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Hybrid
FAQ
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Q:
What is a hybrid rocket motor?
A:
A hybrid rocket motor is a
motor that has its oxidizer and fuel stored in two different states.
All sport rocketry hybrids use the common liquid oxidizer nitrous
oxide (N20) and some form of solid fuel, either a plastic
or a paper. The N20 is stored in one chamber as a liquid,
and the fuel in a different chamber as a solid, until the time of
combustion when the two are mixed creating thrust.
Q:
Why are hybrid motors always so long?
A:
In
a solid motor, the oxidizer is bound to the fuel in such a
way that ensures a consistent oxidizer to fuel ratio, while in a
hybrid motor, the oxidizer is being fed from the top, down along
the axis of the fuel grain. As a result, the fuel grains have
to be long to ensure that the N20 will oxidize the fuel surface
and maintain the proper oxidizer to fuel ratio. The
popular hobby propellant oxidizer ammonium Perchlorate is made up
of 55% oxygen, while N20 is just over 30%. Therefore, to get
the same amount of oxidizer into a hybrid motor, we must add length
there too.
Q:
Why Nitrous Oxide?
A:
Nitrous oxide is a good choice for a rocket motor oxidizer, as it
is cheap, safe, and commonly available. At just over 30% oxygen,
there are better liquid oxidizers to be sure, but the serious upside
to nitrous oxide is that it remains a liquid and still extremely
pressurized at temperatures between 50 and 80 degrees. This
pressurization allows N20 to be fed into a combustion chamber of
a hybrid rocket motor at great pressures, without the use of pumps
or other pressurants. This is something that other, better
performing liquid oxidizers just can't do.
Q:
What happens when the nitrous temperature is too warm?
A:
As temperatures increase, N20 will lose density and trade
its liquid for gas. There is always some gas in the nitrous supply
tank, and that is a good thing. This gaseous vapor is what is used
to push the liquid into the rocket motor during the fill phase,
and what the rocket motor uses to force the liquid out of the oxidizer
tank into the combustion chamber during the burn. While the
vaporous nitrous is good and performs a wonderful task, too much
of it will ruin a good thing. It is important to keep the
N20 mostly a liquid, as liquid is what the motor runs on.
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If we supply our
motor with mostly vapor due to high supply bottle temperatures, at best
we are cheating ourselves out of the thrust that we have paid for.
And in some cases, very bad things can happen. You should never fill
or fire a hybrid if your supply tank pressure is over 900psi.
Q:
What happens when the nitrous temperature is too cold?
A:
In reverse of the problems caused by supercritical nitrous oxide
temps, if the temperature of your supply nitrous it too cold, you'll have
plenty of liquid, but you won't have enough vapor pressure to forcefully
push the liquid into your combustion chamber. The result of this
would be a very underpowered flight, due to an under pressurized combustion
chamber. You should not fire a hybrid motor if your supply tank
pressure is below 500psi.
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Q:
If I cool/warm my motor hardware before I fill it, will it be ok
even when the supply bottle is out of pressure range?
A:
No. This won't at all work.
Q:
I launch rockets in the extreme temperatures. Can I still
fly hybrids?
A:
Sure you can. In higher
summer temperatures, cool the supply bottle in a bucket of water
(ice optional), covered with a solar blanket. For cooler midwestern
winter launches, a bottle warmer can be had that will keep your
nitrous at safe, warm temperatures.
Q:
Are hybrid motors better or worse than solid motors?
A:
Hybrid motors are no better
or worse than any other kind of motor; they are just different.
Some people are drawn to hybrids because of their uniqueness and
quirky complexity, while others shy away from them for the same
reasons. Both hybrids and solids have their upsides
and drawbacks, but the potential of the hybrid rocket motor
has not yet been fully explored. It is interesting to note
that nitrous oxide based hybrids can produce ISP's greater than
APCP motors of the same impulse.
Q:
Hybrid rocket motors are generally rated with lower average impulses
than solid motors. Why is this?
A:
Part 1: Well, there are a couple of things that make
this a reality. In solid motors, when designers want to increase
the average impulse, they can add oxidizer, modify grain geometry
and modify the nozzle throat diameter to influence higher combustion
rates. To add oxidizer in hybrids, the injector can be opened to
a larger diameter thereby causing the N20 to exit the tank at a
higher rate allowing a more rapid oxidization of the fuel source,
resulting in higher average thrust levels. In theory this
sounds great, but in reality there are problems. When dumping a
large amount of oxidizer through a single injector positioned over
the center of a cylindrical grain, a sizeable percentage of the
oxidizer will get spit thru the nozzle without ever oxidizing anything.
