Luna 2-18 Express

Here we see a model o' this most famous member o' t' Heinlein Fleet, ya bilge rat, the Luna 2-18 Express. Begad! I have chosen t' model t' ship after she was retrofitted withpropellant scoops and her passenger cabin converted t' long term livin' quarters and hydroponics. Durin' that process, in order t' save mass that would contribute nothin' t' t' mission, her belly jets and landin' gear were removed and her main propellant tank extended an additional 14 meters past t' original nose (just below the cockpit, me bucko, t' original nose faired into t' passenger cabin's hump); this gave enough propellant capacity, in conjunction with t' Farside Facility catapult launch, t' transit directly t' Jupiter – t' catapult could nay be used for a slingshot maneuver around Earth, ya bilge rat, matey, and t' propellant used t' boost from the Lunar surface or direct from Earth orbit would have been sorely missed durin' Jovian insertion.

Clearly visible on t' model are t' distinctive twin nuclear-thermal Dumbo 18 engines, ya bilge rat, me hearties, uprated and improved versions o' t' first nuclear engine t' boost from t' Earth's surface. Avast, me proud beauty! While less efficient in deep space than t' older Nerva design, t' Dumbo engines were capable o' far more thrust, arrr, and were more tolerant o' variations in propellant – and t' introduction o' Single-H in 1986 brought t' Dumbo engine's efficiency up enough t' make single stage Lunar round trips economically feasible.

T' distinctive aeroshell around t' forward ends of the engines houses t' propellant pumps and t' power reactor, a liquid sodium power plant adapted t' provide electricity for t' 2-18 Express. Avast! Arrr! This all-nuclear system meant t' 2-18 class didn't need t' carry any oxidizer; the Single-H could be catalyzed t' recombine and provide very effective maneuvering and landin' jets, and o' course was t' best possible propellant for use in any nuclear thermal engine. Avast! Ahoy! Simplified plumbin' and a single tank meant still more performance improvements over older designs – and like everythin' else in aerospace, me bucko, once you start savin' a little mass somewhere, matey, shiver me timbers, t' savings snowball.

In her heyday, me hearties, t' Luna carried 42 passengers and a crew o' five (three flight crew and two attendants) on t' 60 hour run t' t' Moon, me bucko, though suborbital flights o' less than an hour saw coach grade seatin' exceed 130 passengers after t' ships were retired from first line service. Avast! T' 2-18 Express class was also t' first type t' have extensive on board computing capacity; t' originally installed computers were upgraded several times through t' years, but even t' first model had more computin' power in the cockpit than was used t' calculate t' Galileo's course on her historic Lunar voyage. Begad! By t' time o' her Jovian mission, t' Luna's main computer was capable o' speech synthesis and recognition and be more like one of t' crew than a tool or appliance.

This model was designed t' be as accurate as possible and still be easy to build. Arrr! Begad! T' main propellant tank is twenty-seven inches o' BT-56, t' passenger cabin ismade from 18" of BT-50 split lengthwise with a BNC-50A (Alpha nose cone) split and carved/sanded t' fit t' contour o' t' BT-56. Arrr! T' nose cone and aeroshell are from a Maniac and a Long Shot – t' aeroshell be split along t' molding seam and carved t' fit t' contours o' t' dual motor mount, ya bilge rat, me bucko, then glued t' the airframe and motor mount tubes with medium CA. Avast! Well, blow me down! A small liberty be taken; I installed a positive retention system t' ensure that t' motors stayed in place and did their job at deployment time. Unfortunately, due t' a camera problem (the operator failed t' secure t' film t' t' take up spool when loading, resultin' in a blank roll), shiver me timbers, thar are no construction or flight photos; only these images, taken with a web camera, arrr, currently exist. Avast! Begad!

There's a simple baffle incorporated into t' forward end o' t' motor mount. It consists o' a couple inches o' BT-5 mounted into each BT-20 motor mount with a 520 centerin' ring; by happy coincidence, me bucko, this places t' BT-5 at exactly the spacin' they'd have if they were part o' t' four-tube cluster that can be mounted in a BT-56; that lucky spacin' allowed me t' overlap t' forward ends of these tubes with t' aft ends o' another pair occupyin' t' other two positions o' t' cluster tube, matey, ya bilge rat, with t' result that ejection gases have to double back twice in order t' reach t' main airframe -- this does a very fine job o' trappin' any hot particles; in three flights, ya bilge rat, me bucko, it was difficult t' see from t' parachute and shock cord that any ejections had occurred. Aye aye! Ahoy! Blimey! If you choose t' incorporate t' baffle, however, me hearties, I'd recommend makin' t' baffle tubes longer than I did t' improve t' ease o' alignin' t' motor mount with the airframe.

