant cartridge is most similar in usage to real base primed, semi-combustible cartridges (E.g. used by T-72 MBT autoloader):
- It can be primed by hitting its rear. This breaks out extruding spring lock, which catches the rear end of barrel. (This makes easier to build working autoloader for the cartridge in the future, as I plan.)
- Its forward part and some of the springs fly out with projectile, but baseplate, and broken out spring lock part have to be ejected from barrel trough breech (It makes designing autoloader somewhat more difficult).
As we cannot build rifled bore from TLG parts, projectile has to be fin-stabilized. Therefore projectile is more darts-shaped instead of classic shell-shape, and it is internally braced by TLG part ‘Outer cable 8 studs’ to prevent it breaking up during the shock of firing. In the reality, smoothbore gun systems are reintroduced in 3rd generation MBTs in 1960s, to facilitate rocket boosting of projectile. Also there are similar shaped projectiles in the reality called “discarding sabot” rounds.

3.2 Barrel, breech and recoil damper


Figure 17: Barrel, Breech, Recoil damper of ASPH
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See loading in LDD
See loaded barrel in LDD

Engineers of TLG did their best to prevent building any strong and long barrels from Lego necessary for modeling modern firearms. Introduction of TLG part ‘Rounded plate 4×4 with 16mm hole’ eased somewhat the situation: barrel with 2 studs smooth bore and 4 studs outer diameter can be built alternating this and ‘Curved brick 2×2’. However this barrel would break off under its own weight, as each section of that held together by 8 studs. Therefore we incorporated ‘1×1 plate with holder’ parts into outer surface of the barrel to hold 4 bracing strings. Correctly tensioned bracing stings will prevent barrel breaking off.

Folding screw type breeches in the reality usually fold aside. As we had to fit breech into very confined space just 5 studs tall, we opted for less handy, but more compact upward folding breech.

Because of the limited power of spring-driven propellant cartridge, recoil force effecting barrel won’t be very serious. Therefore I designed relatively small recoil damper mechanism containing four 1×2 studs springs extracted from TLG part ‘Shock absorber extra hard’. Barrel/breech assembly can slide back 1 stud by recoil on 4 ‘Technic axle 16 studs’, which also hold breech at the rear end of barrel.

3.3 Elevation gear


Figure 18: Elevation gear of ASPH
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Elevator gear in the reality is placed normally under barrel. But in ASHP, the whole gun turret placed on 6×6 chassis can be only 5 studs tall to fit in the cargo bay, we distributed elevator gear on left/right sides of barrel. 2 PF M-motor drives 2 linear actuators elevating barrel from 0..+20 degrees, which is considerably less than maximal elevation of a howitzer in the reality. To reduce the immense load on linear actuators, we used 4 barrel mass compensator springs made from ‘Assembly shock absorber’. Both elevation motors are controlled by Channel Red of left IR receiver.

3.4 Traverse gear and gun turntable


Figure 19: Traverse gear and gun turntable of ASPH
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Creating turntable of the gun with pivot points of 6×6 chassis, barrel, linear actuators and compensator springs was a real pain. We had to “flow” it into the residual space left around the parts mentioned above, however it has to be reasonably strong to tolerate forces of elevator gear and recoil. To make things happier, when independent suspensions of 6×6 chassis are retracted for airlift, top of the 6 shock absorbers protrude upward from the chassis 1.5 studs. That’s why we have holes in the baseplate of turntable, to let them through.

Traverse gear is normally built inside the chassis in the reality. But there was no space left because of its compactness. Therefore, 2 PF M-motors with Z8 pinion gear are placed on a platform protruding from gun turntable, and pressed by 1×2 springs to toothed bars fixed on the top of chassis. Both traverse motors are controlled by Channel Blue of left IR receiver.

