Please Note: This is an old version of the FAQ and will be updated soon.
What is the Spacecub?
The Spacecub is a manned, suborbital rocket meant to be built, operated
and flown by individual hobbyists. It is planned to be fully reusable
and not to drop pieces of itself along the way. In many ways the
Spacecub can be thought of as a fully reusable, manned version of the
Viking rocket of the early '50's since it has the same basic performance
and general capability.
In addition to the hobbyist market, SpaceCub could also be attractive to "adventure
tours" types of businesses (such as the outfit that sells the opportunity to fly Russian
fighters or Interglobal Space Lines
which sells, among other things, zero g aircraft
flights) and those who currently launch sounding rockets.
With the exception of the computers and electronics, most of the
features of the Spacecub are rather old technology. Most of the
technical issues have been dealt with thirty or more years ago.
What are the numbers for the Spacecub?
The Spacecub will have the following dimensions:
How much will the Spacecub cost?
We're aiming at keeping the cost of the Spacecub down to about
$250,000-500,000 in kit form with a fully constructed version at
perhaps twice that. This is comparable to at least one high-end
kit plane, a plane that has sold upwards of 50 kits to date.
(According to figures listed in Jane's All the World's Aircraft.)
How many passengers will it carry?
The current version of the Spacecub is a four seat vehicle, but there are
plans to use the experience gained in the first Spacecub to design and
build new vehicles with greater range, more passengers, and larger
payload capacity.
Why is the Spacecub suborbital? Why not make an orbital vehicle?
It takes a speed of nearly 8000 m/s to make orbit. However, fighting
gravity and shouldering aside the atmosphere uses power that makes the
rocket require a velocity potential (a "delta v") of about 9500 m/s. With
kerosene and oxygen as propellants the rocket would need 20 times as much
propellant as the empty weight of the rocket. The suborbital Spacecub only
makes a speed of 1850 m/s but fighting gravity and the atmosphere requires
a "delta v" of about 4000 m/s. To do this the rocket needs fuel of 4.5
times the rocket's empty mass. This is much, much easier to do than 20
times. However, all is not lost. As more experience is gained with
vehicles like SpaceCub, it will be possible to design larger and faster
versions, leading up to an orbital vehicle. All will be done in easy
stages, making money with each step. This minimizes both market
and technical risks at each stage.
Have any been built or flown yet?
No. It's just finished conceptual development [Note: Since the time this
was written, the concept has been "back burnered." Further
develop should be forthcoming.]. However, a series of scale models
have flown, testing the stability of an early configuration in subsonic flight.
These model flights have provided valuable information toward the eventual
building of a flying models. (Many thanks to Brent for the work.)
The latest configuration, aimed at possibly winning the X-Prize, has not yet
had these model flights, however work toward doing so is underway.
Current plans call for seeking financing in 1 - 2 years, with a prototype
about 2 years after that, and first kit sales 6 months to a year after that
depending on how flight tests go.
What is the X-Prize?
The X-Prize is a prize of the same nature of the old Ortig prize offered
for the first non-stop transatlantic flight (won by Charles Lindberg), and
the Kremer prizes for various man-powered aircraft milestones. It is to
be a prize of $5-$10 million to the first private vehicle (government
programs need not apply) to fly twice within fourteen days (same vehicle,
so it must be reusable) to an altitude of 100 km. The vehicle must be
able to carry three people, which was the reason for the recent change
in SpaceCub design.
What does the Spacecub look like?
The current design has a ) main body seven meters (21') long, an elliptical
in cross section--1.5 meters high and 3 meters across. The nose is conical
and two meters long with a rounded tip and rounded "shoulders" where it
meets the main body. The rocket has swept wings with root chord of 4
meters and a tip chord of 1 meter with the trailing edges swept back to a
point 1.5 meters behind the mainbody. The span is 11.5 meters (33').
There are two vertical fins, one dorsal and one ventral, swept, two meters
in root chord, 1 meter tip and with trailing edges also swept back 1 meter.
Tip to tip span of the vertical fins is 4 meters.
How does the Spacecub take off? Vertically or horizontally like
an airplane?
