Rocket Science E07 - Hybrid Propulsion

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Hybrid engines use both a solid propellant and a liquid or gaseous propellant.
For instance, a solid fuel is stored in the combustion chamber and a liquid oxidizer is
added from a tank.
Usually it's the fuel that's solid, but it can also be the other way around.
The advantage of having a liquid oxidizer is that it's safer and more efficient than
having a liquid fuel.
A valve is used to control the flow of the oxidizer.
The valve controls how fast the oxidizer is allowed to interact with the fuel, meaning
that it can control the thrust.
Before the liquid propellant enters the chamber, it passes an injector to atomize the liquid,
essentially turning it into tiny pieces.
This is necessary to get a stable and homogeneous burn without the chaos you would expect of
a liquid.
The liquid propellant is pushed into the combustion chamber using an inert gas tank or a turbopump
system, just as is the case with liquid propulsion.
The turbine of such a turbopump system can be powered in a number of ways.
A solid rocket motor can be used for instance.
A bit of an odd application of hybrid engines is the Bloodhound SSC.
This vehicle will attempt to reach 1,000 mph, breaking the current land speed record of
763 mph, set by the same group of people in 1997.
Here we have a tank carrying about 2100 lbs of hydrogen peroxide.
This will act as the oxidizer of the hybrid propulsion system.
Notice how there are these perforated walls inside of the tank.
Some tankers have these too.
They help evenly distribute the force from the sloshing liquid across the wheels.
Here we see the oxidizer pump.
It will be spinning at about 10,000 RPM.
Here we have a better view of the impeller.
It has 3 inducers at the top which split into 6, then 12 and eventually 24 separate blades.
The pump is driven by a 550 break horsepower supercharged Jaguar V8 engine that can somehow
run at 18,000 RPM.
Here we see the rather long hybrid motor.
Long motors have a high surface area grain that doesn't burn for very long.
This results in higher thrust for a shorter period of time.
Less burn time is useful because then the drag of the air and wheels doesn't have
time to slow down the vehicle as much, so the top speed goes up.
The vehicle is also powered by a jet engine which takes it to about 650 mph.
A butterfly valve opens allowing the pump to fill up with hydrogen peroxide from the
tank.
The pump and engine are linked.
The clutch is still disengaged.
A small amount of hydrogen peroxide now passes the injector and the hybrid engine starts
burning.
Now the engine racks up from 3,000 to 18,000 RPM.
The clutch engages and the pump rapidly accelerates.
Now the hybrid motor is at maximum thrust, accelerating the vehicle to 1,000 mph.
Let's go over the advantages and disadvantages of a hybrid engine.
We'll start with the advantages.
The valve is used to control the thrust.
By closing the valve entirely, the engine can also be shut off completely.
Solid rocket propulsion can't do this.
Liquid and hybrid propulsion can.
The manufacturing process of a hybrid propulsion system is safer than that of a solid propulsion
system.
In solid propulsion, the fuel and oxidizer are mixed.
A spark can ignite the propellant.
Here we have a video of the boosters of NASA's Space Launch System being molded.
This is a very dangerous process.
In hybrid propulsion, the fuel and oxidizer are separate.
If the fuel is ignited, it only has the oxygen in the air to burn.
The burning is a lot slower, if there is any burning at all.
This makes the manufacturing process of hybrid propulsion much safer.
Hybrid propulsion is simpler than liquid propulsion, but more complex than solid propulsion.
The cost lies somewhere between those of solid and liquid propulsion.
Now, solid fuels tend to be denser than liquid fuels.
Let's take a look at some examples of fuel densities.
In model rocketry sucrose sugar is often used as a fuel, which has a density of about 1.59
grams per cubic centimeter.
The most popular liquid fuel is RP-1.
It's a sort of refined petroleum very similar to jet fuel.
This one's density is merely 0.81 grams per cubic centimeter.
Other liquid fuels like liquid methane, which is getting some popularity and liquid hydrogen
are much less dense still.
Hybrid propulsion takes up less space than liquid propulsion does due to the higher density
of the fuels.
This is a major advantage.
Hybrid propulsion can be very efficient.
Much more efficient than solid propulsion.
This is because the most efficient oxidizers are the liquid ones.
Primarily liquid oxygen, but also liquid fluorine.
In solid propulsion the oxygen is bonded to other atoms so that it remains a solid, which
results in a loss in efficiency.
For example, in model rocketry potassium nitrate is used as the oxidizer and sucrose sugar
as the fuel.
Three oxygen atoms are bonded to a nitrogen and a potassium atom.
The additional potassium and nitrogen atoms are required to keep the material solid.
In liquid propulsion there's no need for these additional atoms.
Take liquid hydrogen fuel and liquid oxygen oxidizer, for instance.
The oxidizer is just the oxygen in liquid form.
The efficiency, just as the cost, typically lies between solid and liquid propulsion.
As a reminder, by a high efficiency we mean a high velocity of the exhaust gas.
The efficiency does vary a lot depending on the mixture ratio, meaning the fuel to oxidizer
ratio.
When the mixture ratio is far from ideal for the reaction, so when the system is throttled
down significantly using the valve, the efficiency is low.
We can see this on the graph.
Along the horizontal axis we have the mixture ratio.
When it is very low, so on the left side, the efficiency is also low.
Also notice how different fuels have different curves.
The grains used in hybrid propulsion tend to be quite complicated.
The result is that some of the solid propellant is usually left unburnt.
This is unfortunate, because that unburnt propellant would have resulted in significant
acceleration since the mass is low at engine cutoff.
The best hybrid propulsion grains are borderline impossible to mold.
3D-printing does allow to make them, but then size and fuel choice become a problem.
Still, the technology might see large-scale application in the future as 3D-printing advances.
So, here is a picture of a transparent 3D-printed grain.
It uses a sort of double helix design which has a large surface area, so a high thrust,
but still consists of very little empty space, so it has a high density as well.
High thrust and high density make it ideal, but it's clearly not feasible to mold.
The Space Ship One aircraft was the first privately funded program to get a human to
space.
It was able to carry 3 people.
Space Ship One attached to another aircraft called White Knight One to lift off.
At 50,000 feet, the two separated.
Space Ship One would use a hybrid engine to get to space, which is defined as 100 kilometers
altitude.
Both Space Ship One and White Knight One were able to land and be rapidly reused.
About a month ago as of this video, a newer version of this aircraft called VSS Unity
had a successful test flight.
Virgin Galactic, the company now developing it, wants to use it to take customers on trips
to space.
After detaching from VMS Eve, VSS Unity reached a speed of 1,400 mph in about 30 seconds.
The next episode will be about orbits.
Thanks for watching.
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The sources for this video and other links are in the description.
I'll see you next time!

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Rocket Science E07 - Hybrid Propulsion

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