To fly much farther away
In order to move ahead in space, reaction of mass ejection,
or jet power, is utilized. To fly even further away, a stronger jet is needed.
Jet strength is a product of ejection mass (i.e., propellant amount) and ejection
speed. As space is a vacuum and there is nothing to be consumed, a space vehicle
has to carry all its necessary propellant from the ground. Space vehicle load,
however, is limited. If you want to load more propellant, you have to unload something
else from vehicle.
To that end, we have tried to produce a faster jet instead of increasing propellant
amount. Ejection velocity of the conventional hydrazine thruster is 3km per second.
The ion engine has a capability to produce jet of 30km per second, 10 times faster
than that of hydrazine thruster. Faster ejection velocity is called “high
specific impulse.” By installing a high specific impulse engine on a lighter
space vehicle, it becomes possible to fly much farther away.
Figure 2 shows fuel (propellant) weight occupation ratio and orbital maneuver
capability of the satellites and rockets launched in the past by ISAS. Since all
the space systems before HAYABUSA used hydrazine thrusters, orbital maneuver capability
is around 1km/s whereas the propellant takes up to 50% of the vehicle’s total
weight. Meanwhile, in the ion-engine equipped HAYABUSA, propellant accounts for
only 13% while it can realize 4km/s in orbital maneuver capability. This capability
of HAYABUSA exceeds that of a rocket’s single stage. Unlike past satellites
that fly in space with inertia after release from their launchers, HAYABUSA is
a “space ship” that can change its trajectory by itself to fly to its
destination. Its main thrust, or driving force, is the microwave-discharge-type
ion engine, _-series. We can call the engine, quoting the words of Dr. Von Braun,
“a foothold in deep space.”
The ion engine is the first to ionize the propellant xenon by electrical discharge
and to accelerate the ions by electrodes biased at 1.5kV. The ions are then mixed
with separately produced electrons and are ejected as high-speed plasma beams.
The plasma-state xenon glows purple. This “microwave-discharge-type ion engine”
electric propulsion system developed by ISAS entirely eliminates the need for
limited-durability discharge electrodes.
Now, let’s talk about origin of the name “_10,” which implies both
micro (_) wave driving and the highest specific impulse motor Mu-Rocket. The number
10 indicates its effective diameter.
Opening the way to new, never-before-seen worlds
By making the onboard-propulsion system higher-specific impulse instead of using
large scale launchers, it becomes possible to escape from corridor unlimitedly
extended around the earth with the medium scale rocket M-V and to achieve deep
space cruising across interplanetary space. The ion engine lures us to even more
distant “epic space voyages” and will open the way to new worlds that
have never been seen before.
I briefly studied the Age of Geographical Discovery (15th to 16th centuries) on
earth. In the search for spices, Spain and Portugal looked for new sea routes
as an alternative to the land routes. Their voyages were supported by the nautical
technology. Previously, the oar-powered galley was used. Since it needed many
slaves to provide power, long cruises without frequent port stops were impossible.
In addition, the ship’s draft was very low to keep the oars at water level
and therefore performance against waves was very poor. It took the wind-powered
galleons to make ocean crossing a reality. Like the famous pirate ships, it was
shaped with a gradually elevated stern. The ship had square sails for tailwind
and triangular sails for moving to windward, with three or four sails hoisted
on each mast. Sand and/or stones were loaded into the ship’s hull to balance
with high center of gravity due to the tall masts.
In addition to improved shipbuilding technology, sailing techniques also advanced.
The compass indicating north was introduced, and the astrolabe measured latitude
based on the angle of elevation of Polaris. The ship’s speed was calculated
by throwing a log, a rope with a wooden weight on its end, into sea and measuring
the length as it played out. Charts were also used. With these new tools and techniques,
explorers could set out across the broad ocean and leave the old coastal navigation
Voyages in those days were still far from safe however. The threat of shipwreck
was constant, and diseases like scurvy killed great numbers of the crew. Vasco
da Gama and his crew of 170 sailed off to find a sea route to India in 1498, but
it is said that only 44 returned alive. Magellan and a crew of 250 succeeded in
circumnavigating the globe in 1519, but only 18 returned alive. Despite such dangers,
Columbus proposed his plan to sail to the New World to Portugal, England and France,
and finally persuaded the Queen of Spain and accomplished it in 1492.
| Fig.2 Propellant weight occupation ratio and orbital
maneuver capability of deep space explorers operated by ISAS and M-V rocket