The Spacecub both takes off and lands vertically. This means that some
fuel must be reserved for landing. This also means that virtually any
flat surface twelve to fifteen meters across can serve as an emergency
landing point.
If It's VTOL, why wings?
Suborbital vehicles reenter on much steeper paths than do orbital vehicles
this means that they dive into deeper atmosphere while retaining more of
their velocity than orbital vehicles. High speed and denser atmosphere add
up to strong G forces. Computer models show that a capsule configuration,
even a lifting capsule like Gemini or Apollo would experience reentry
accelerations of 12-15 G's. This was considered too high for a hobbyist
spacecraft.
The only other option was a lifting reentry to keep the vehicle at higher
altitude, and in thinner atmosphere, until it had lost more of its speed.
Lifting bodies were one possibility but problems with layout and lofting,
particularly when it was considered that SpaceCub was meant to be built
by hobbyists, arose. That left wings as the only other option. Wings, of
course, meant a penalty in such areas as weight and drag during ascent,
but they proved the only viable option for the initial SpaceCub design.
What will the experience of flying the Spacecub be like?
The flight will start, with the Spacecub fueled and checked, with
the pilot laying back in the seat, with the nose of the Spacecub
pointed at the sky. Mist, visible through the Spacecub's canopy, will
drift around the front half of the Spacecub, condensation around the
craft's liquid oxygen tank.
Then the engines will light. The engine's roar will shake the
cockpit as the Spacecub begins to climb slowly into the sky.
Acceleration will be gentle, a slight heaviness at first that will
slowly increase until the pilot feels three times his own weight. In
just over two minutes the Spacecub is over twelve miles up and
breaking through the sound barrier. The engine noise vanishes and
all becomes relatively quiet, with the only noise being that carried
through the body of the SpaceCub itself.
Outside the sky becomes darker, first purple, then black. The stars
become visible against the black.
Then, the engines cut out, less than four minutes after launch.
All feeling of weight vanishes as the Spacecub begins its ballistic
arc. For the next two minutes the Spacecub continues to coast upward
before the ever-present pull of gravity begins to drag it back down.
Then three minutes of falling and the rocket's wings begin to again
feel the bite of the atmosphere. The air around the Spacecub begins
to glow with heat. The skin of the Spacecub begins to warm as
acceleration builds. The airflow over the wings generates lift,
and the Spacecub starts to pull up from its dive. At an altitude of
more than 20 miles the Spacecub comes level, still moving more than 3
times the speed of sound. The Spacecub slows, shedding energy and
begins a gentle descent. At about 12 miles it drops back through
the speed of sound.
The Spacecub glides to the landing target and pulls up in an
increasingly steep climb, trading speed for altitude. When it
comes vertical the engines light and the Spacecub sinks
vertically on its tail.
Why not use a parachute for landing, like the old Space Capsules?
Those old capsules set down at sea for a reason. Landing a large
vehicle by parachute is hard on anyone in it. Such landings can end
in broken bones and internal injuries. The Russian Soyuz capsules use
a parachute landing, but even they use braking rockets to slow them at
the last moment.
Well, why not a horizontal landing, like an airplane?
The Spacecub would have a stall speed of nearly 80 miles an hour,
a power-off sink rate, in stable glide, of over 1000 feet per
minute. That falls somewhere between a very bad glider and a very bad
parachute. The key phrase there is "very bad." That combination would make for a difficult and dangerous
landing. The Spacecub is aimed at hobbyists and pilots with a
moderate amount of experience, not highly trained and
experienced jet pilots.
Other problems arise from the need to flight test SpaceCubs. Each
builder will have to test his own, or have it tested for him. With
a vertical takeoff/horizontal landing there is no provision for
taxi tests, hover tests, short hops, or other steps that can be made
for either VTVL or HTHL modes. This makes the testing process more
dangerous.
Other problems with horizontal landing include a second set of,
heavier, landing gear for horizontal landing, accessory equipment
required at the landing site to erect the rocket for takeoff, very
large peak structural loads on the airframe during landing (that
descent rate is hard on the structure), protecting tires from
reentry heat, and reduced options for landing sites in case a
navigation error or emergency prevents reaching the original target
site.