3.5 Spades and stands


Figure 20: Retractable spades and stands of ASPH
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See folded model in LDD
See deployed model in LDD

Spades and stands are where the compactness requested by air transportability really did take the toll. These are normally hydraulically folded, but we could solve here only manual folding. Deployed spades can be fixed with locking pins and can be leveled by linear actuators. They help to press down spades to the ground, so backing with the 6×6 chassis will dig in spades.

3.6 6×6 chassis, drivetrain and suspension


Figure 21: Drivetrain of ASPH
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See extended model in LDD
See folded model in LDD

We tried to create the most compact 6×6 layout possible from 6 PF XL wheel hub motors and 2 PF Battery packs with independent trailing/leading arm suspension. Top pivot points of the 6 shock absorbers are detachable, pulling out locking pins, enabling chassis to “sit down” for air transport. Left 3 PF XL motors are controlled by Channel Blue of right IR receiver, right 3 motors by Channel Red.


Figure 22: Independent retractable suspension of ASPH
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Leading/trailing arms of independent suspension can work from 0 to -21 degrees, facilitating 2.75 studs spring path for 13 studs diameter ‘Motorcycle wheels’ with ‘Off-road tires’. Wheel base is 28 studs, while trench bridging capacity is 14 studs. Pivot axis of suspension swingarms is incorporated into belly armor of PF battery packs.

3.7 Cockpit interior and pilot figures


Figure 23: Cockpit interior and pilot figures of ASPH
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Creating the cockpit, the main difficulty was to squeeze two 24 studs tall Bionicle/Technic figures into a 10 studs tall interior. Therefore we used 45 degrees backward tilted pilots’ seats with safety belts from ‘Motorcycle chain’. Luckily, ASPH is controlled by 2 joysticks instead of steering wheel, and this reduced the space consumption of dashboard considerably. Left seat’s joysticks control propulsion and steering, while right seat’s joysticks control gun turret movement. In the space left behind the back of seats we placed 2 fire extinguishers and 1 spring-shooting, auto-loaded M-4 Assault Rifle Short version with 9 round box magazine of 1.5×7.5mm projectiles for self-defense.

3.8 M-4 Short Assault Rifle for self-defense


Figure 24: Firing cycle of M-4 Short assault rifle for self-defense
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3.9 Amphibious ammo van


Figure 25: Amphibious ammo van
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Because of the dimensional constraints of ASPH, it cannot carry any ammo, and even there was no space left for its foldable panoramic targeting telescope. Therefore I designed an airborne amphibious ammo van using the same 6×6 chassis with retractable suspension. I experimented here creating a collapsible body for air transportability. 20 projectiles and their propellant cartridges are stored in a removable 20-round manual ammo rack.


Figure 26: 20-round manual ammo rack
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4 Dimensions of ASPH

- Barrel inner bore: 2 studs/ 16 mm/ 0.6in, Real: 160 mm/ 6in
- Barrel outer diameter: 4 studs/ 32 mm/ 1.26in, Real: 320 mm/ 12in
- Barrel length: 70 studs/ 560 mm/ 22.05in, Real: 5.6m/ 18 ft 4.33in
- Barrel recoil path: 1 studs/ 8 mm/ 0.31in, Real: 80 mm/ 3.15in
- Barrel elevation range: 0°..+20°
- Barrel traverse range: -5°..+5°
- Deployed length: 110 studs/ 880 mm/ 34.65in, Real: 8.8m/ 28 ft 10.24in
- Road/ folded length: 71 studs/ 568 mm/ 22.36in, Real: 5.68m/ 18 ft 7.48in
- Chassis length: 61 studs/ 488 mm/ 19.21in, Real: 4.88m/ 16 ft
- Chassis height road: 18.75 studs/ 150 mm/ 5.91in, Real: 1.5m/ 4 ft 11.02in
- Chassis height folded: 16 studs/ 128 mm/ 5.04in, Real: 1.28m/ 4 ft 2.36in
- Chassis width: 17 studs/ 136 mm/ 5.35in, Real: 1.36m/ 4 ft 5.51in
- Ground clearance: 5.75 studs/ 46. mm/ 1.81in, Real: 0.46m/ 1 ft 6.1in
- Suspension spring path: 2.75 studs/ 22 mm/ 0.87in, Real: 0.22m/ 8.66in
- Wheel diameter: 13 studs/ 104 mm/ 4.09in, Real: 1.04m/ 3 ft 4.92in
- Wheelbase: 28 studs/ 224 mm/ 8.82in, Real: 2.24m/ 7 ft 4.13in
- Trench bridging: 14 studs/ 112 mm/ 4.41in, Real: 1.12m/ 3 ft 8.07in
- Forward ramp angle: 20°
- Rear ramp angle: 60°

