Luna 2-18 Express
Model designed and built by Donald Qualls
The Heinlein Fleet entered service in
1991 with the launch of the Terra.
The class eventually consisted of eight ships, and served with distinction
on suborbital, orbital, and transLunar routes until being phased out of
first line service in favor of newer, more efficient designs beginning
in 2019. The ships continue in second line and feeder service to the present
day. Powered by twin Dumbo 18 nuclear engines, the 2-18 Expresses were
capable of single stage operation round trip from Earth to the Lunar surface
with a full passenger complement, or heavy lift operation to orbit or suborbitally.
Equipped with belly jets as well as wings, they could be flown off the
ground horizontally (with 65% propellant load) from simple concrete runways,
short landed in less than two kilometers, boosted in landing attitude under
Lunar gravity, or hoisted and launched vertically from towers like older
transport designs for maximum delta-vee.
second in class, is famous not for what she did in her original role, but
for what she was called on to accomplish after she was officially retired.
Purchased as scrap after being taken off the Lunar route by Pan Am Cunard
in 2021, she was refitted for long endurance, her engines modified to use
any combination of water, methane, or ammonia (or, in practice, nearly
anything that would liquify at tank pressure and temperature) instead of
Single-H (stabilized monatomic hydrogen), and catapulted from the then-new
Farside Facility to carry the first manned mission to the outer planets.
After four years exploring the Jupiter system, scooping reaction mass from
the atmosphere of the giant planet at intervals of a few months, she was
lost with all hands, presumably to a meteoroid impact, during transit of
the main Belt en route back to Earth.
Despite her tragic end, the Luna
proved beyond any doubt that the 2-18 Express class was capable of far
more than their designers envisioned. The Luna
made the entire expedition possible, since her combination of low cost,
high capacity, and excellent record meant money could be saved for other
things; without the lucky find, the mission would have had to be postponed,
possibly for many years, while more funds were raised and a ship built.
-- Jane's All the System's Spaceships,
Here we see a model of this most famous
member of the Heinlein Fleet, the Luna 2-18 Express
. I have chosen
to model the ship after she was retrofitted with
scoops and her passenger cabin converted to long term living quarters and
hydroponics. During that process, in order to save mass that would contribute
nothing to the mission, her belly jets and landing gear were removed and
her main propellant tank extended an additional 14 meters past the original
nose (just below the cockpit, the original nose faired into the passenger
cabin's hump); this gave enough propellant capacity, in conjunction with
the Farside Facility catapult launch, to transit directly to Jupiter –
the catapult could not be used for a slingshot maneuver around Earth, and
the propellant used to boost from the Lunar surface or direct from Earth
orbit would have been sorely missed during Jovian insertion.
Clearly visible on the model are the distinctive
twin nuclear-thermal Dumbo 18 engines, uprated and improved versions of
the first nuclear engine to boost from the Earth's surface. While less
efficient in deep space than the older Nerva design, the Dumbo engines
were capable of far more thrust, and were more tolerant of variations in
propellant – and the introduction of Single-H in 1986 brought the Dumbo
engine's efficiency up enough to make single stage Lunar round trips economically
distinctive aeroshell around the forward ends of the engines houses the
propellant pumps and the power reactor, a liquid sodium power plant adapted
to provide electricity for the 2-18 Express. This all-nuclear system meant
the 2-18 class didn't need to carry any oxidizer; the Single-H could be
catalyzed to recombine and provide very effective maneuvering and landing
jets, and of course was the best possible propellant for use in any nuclear
thermal engine. Simplified plumbing and a single tank meant still more
performance improvements over older designs – and like everything else
in aerospace, once you start saving a little mass somewhere, the savings
In her heyday, the Luna carried
42 passengers and a crew of five (three flight crew and two attendants)
on the 60 hour run to the Moon, though suborbital flights of less than
an hour saw coach grade seating exceed 130 passengers after the ships were
retired from first line service. The 2-18 Express class was also the first
type to have extensive on board computing capacity; the originally installed
computers were upgraded several times through the years, but even the first
model had more computing power in the cockpit than was used to calculate
Galileo's course on her historic Lunar voyage. By the time of
her Jovian mission, the Luna's main computer was capable of speech
synthesis and recognition and was more like one of the crew than a tool
This model was designed to be as accurate
as possible and still be easy to build. The main propellant tank is twenty-seven
inches of BT-56, the passenger cabin is
from 18" of BT-50 split lengthwise with a BNC-50A (Alpha nose cone) split
and carved/sanded to fit the contour of the BT-56. The nose cone and aeroshell