All that said, however, the vertical landing mode is one of the greater
technical risks for SpaceCub. In particular, difficulties may arise in
converting the engines chosen for SpaceCub for both reliable in-flight
restart and continuous and precise throttling. While there is no doubt
that both of these problems can be solved, there is a question about
whether they can be solved within the projected budget. As a hedge
against that possibility a horizontal landing option using an air-cushion
landing gear to minimize stress to the airframe is under investigation.
Such a system will involve a severe penalty in overall vehicle
performance for the weight of the landing gear plus the weight of a separate
"go around" engine (more important for HL than for VL). However, the
performance should still be sufficient to meet the X-Prize requirements.
What about terrorists? Won't they be able to use SpaceCubs as
weapons?
Short answer, perhaps they could. But in a bit longer answer, they'd
be really stupid to try. The SpaceCub has rather short range as
missiles go. It has extremely limited payload. And its liquid
propellant engines, in particular using liquid oxygen, make launch
operations extremely hard to conceal. Furthermore, it comes down as a
large, essentially empty vehicle. With the large wings and empty tanks
it's speed in the low atmosphere is something on the order of 150 miles
per hour. It's all metal construction, still hot from reentry, make it
a perfect target for both IR and radar guided counterweapons.
There are plenty of better choices if one is seeking a terror weapon, or
a battlefield weapon.
How much does fuel cost for each flight?
A flight of maximum performance requires 1200 gallons of kerosene and
2200 gallons of liquid oxygen. With kerosene at $2.00 per gallon and
the price of liquid oxygen as quoted by a local supplier of $0.35
per gallon a fully fueled flight would require $3200. That's expensive,
but not out of reach of those who can afford to buy a SpaceCub in the
first place.
How many flights will the Spacecub be able to make overall?
With regular inspections of flight critical hardware and occasional
overhauls of the engines, just as on aircraft, there's no particular
reason the Spacecub cannot last for years and hundreds, or thousands
of flights. SpaceCub is designed with 50% margins throughout, which
is a general standard and helps to keep reliability and lifetime high.
What preparations are needed to fly the Spacecub again?
The Spacecub will require refueling, recharging of the batteries,
resupply of the pressurized helium tanks (used to pressurize
propellant tanks and to power the RCM's during ballistic flight),
and a preflight checkout taking no more than an hour or so to be
ready to fly again. This is an off the cuff estimate, though, until we
actually have flight experience with the vehicle.
What kind of kit will the Spacecub be?
As presently conceived the Spacecub will be available in a
"materials and components" kit. Actuators, sensors, engines, and
complex parts such as those containing compound curves or taking
high stresses (nose cone and wing spars as examples) will be
pre-formed. Titanium parts will also be pre-cut to relieve the
builder from working this difficult material.
What kind of materials will the Spacecub be made of?
The Spacecub uses mostly aluminum and titanium in its structure. The
propellant tanks are sheet aluminum. The tank support structure is
aluminum. The skin and skin support structure is titanium. Should
titanium prove to be untenable, it would be possible to use steel
instead, but the result would be either a more fragile vehicle
(thinner skin and framing members) or a heavier vehicle, or both.
Either result would reduce the performance of the spacecraft.
What will be used for reentry heat shielding?
At the speeds of the Spacecub, its size, and its mass, the skin of
of the vehicle itself is adequate to survive reentry. The maximum
temperature of the craft is less than 800 K (980 F).
What kind of redundancy will the Spacecub have? How will you
handle an engine out on landing?
The Spacecub is designed to have three to five (not yet finalized)
engines. The rocket engines we are most interested in are the verniers from
the Russian RD-107 or RD-108 engines. These engines have been around
since the days of Sputnik and have established a simply incredible
reliability. Other engines, mostly of Russian make, are also under
consideration. In all cases, the ability to make a safe landing, at
any point in the flight, should one engine fail, is a major design
criterion. In the landing phase, the rocket has shed enough mass that
any one engine is sufficient for landing.