5 Unsolved issues

- The weakest point of ASPH is elevator gear, which had to fit in very confined space, so arms of force are very short compared to mass of barrel, and this can lead break up of ABS parts. I designed an alternative swingarm + linear actuator type elevator gear, but that would further reduce the already confined working range of 0..+20 degrees.
- Further complicates problems that pivot point of barrel is 0.5 studs under centerline of barrel, so recoil force will generate torque further stressing elevator gear. I could not solve alignment of barrel pivot on its centerline because of space limitations.
- 13 studs diameter motorcycle wheels with off-road tires are too close to stands a nd spade fixtures. At -15 degrees position of suspension swingarms, tires even slightly touch them. In muddy environment it can cause serious problem, as mud will clog that small gaps. This is another point where compactness requested by air transportability took its toll.
- In the future, I will work on fixing the problems above and will try to incorporate a very compact autoloader mechanism, which works from box magazine storing projectiles/propellants in alternating order.

6 SAFETY CAUTION, READ CAREFULLY

1.NEVER LEAVE ASPH UNATTENDED TO PREVENT ITS ABUSE BY INAPPROPRIATE USERS!
2.INAPPROPRIATE USAGE OF ASPH MAY NOT LEAD TO DEATH OR SERIOUS INJURY OF THE TARGET.


Figure 27: Example for inappropriate usage of ASPH
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See model in LDD


Building instructions
Download building instructions (LEGO Digital Designer)

Comments

 I made it 
  June 19, 2015
Quoting Nick Barrett Really impressive and an excellent array of functions. XLs on each wheel will make it plenty quick enough too!
Thanks. Using XL Motor as wheel hub motor was a very strong compromise, because of the lack of any other suitable motor. The price will be paid in unsprung mass.
 I like it 
  June 18, 2015
Really impressive and an excellent array of functions. XLs on each wheel will make it plenty quick enough too!
 I made it 
  June 17, 2015
Quoting c bigboy99899 Great stuff again!
Thanks.
 I like it 
  June 17, 2015
Great stuff again!
 I made it 
  June 14, 2015
Quoting Centurion Cone Wow awesome SPG! great job!
Thanks.
Gabor Pauler
 I like it 
Matt Bace
  June 13, 2015
Wow! Another technical tour de force. The idea for the propellant cartridges is very clever. It's amazing that you were able to include that functionality while maintaining an overall authentic look. Great work once again.
 I like it 
  June 13, 2015
Wow awesome SPG! great job!
 I made it 
  June 13, 2015
Quoting Kurt's MOCs Another stunning project! Great work and I love how it fits into your helicopter. Amazing!
Thanks.
 I made it 
  June 13, 2015
Quoting Polino zs Everything you did, you just made it an insuperable creation ... brilliant job, man!
Thanks.
 I made it 
  June 13, 2015
Quoting Matt Bace Wow! Another technical tour de force. The idea for the propellant cartridges is very clever. It's amazing that you were able to include that functionality while maintaining an overall authentic look. Great work once again.
Thanks!
 I like it 
  June 13, 2015
Another stunning project! Great work and I love how it fits into your helicopter. Amazing!
 I like it 
  June 13, 2015
Everything you did, you just made it an insuperable creation ... brilliant job, man!
 
By Gabor Pauler
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