The fuel can be placed into the flow of the oxidizer, but that involves
expensive fuel grain manufacturing techniques that, to this point,
have not been explored by manufacturers in the sport rocketry hybrid
market. It is also worth noting that because the nominal pressure
of the nitrous in a typical hybrid motor is 650psi, the combustion
chamber can't be run at any higher than 600psi. In fact, most
hobby hybrids run very much lower than this. Having explained
that, let me tell you why I believe Hybrid motors get a bad rap
when it comes to 'average' impulse below in part 2..
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A: Part
2: Hybrid rocket motors have two distinct 'phases' of operation.
There is the liquid phase, when the motor uses the N20 in liquid form
to create thrust, andthere is the gas or 'blow down' phase that
occurs after all of the liquid is expelled from the tank. Most of
the total thrust from the motor is produced during the liquid phase.
The blow down produces very little thrust, but does produce enough to
show up in the thrust trace, thereby elongating the burn time. In
many cases, as much as half of the total burn time of a hybrid motor is
the result of the blow down phase. Since we derive the average impulse
ratings of rocket motors by dividing total thrust by the burn time,
the blow down phase really brings the average impulse of hybrid motors
down sharply.
Q:
I don't have a lot of money to invest on Ground Support Equipment. Should
I even think about messing with hybrids?
A:
Absolutely. Today, many clubs
have hybrid GSE available for use by club members for just the cost
of normal range fees and whatever nitrous oxide your flight consumes.
If you happen to belong to a club that does not yet have hybrid support,
consider putting it together yourself. For less than 200 dollars, you
can create support equipment capable of filling monotube hybrid motors
like RATTworks, Propulsion Polymers, West Coast Hybrids, and Sky Ripper
Systems. Look here for info.
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Q:
What happens during the fill and launch of a U/C valve monotube
hybrid rocket motor?
A:
A solenoid valve is activated, opening the passage way between the
supply bottle and rocket motor tank. The tank begins to fill
with liquid N20 from the bottom up. During the fill, a small
vent hole at the top of the motor allows air to evacuate from the
tank as it is displaced by the more dense nitrous liquid.
This is what causes the 'hissing' sound you hear when hybrid motors
are being filled. When the liquid N20 reaches the top of the
motor, a plume of white fog will be seen exiting the vent
hole. This is the liquid N20 boiling off as it hits atmospheric
pressure, and our signal that the motor is full and the ignition
sequence can be started. During the ignition sequence,
a heat source is introduced into the combustion chamber near the
bottom of the U/C valve. This heat source may be delivered
by a piece of APCP rocket propellant and an igniter, or by a 'pyro
free' method, using gaseous oxygen and an electric spark.
The heat source allows two things to happen. First, it melts away
the plastic fill tube, causing the liquid ntrous oxide to pass into
the combustion chamber. Secondly, it heats the ntrious as
it passes through the flame front, forcing the decomposition of
the liquid N20 into it's raw components. The resulting oxygen
passes over the burning fuel grain, causing it to burn vigorously.
Q:
Sometimes, it seems like it takes longer than it should to fill
a hybrid motor. Is it ok to start the ignition sequence before
seeing the plume of liquid from the vent hole?
A:
No. This is one of my
pet peeves. If you don't see a vent after a very long fill,
something is wrong. Abort the launch, and trouble
shoot the problem.
Q:
Aren't hybrid motors complicated to assemble?
A:
Most hybrids are less complicated to assemble than an average solid
propellant high power reload.
Q:
Can I certify for high power rocketry with a hybrid rocket motor?
A:
Yes. Both TRA and NAR
allow its members to certify all three levels with hybrid rocket
motors.
Q:
I have heard that hybrid motors are cheaper to fly than solids,
is that true?
A:
Yes, and no. There are
some real 'investments' that must be made to buy and fly hybrid
rocket motors. First and foremost is the hardware. For
example, the hardware for a 38mm solid rocket motor in the 'H' range
from a popular rocket motor manufacturer is $88.00, and a
check on the average cost of a 38mm H impulse hybrid motor is $127.00.
More rocket clubs are acquiring the support equipment needed to
accommodate hybrid flyers, but many have not.
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So, one may also have to consider the added cost of hybrid specific ground
support equipment. This can range from a very simple $200 rig, to
a very complex $800 setup depending on the style of hybrid motors you
plan to support. In general, as the total impulse increases,
the cost per flight of hybrids as compared to solids begins to look attractive.
Another thing to consider;
a composite motor has an delay element and an ejection charge for recovery
deployment. With the hybrid, you are going to have to do some kind
of electronic ejection, be it by timer or altimeter. That means
added cost and complexity.
Photos
by Greg Rothman, and Jamie Black.
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