are from a Maniac and a Long Shot – the aeroshell was split along the molding
seam and carved to fit the contours of the dual motor mount, then glued
to the airframe and motor mount tubes with medium CA. A small liberty was
taken; I installed a positive retention system to ensure that the motors
stayed in place and did their job at deployment time. Unfortunately,
due to a camera problem (the operator failed to secure the film to the
take up spool when loading, resulting in a blank roll), there are no construction
or flight photos; only these images, taken with a web camera, currently
There's a simple baffle incorporated into
the forward end of the motor mount. It consists of a couple inches
of BT-5 mounted into each BT-20 motor mount with a 520 centering ring;
by happy coincidence, this places the BT-5 at exactly the spacing they'd
have if they were part of the four-tube cluster that can be mounted in
a BT-56; that lucky spacing allowed me to overlap the forward ends of these
tubes with the aft ends of another pair occupying the other two positions
of the cluster tube, with the result that ejection gases have to double
back twice in order to reach the main airframe -- this does a very fine
job of trapping any hot particles; in three flights, it was difficult to
see from the parachute and shock cord that any ejections had occurred.
If you choose to incorporate the baffle, however, I'd recommend making
the baffle tubes longer than I did to improve the ease of aligning the
motor mount with the airframe.
The wings are mounted at 15 degrees anhedral
– that is, drooping slightly. The wing planform is a 60 degree leading
edge sweep, tapering from a 6 inch root edge (this can be cut from 3 inch
wide fin stock) to a two inch tip, in 1/8" balsa. The rudder fins are mounted
far enough inboard that there's a flat fin surface to glue to – on the
original ship, the ventral fins would hinge outward, flat to the bottom
of the wing tip, to provide ground clearance for landing. I haven't modeled
the belly jets or landing gear, of course, because they were removed during
the refit to this configuration, but you can see the propellant scoops,
in their retracted position, at the extreme forward end of the engine nacelles.
Radially mounted on the aft ends of the
nacelles are the radiators for the electric power reactor; these dump waste
heat overboard by radiating it away to space or conducting it to the atmosphere.
As you can see, they're mounted in such a fashion as to minimize the amount
of exposure to each other, in order to get maximum efficiency with minimum
mass; they're modeled in 1/16" balsa.
issue of paint schemes is a sensitive one – the Luna
wore the Pan Am Cunard colors, but that paint scheme wasn't preserved when
she was taken out of service, and the extensive refit prior to the Jupiter
mission didn't originally include a repaint – paint, again, was excess
mass. The actual color scheme, then, is a combination of whatever was left
of the original PAC and whatever natural finish was present on new parts
(such as the propellant tank extension and ram scoops) – the latter frequently
bare metal, since the ship wasn't expected to need protection from oxidation.
Just before launch, however, someone pointed
out that publicity images would show the ship in a bad light, looking like
something jury rigged from a wreck instead of a pioneer, so the expedition
management had a fast coat of whatever they could find applied to cover
the refits, with no expectation of good adhesion due to complete lack of
surface preparation. Unfortunately, the only images of the ship in this
configuration are those publicity shots – so that's the color scheme I've
modeled, a plain white hull and wings, flat black radiator surfaces, and
whatever windows were left after the refit. The dark band just aft
of the nose, unfortunately, isn't part of the modeled color scheme -- it's
the minor damage from a core sample on the first flight of the model.
Please see my flight report for the first
three launches on my regular launch log pages: Here
Recommended motors: 2x B4-4, 2x B6-4, 2x C6-5
Building -- Parts and Plans
To built the Luna 2-18 Express
you'll need the following parts:
27" of BT-56
1 or 2x BT-56 couplers (mine came with an 18" section in the bag, so
I only needed one coupler, but the standard length seems to be 9" and will
2x PNC-56, same as those in the Maniac and Long Shot kits
18" of BT-50 (for passenger cabin)
1 BNC-50, balsa version of an Alpha nose cone (plastic will work also,
but it's a bit harder to split and sand to contour)
2x 6" sections of BT-20
4x 520 centering rings -- two for the baffle, two as thrust rings
4x 4" (or longer, as desired) sections of BT-5 -- or only two if you
don't build the baffle
Card stock for cutting the gas seals on the baffle
1 sheet 1/8" balsa, 3" wide, medium-hard
suitable scrap or sheet of 1/16" balsa at least 1.6" wide, medium
3/16" launch lug
Recovery system of your choice (I used a 14" Quest parachute left over
from my Big Betty's conversion to an egg lofter)
Optionally, some sort of motor retention system
Please see the following fin alignment diagram, including baffle layout,
and dimensioned fin pattern:
To construct the Luna 2-18 Express model, first join the sections
of BT-56 airframe. Ensure that the sections are straight by rolling
the tube on a flat surface such as a table top before the glue sets.