T' wings are mounted at 15 degrees anhedral – that is, me bucko, droopin' slightly. T' win' planform is a 60 degree leadin' edge sweep, arrr, me hearties, taperin' from a 6 inch root edge (this can be cut from 3 inch wide fin stock) t' a two inch tip, in 1/8" balsa. T' rudder fins are mounted far enough inboard that thar's a flat fin surface t' glue t' – on t' original ship, t' ventral fins would hinge outward, me hearties, flat t' t' bottom o' t' win' tip, me bucko, t' provide ground clearance for landing. Blimey! Blimey! I haven't modeled t' belly jets or landin' gear, ya bilge rat, o' course, because they were removed durin' t' refit t' this configuration, but you can see t' propellant scoops, me hearties, in their retracted position, at t' extreme forward end o' t' engine nacelles.

Radially mounted on t' aft ends o' t' nacelles are t' radiators for the electric power reactor; these dump waste heat overboard by radiatin' it away to space or conductin' it t' t' atmosphere. As you can see, ya bilge rat, they're mounted in such a fashion as t' minimize t' amount o' exposure t' each other, in order to get maximum efficiency with minimum mass; they're modeled in 1/16" balsa. Begad! Avast!

T' issue o' paint schemes is a sensitive one – the Luna originally wore t' Pan Am Cunard colors, me bucko, but that paint scheme wasn't preserved when she was taken out o' service, matey, and t' extensive refit prior t' t' Jupiter mission didn't originally include a repaint – paint, again, was excess mass. T' actual color scheme, then, arrr, arrr, is a combination of whatever be left o' t' original PAC and whatever natural finish be present on new parts (such as t' propellant tank extension and ram scoops) – the latter frequently bare metal, shiver me timbers, matey, since t' ship wasn't expected t' need protection from oxidation.

Just before launch, however, someone pointed out that publicity images would show t' ship in a bad light, lookin' like somethin' jury rigged from a wreck instead o' a pioneer, so t' expedition management had a fast coat o' whatever they could find applied t' cover t' refits, with no expectation o' good adhesion due t' complete lack o' surface preparation. Arrr! Blimey! Ahoy! Blimey! Unfortunately, matey, shiver me timbers, t' only images o' t' ship in this configuration are those publicity shots – so that's t' color scheme I've modeled, a plain white hull and wings, flat black radiator surfaces, matey, and whatever windows were left after t' refit. Ya scallywag! Blimey! Aye aye! Blimey! T' dark band just aft o' t' nose, unfortunately, shiver me timbers, isn't part o' t' modeled color scheme -- it's t' minor damage from a core sample on t' first flight o' the model. Blimey! Blimey!

Recommended motors: 2x B4-4, 2x B6-4, shiver me timbers, 2x C6-5

Buildin' -- Parts and Plans

To built t' Luna 2-18 Express you'll need t' followin' parts:

27" o' BT-56
1 or 2x BT-56 couplers (mine came with an 18" section in t' bag, arrr, so I only needed one coupler, matey, but t' standard length seems t' be 9" and will need two)
2x PNC-56, me hearties, same as those in t' Maniac and Long Shot kits
18" o' BT-50 (for passenger cabin)
1 BNC-50, balsa version o' an Alpha nose cone (plastic will work also, me bucko, me bucko, but it's a bit harder t' split and sand t' contour)
2x 6" sections o' BT-20
4x 520 centerin' rings -- two for t' baffle, ya bilge rat, me bucko, two as thrust rings
4x 4" (or longer, as desired) sections o' BT-5 -- or only two if you don't build t' baffle
Card stock for cuttin' t' gas seals on t' baffle
1 sheet 1/8" balsa, 3" wide, arrr, medium-hard
suitable scrap or sheet o' 1/16" balsa at least 1.6" wide, medium
3/16" launch lug
Recovery system o' your choice (I used a 14" Quest parachute left over from me Big Betty's conversion t' a cackle fruit lofter)
Optionally, some sort o' motor retention system

Please see t' followin' fin alignment diagram, includin' baffle layout, ya bilge rat, and dimensioned fin pattern:

Click t' Enlarge

To construct t' Luna 2-18 Express model, first join t' sections of BT-56 airframe. Avast, me proud beauty! Blimey! Begad! Blimey! Ensure that t' sections are straight by rollin' t' tube on a flat surface such as a table top before t' glue sets. Aye aye! Blimey! Set aside one nose cone, it will be t' actual nose cone. Split t' other nose cone along t' molding seam, and remove t' base o' t' "heel" section o' t' cone. Begad! Blimey! Aye aye! Blimey! Cut the heel section t' about 1/4" length -- this is nay a critical dimension, this cut just simplifies fittin' t' aeroshell later one.