Control systems and electronics will be fully redundant, with nice,
simple, foolproof mechanical switches for shifting from main to backup
systems. Three computers, two GPS receivers, and two inertial
platforms will give redundancy in navigation. Even primary structural
member have redundancy. The three pressurized propellant tanks carry
fuselage loads. Each one is sized, individually, to carry the entire load so
that loss of pressure in one, or even two, would not result in loss of the
vehicle or its crew.
How automated will the Spacecub be? Will the pilot be able to
take control?
The Spacecub is designed so that the entire flight can be made with the
pilot doing no more than pushing the "go" switch, or, alternately, with the computer
doing nothing but provide data to the pilot who controls the entire
flight manually, or any level of automation in between. This will allow
pilots with a wide variety of different abilities and experiences to
fly the Spacecub. For low experience levels the Spacecub would operate
in a highly automated mode. As the pilot gains experience, the level
of automation could be dropped until the pilot is flying manually. This makes
the SpaceCub its own trainer.
What kind of license will be required to fly the Spacecub?
This is one of a number of yet unresolved legal issues. The Office of
Commercial Space Transportation controls private space flights but
their regulations, as yet [at the time this was written, 1994], contain no provision for manned vehicles.
Launching overseas or in International waters is no help. The relevant
laws (chiefly Public Law 98-575) applies the restrictions to US persons
even when the activities take place outside the US.
Currently, there is work underway to get this situation changed and to get
a licensing procedure based loosely on aviation regulations
enacted which will allow Certificates of Flightworthiness and Spacecraft
Pilot's licenses which will let you file a flight plan and go.
The level I have been recommending for piloting the Spacecub is
250 hours of flight time, instrument and multi-engine ratings, and 10 hours of aerobatic instruction, plus either five hours of dual instruction
"in type" (i.e. a supersonic, VTOL, rocket plane), or twenty hours in
a high-fidelity simulator, or some combination of the two.
Will the FAA allow the Spacecub to be launched?
The Office of Commercial Space Transportation is now an office
within the FAA. It's mission is explicitly to provide for commercial
access to space. While the legal structure does not, at present
allow for manned vehicles such as SpaceCub since no commercial
manned vehicles exist at present, there are indications that they are
amenable to working on developing such a structure if necessary.
SpaceCub could be launched using the same regulations as for
unmanned vehicles, but that would not be terribly practical at
present. In particular, the liability insurance requirements are
prohibitive.
What kind of launch and landing sites will be required?
The Spacecub needs a solid surface to launch from, a supply of liquid
oxygen and kerosene (which can be trucked in), compressed helium,
and a supply of electrical power to charge batteries. No specialized
gantries, launch pads, or other such facilities will be required.
For landing, you need a flat spot to set it down, although it might
be nice to have the facilities to take off again.
What is the liability exposure of the Spacecub?
This is an issue for the lawyers to wrangle over. The actual risk
factor (to anyone but the pilot within the vehicle) is actually quite
low. The total amount of fuel is 1200 gallons. The fuel we are
considering is the low volatility Jet A, or JP-5 (military version of
the same stuff) so even a complete, catastrophic failure of the
fully fueled rocket on takeoff would be less disastrous than a
similar failure in a corporate jet. Further, the terminal velocity of the
falling rocket in the lower atmosphere is about 300 mph, and the
rocket is quite lightweight at that point so a total failure here is limited
in extent. Finally, the redundancy and engine out capability of the
ship makes the occurrence of such a complete and total failure (or
explosion) a very low likelihood event. In practical terms the rocket
is little more hazardous, to bystanders, than aircraft of similar fuel capacity.
Some have expressed concern about the presence of liquid oxygen, and the
dangers it might add. This effect should be minimal. Again, the amounts
involved are small on an industrial scale. Also, the physical separation
of fuel and oxygen tanks makes the chance of sufficient mixing to cause
a catastrophic explosion remote.