Set aside one nose cone, it will be the actual nose cone. Split the
other nose cone along the molding seam, and remove the base of the "heel"
section of the cone. Cut the heel section to about 1/4" length --
this is not a critical dimension, this cut just simplifies fitting the
aeroshell later one.
Sand the glassine off all the tubes to facilitate glue adhesion -- doing
this now will avoid frustration later.
Mark the motor mount tubes for the locations of the radiator fins and
for their glue join line. Install the 520 centering rings, two as
thrust bearings about 2 1/2 inches from the rear of the tubes, the others
exact at the forward ends of the tubes. Glue one BT-5 section into
each forward centering rings, taking care to align the BT-5 parallel to
the BT-20. Join the two BT-20 tubes side by side, with both BT-5
at the same end. Placing the tubes on a flat surface like a table
top while gluing will help ensure the motor mounts are parallel.
If you're making the baffle, cut the centering rings to fit the BT-56
and the BT-5 baffle tubes, and assemble the baffle as shown in the diagram
-- the BT-5 for the forward half should overlap the BT-5 from the motor
mounts by about one inch, to allow for good gluing. Start by sliding
one centering ring onto the BT-5 in the motor mount, leaving about 1/2"
clearance from the motor mount to allow for attaching the aeroshell later;
glue it in place. Slide the other centering ring over the other pair
of BT-5, apply glue, and then glue those tubes to the tubes from the motor
mount to for a cluster of tubes with about an inch of overlap and a baffle
chamber with adequate space to avoid restriction of the ejection gas.
Be very careful of alignment; roll the assembly on a table with the motor
mounts hanging off the side to ensure it's all straight. Once the
glue is dry, mount the assembly into the BT-56 airframe, butting the forward
end of the BT-20 against the aft end of the BT-56. If you've built
everything straight up to now, the motor mounts will align parallel to
the airframe; if not, you can flex the mount a bit at this stage, and when
the glue sets up where the BT-5 contacts the inside of the BT-56, it will
lock your adjustments in place.
Once the glue is dry, carefully carve each half of the split nose cone
to fit the contour of the motor mount tubes. The aeroshell halves
must lie along the motor mount tubes and the shoulder end must fit into
the airframe. Once you have a good fit, glue the aeroshell in place.
If you'll be using a motor retention device or system, be sure to install
it before permanently mounting the aeroshell, as you won't be able to afterward.
Cut all the fins. Plane and/or sand the main wings to a symmetrical
airfoil, then do the same for the rudder fins and radiators. Use
your choice of methods to fill the grain and tube spirals; I used Finishing
Wood Filler before gluing the parts together, but everyone has their own
preference. Attach the wings first, then put the rudders on the wings,
and finally attach the radiators to the motor mount tubes. If you
use double glue technique, white glue is more than adequate, though I recommend
you use a sling type shock cord attachment to lower the rocket horizontally
or slightly nose down. Don't forget to fillet -- those are very,
very large fins with a surface mount.
Once the fins are in place, mark the BT-50 full length on both sides,
and carefully split it along the mark. If you use a metal angle or
channel as a guide, beware of the tube closing up after the first side
is split, and leaving you with a helical second cut -- this is bad.
Split the BNC-50 and glue the halves into the ends of the split tube.
Use the airframe to mark the arc on the butt end of the nose cone halves,
and then use a gouge or power sander to rough cut before sanding to final
fit. Glue the passenger cabin to the airframe, ensuring the BT-50
is adhered all along both edges. Fillets here are cosmetic, but they
have a big effect on the appearance of the model.
Hang in there, you're almost done! All that's left is filling
and sanding, if you haven't, attaching the launch lugs, installing the
recovery system, then finishing by your choice of methods. Now go