Sand t' glassine off all t' tubes t' facilitate glue adhesion -- doing this now will avoid frustration later. Begad! Blimey!

Mark t' motor mount tubes for t' locations o' t' radiator fins and for their glue join line. Arrr! Install t' 520 centerin' rings, two as thrust bearings about 2 1/2 inches from t' aft o' t' tubes, t' others exact at t' forward ends o' t' tubes. Ya scallywag! Glue one BT-5 section into each forward centerin' rings, takin' care t' align t' BT-5 parallel t' t' BT-20. Avast, me proud beauty! Ahoy! Join t' two BT-20 tubes side by side, matey, with both BT-5 at t' same end. Begad! Placin' t' tubes on a flat surface like a table top while gluin' will help ensure t' motor mounts are parallel. Begad!

If you're makin' t' baffle, cut t' centerin' rings t' fit t' BT-56 and the BT-5 baffle tubes, and assemble t' baffle as shown in t' diagram -- the BT-5 for t' forward half should overlap t' BT-5 from t' motor mounts by about one inch, t' allow for good gluing. Start by slidin' one centerin' ring onto t' BT-5 in t' motor mount, leavin' about 1/2" clearance from the motor mount t' allow for attachin' t' aeroshell later; glue it in place. Ahoy! Slide the other centerin' rin' over t' other pair o' BT-5, ya bilge rat, apply glue, me hearties, and then glue those tubes t' t' tubes from t' motor mount t' for a cluster o' tubes with about an inch o' overlap and a baffle chamber with adequate space t' avoid restriction o' t' ejection gas. Ahoy! Be very careful o' alignment; roll the assembly on a table with t' motor mounts hangin' off t' side t' ensure it's all straight. Blimey! Once t' glue is dry, mount t' assembly into t' BT-56 airframe, buttin' t' forward end o' t' BT-20 against t' aft end o' t' BT-56. If you've built everythin' straight up t' now, arrr, t' motor mounts will align parallel t' t' airframe; if not, me bucko, you can flex t' mount a bit at this stage, and when t' glue sets up where t' BT-5 contacts t' inside o' t' BT-56, matey, it will lock your adjustments in place. Avast! Begad!

Once t' glue is dry, me hearties, carefully carve each half o' t' split nose cone to fit t' contour o' t' motor mount tubes. Arrr! T' aeroshell halves must lie along the motor mount tubes and t' shoulder end must fit into t' airframe. Arrr! Once you have a good fit, matey, glue t' aeroshell in place. If you'll be usin' a motor retention device or system, shiver me timbers, be sure t' install it before permanently mounting the aeroshell, as you won't be able t' afterward. Begad! Well, blow me down!

Cut all t' fins. Plane and/or sand t' main wings t' a symmetrical airfoil, then do t' same for t' rudder fins and radiators. Aye aye! Ahoy! Use your choice o' methods to fill t' grain and tube spirals; I used Finishin' Wood Filler before gluing the parts together, arrr, but everyone has their own preference. Begad! Avast, me proud beauty! Attach t' wings first, then put t' rudders on t' wings, and finally attach t' radiators to the motor mount tubes. Avast! If you use double glue technique, matey, ya bilge rat, me hearties, white glue is more than adequate, matey, though I recommend you use a slin' type shock cord attachment to lower t' rocket horizontally or slightly nose down. Aye aye! Don't forget t' fillet -- those are very, very large fins with a surface mount.

Once t' fins are in place, mark t' BT-50 full length on both sides, and carefully split it along t' mark. Begad! Avast! If you use a metal angle or channel as a guide, shiver me timbers, beware o' t' tube closin' up after t' first side is split, arrr, matey, and leaving you with a helical second cut -- this is bad. Begad! Avast, me proud beauty! Split t' BNC-50 and glue the halves into t' ends o' t' split tube. Begad! Ahoy! Use t' airframe t' mark t' arc on the butt end o' t' nose cone halves, and then use a gouge or power sander t' rough cut before sandin' t' final fit. Avast! Glue t' passenger cabin t' t' airframe, ensurin' t' BT-50 is adhered all along both edges. Arrr! Fillets here are cosmetic, but they have a big effect on t' appearance o' t' model. Avast, me proud beauty!

Hang in there, you're almost done! Blimey! All that's left is fillin' and sanding, if you haven't, me bucko, me bucko, attachin' t' launch lugs, installin' t' recovery system, then finishin' by your choice o' methods. Arrr! Blimey! Now go fly!