# The hybrid rocket

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#### bearnard66

##### Active Member
Talking about modern space technology, I am really interested for a few days about hybrid rocket. I only found the information how it works, I mean why the term hybrid was given to it because it combines solid Hydroxyl Terminated Polybutadiene (HTPB) fuel and 90% Hydrogen Peroxide (H202) liquid oxidiser. But now I am looking for the answer what this kind of rockets is used for?

#### Antares JS

##### Well-Known Member
SpaceShipOne used a hybrid, SpaceShipTwo uses one, and hobbyists use them on occasion, but their popularity among hobbyists took a nosedive after the court case over APCP was resolved in Tripoli/NAR's favor. I don't know of anyone else that uses them.

Hybrid motors don't scale well, as Virgin Galactic found out when they tried to upscale SpaceShipOne's motor into SpaceShipTwo. Manufacturing the fuel grain becomes difficult when you go big because it becomes more and more difficult to machine the proper geometry of channels out of it to get the performance you want, to say nothing of having to design the fuel grain in the first place, plus design and build your oxidizer injector to apply oxidizer to those channels just right. The complexity is such that you might as well just built a liquid rocket engine and get better performance.

The only real advantage of hybrids is that because your fuel grain is a piece of plastic, they are totally inert until oxidizer is loaded. This is why hobbyists used them more when APCP was being restricted by the ATF. However, they came with the large up-front cost of having to have an expensive GSE setup to load the oxidizer. This is probably why hobbyists mostly stopped bothering with them after restrictions were lifted from APCP.

Hybrids can be interesting to hobbyists, but ultimately are not very practical for professional use.

#### G_T

##### Well-Known Member
The hybrid fuel grain need not be plastic, and can be cast in complex geometries or even 3D printed in such. There are no real issues scaling the grain up to whatever size you want.

There are issues scaling the grain if the material is such that it does not scale well, such as a wax. At about 6", grains composed primarily of wax tend to suffer slump failure during burns.

Practical is sort of a cost/benefit equation. Since AP has way more than doubled in price these last few years the financial appeal of hybrids increases. Hybrids are MUCH cheaper to launch per newton second.

The real problem is most clubs got rid of, or mothballed, their GSE. GSE for hybrids is additional hardware that is not needed for solids. This can be a fill station of sorts - for pre-fillable hybrids, or solinoids and control box etc for those that fill at the pad. In all cases a nitrous source tank is required. For hobby motors, the oxidizer is required to be N2O.

At the moment, I think hybrid GSE doesn't cost more than one decent sized solid propellant reload, say, for a L3 cert flight. Prices have changed.

It's a chicken and egg thing. Clubs don't set up GSE at the fields. Manufacturers aren't making much in the way of kits for hybrid motors since they aren't selling many. Not many would make a hybrid rocket when their club doesn't provide any support. Let's face it, HTPB solids are easier to deal with at the field. But they definitely cost a whole lot more in the long run. That cost is going to push people out of this hobby and provide more of a barrier of entry for others.

If you don't know, CG shift during the burn is a bit different for hybrids than for solids. Hybrids also tend to have a greater L/D (length to diameter ratio). These things are the differences that need to be taken into account when making a rocket for a hybrid motor.

Sugar motors, and N2O hybrid motors, are pretty cheap to fly on a per-flight basis. HTPB, not so much so.

Picture is of an EX hybrid motor I made, mock assembled, that I hope to fly some day. I have my GSE, just no nitrous supply tank and nitrous. And COVID has put a crimp on many things.

Gerald

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#### G_T

##### Well-Known Member

N2O itself definitely has scaling issues. When plumbing lines get larger in diameter, water hammer is sufficient to detonate it. That is, the channel diameter is sufficient to propagate a blast wave. N2O is a monopropellant by itself. That adds additional safety concerns. Hobby size plumbing doesn't really have that issue.

Other good oxidizers such as H2O2, LOX, etc, have their own handling concerns - cryogenic, corrosive, hypergolic with lots of organics, etc.

Solids are just simpler to handle once they are cured, and are a very mature technology. In the large scale end of things, as soon as one goes to a liquid oxidizer for ISP improvement with all the associated handling and logistics, one might as well go fully liquid. The fuel is usually easier to deal with than the oxidizer.

I's in the small to intermediate size range where hybrids may make the most sense commercially (say roughly, O through S range, and of course down to a fair bit smaller for hobbyists). But there isn't much of a market for intermediate size rockets at the moment, non-military, though I could be mistaken. The cost to hitch a payload ride on a larger rocket has decreased a lot in the last few years.

Gerald

#### bearnard66

##### Active Member

N2O itself definitely has scaling issues. When plumbing lines get larger in diameter, water hammer is sufficient to detonate it. That is, the channel diameter is sufficient to propagate a blast wave. N2O is a monopropellant by itself. That adds additional safety concerns. Hobby size plumbing doesn't really have that issue.

Other good oxidizers such as H2O2, LOX, etc, have their own handling concerns - cryogenic, corrosive, hypergolic with lots of organics, etc.

Solids are just simpler to handle once they are cured, and are a very mature technology. In the large scale end of things, as soon as one goes to a liquid oxidizer for ISP improvement with all the associated handling and logistics, one might as well go fully liquid. The fuel is usually easier to deal with than the oxidizer.

I's in the small to intermediate size range where hybrids may make the most sense commercially (say roughly, O through S range, and of course down to a fair bit smaller for hobbyists). But there isn't much of a market for intermediate size rockets at the moment, non-military, though I could be mistaken. The cost to hitch a payload ride on a larger rocket has decreased a lot in the last few years.

Gerald
I found one of these hybrid rockets on SkyHy website It`s a two-stage unguided launch vehicle mostly used for microgravity missions and experiments. As I have already mentioned it combines solid Hydroxyl Terminated Polybutadiene (HTPB) fuel and 90% Hydrogen Peroxide (H202) liquid oxidiser. Each stage uses a helium pressure feed system and both vehicle stages are stabilised by fins