The main causes of explosive failures in rockets are combustion
instabilities, destruction by Range Safety Officers (using bombs planted
in the vehicle), and hard impacts (crashes). By using well established
engines, combustion instabilities can be avoided. Airplane, rather
than missile, style operations, will eliminate the need for RSO's and
their bombs (after initial testing). That leaves crashes. Multiple
engine redundancy, computer enhanced, fly-by-wire control, and strict
operational guidelines (to be developed) will minimize that risk.
What environmental effects will a launch cause?
This was one of the reasons for the choice of propellants for the rocket:
kerosene and oxygen. This was to avoid highly corrosive and toxic
propellants such as hydrazine or Red Fuming Nitric Acid. The environmental
effect would be that of burning 1200 gallons of kerosene in a reasonably
efficient system. There appears to be some question, however, over
just how efficient that system is. Rockets generally run fuel rich
since that provides best performance, and so would burn "dirtier"
that kerosene burners on the ground. However, the exhaust is hot enough
that the unburned kerosene products would burn quickly in the air.
The detailed amounts of pollution would have to be investigated. Still,
flight rate is likely to remain low enough for some time that total
environmental effect will be negligible.
What about damage to the launch site from the rocket exhaust?
This is still an unknown. While the rocket exhaust is very hot, the
total mass and exposure time is low. One trick under consideration
for protection of the launch surface is to have the Spacecub carry a water tank that sprays water into the exhaust to carry off the
heat. About 120 gallons of water would be enough to carry off all
the heat generated in the first ten seconds of flight. After that,
the rocket is more than 100 meters up and no longer a danger to the
surface below.
What about noise problems on launch and landing?
This is another unanswered question. The reputation rockets have for
being really loud comes from the big rockets people are most familiar
with. For instance, the Atlas rocket has about 400,000 lbs of thrust on
takeoff. The Spacecub has 20,000 lbs or thrust or so on takeoff. The
bigger the rocket, the noisier. Just how loud the Spacecub will be is
still not known though. From theoretical indications, the SpaceCub should
be 1/20th, or 13 dB less loud. It appear that here SpaceCub resembles
aircraft more than it does spacecraft. Also, the same water spray under
consideration for launch-stand protection would help to deaden this
noise.
What International Treaties Affect the SpaceCub?
This is another sticky issue. The main treaties are the 1967 Outer
Space Treaty and the 1973 Convention of International Liability for
Damage Caused by Space Objects. However, most of the provisions of these
treaties can be neatly avoided by simply restricting flights to over the
US. Flights to higher altitude will also need some check to avoid any
risk of collision, yet such a risk is minute for the altitudes and
flight times of the Spacecub.
Why would one want to use or buy SpaceCub?
If one needs a "practical" reason the Spacecub is the fastest means of
going up to 600 km. However, "practical" reasons aren't really what
the Spacecub is about. It's meant for recreational flying. In the
Spacecub one could see the sun against a black backdrop. One could
experience weightlessness. One could see the Earth from above its
atmosphere.
What about emergency considerations? Can the pilot extract
himself from the rocket in an emergency?
In most possible failure modes, the pilot would be best advised
to stay with the rocket. In particular, the pilot would have to stay
with the rocket at supersonic or extra-atmospheric flight. The multiple
engines provide the ability to the Spacecub to continue flying and either
abort to a controlled landing or complete the flight even should an
engine fail.
The prototype, however, will probably have some sort of escape
system.
Perhaps the most serious potential problem for pilot safety is
loss of cabin pressurization. As a shield against this a look is
being taken at the Space Activity Suit, a skintight elastic garment
that provides pressurization by direct pressure of the fabric. The
suit has been tested to pressures of at least 3.5 psi and appears to be
adequate for emergency use until reentering the lower atmosphere.
Where does it go from here?
At this point, SpaceCub has gone about as far as paper analysis can
carry it. Most of what remains in that area is just fiddling with details.
The next step is to get actual experience with hardware. The following
projects have been suggested to continue:
How can I find out more?
One way would be to stop in at sff.people.dburkhead
and ask.
SpaceCub is always looking for participants, people who have skills and abilities they can bring to the project. What are being sought are not employees, but the initial core of people around which to build the project. You can discuss the project at the private news site sff.people.dburkhead or write to David L. Burkhead to find out more about these opportunities.