Student Asking: Pros and Cons of. HPDE/N2O Hybrids?

[Elj4y]

Hi folks, journalism student here.
Looking for some community perspectives on the hybrid motors, specifically the HDPE/N2O type, as my assessment is a short piece about whether they're the future of spaceflight or just another propulsion method.
Main things I'm interested in hearing about:
- Environmental impacts
- Mechanical lements (do they require more or less work to be fitted, do they wear out faster, etc)
- Results (do they achieve better results in any specific area? Do they fall short on other areas?)
- Are hybrids better for use at large or small scales and are the challenges faced at these scales different?
- Overall viability at all scales (small, mid, large)

Assessment also requires us to interview at least one person for the piece, if anybody here has experience in the commercial side and knows a fair bit about these, let me know if you'd be interested in further contact.

Also feel free to link to research and study papers on the matter (preferably with summaries, I've got three days to get this done)

Thanks for any advice or help you can offer

 

[dhbarr]

Nitrous is not nearly as tame as it seems.

Delivered real world specific impulse is fairly low.

They don't scale as small as solids.

I'm just an internet armchair rocketeer, no professional experience of any kind.

 

[rocket_troy]

I've made and flown about 50-100 of them, but can't really assist much with these questions. The combination is reasonably effective if performance (particularly density performance) isn't critical important. I mostly use HDPE for an N2O fuel for its acoustic note if pushed well beyond N2O hybrid design limitations recommendations ie. for crowd-pleasing effects

TP

 

[Elj4y]

Nitrous is not nearly as tame as it seems.

Delivered real world specific impulse is fairly low.

They don't scale as small as solids.

I'm just an internet armchair rocketeer, no professional experience of any kind.

Any knowledge is still valuable, especially since this isn't really my field of expertise - I'm just writing an article about it

 

[Elj4y]

I've made and flown about 50-100 of them, but can't really assist much with these questions. The combination is reasonably effective if performance (particularly density performance) isn't critical important. I mostly use HDPE for an N2O fuel for its acoustic note if pushed well beyond N2O hybrid design limitations recommendations ie. for crowd-pleasing effects

TP

What drew you to them initially?

 

[Antares JS]

Hybrids sacrifice the simplicity of a solid rocket motor for a small theoretical increase in specific impulse that doesn't usually pan out in the real world. A hybrid also has the same design problems as a solid motor with determining what grain geometry to use, with the added complexity of having to plumb your liquid oxidizer towards whatever surfaces you want to burn. The cost/benefit of hybrids really doesn't pan out in the real world.

I think it's pretty telling that the only major commerical user of a hybrid motor I can think of is Virgin Galactic with their long, LONG-delayed SpaceShipTwo.

Hybrids saw significant usage with hobbyists in the 2000's when the ATF was enforcing a requirement to have a low explosive user permit to acquire solid composite propellant. Hybrids, since they are totally inert until they are loaded with oxidizer, were a way to get around these restrictions, but after Tripoli Rocketry Association and the National Association of Rocketry won their court case against the ATF and APCP solid propellant was taken off of the ATF explosives list*, use of hybrids in hobby rockets dropped off dramatically.

I have not worked with hybrids since my college days, but I do work in the space launch industry.

*TRA and NAR were able to demonstrate that APCP did not meet the ATF's own definition of an explosive due to its relatively slow burn rate.

 

[rocket_troy]

What drew you to them initially?

N2O Hybrids or N2O-HDPE Hybrids?

Hybrids in general: I used to make my own solids (APCP, Candy, Chlorate-Epoxy, KPCP, PSANCP and others - literally over a thousand of them. When you're young and passionate and full of beans, casting composite propellant is enjoyable, but after a while you do get sick of the prep and clean up over and over.
So, when I was introduced to N2O hybrids, I was rather impressed at the simplicity of making them for me. That's coming from the solids background. Particularly, avoiding casting really appealed to me and when I do cast, it's paraffin wax and hot melt glue binder, so there's no clean up and anything leftover can be remelted for the next casting.
What also appealed to me about N2O hybrids was the philosophy I went in with. I was always chasing bleeding edge performance with the solids whereas the hybrids... well... it's a bit pointless doing them for raw performance, so I did them for HPR wow factor, which also made the development process less onerous and more enjoyable.
If you were dead-set on a hybrid config for a high performance application, like an orbital stage, then you wouldn't be using N2O. N2O has its virtues in the hobby domain, but isn't as practical for larger rockets/stages. HDPE is also not a fantastic fuel in terms of performance ie. the specific impulse possible mixed with N2O is comparable to many other engineering and printable plastics, but its specific gravity is less than 1 so using it is like wasting volume and (in turn) dry mass to house it.
Virtues of N2O-HDPE:
Solid performance over a wide range of oxidizer-fuel ratios ie. 5:1 to 10:1 allowing less complicated injector design and core geometries.
Burns clean over a wide range of O:F ratios and minimal environmental impact
Easy to fabricate and manufacture both the motor and the fuel grain
Oxidizer is self pressurizing allowing simple blowdown configurations
Oxidizer can be multi phase across injector allowing for easier ignition and flame holding
Oxidizer is a monopropellant which is virtuous for the above but also a potential hazard
Oxidizer is easily stored and transported - no dewar or cryogenics or acids or acutely toxic fumes or hazmat kit
+ the virtues hybrids have in general eg. easily to terminate, possible to throttle, inherent separation of oxidizer and fuel, can utilise the fuel grain as an insulator, can easily incorporate reactive metals into the fuel and others.

Cons:
Raw Performance
N2O requires respect (like all rocket grade oxidizers). It is a monopropellant that is sensitised with the combination of organic contamination and pressure.
Not really suited for chilled liquid injection, so achieving high densities (whilst possible) will generally require much more development to nail down.
Hybrids in general have a tendency to underachieve to their theoretical c*
Don't pack as well as solids (volumetric loadings will nearly always be lower)
More complex than solids and more development needed to make them
More ground support equipment needed than solids
Not as reliable as solids due to the extra complexity
More (ambient) temperature affected than solids
+ others

TP

 

[Rob Campbell]

Hybrids are also loud. They sound like a wet fart blasted through Led Zeppelin's amplifiers.

 

[jderimig]

Hybrids are also loud. They sound like a wet fart blasted through Led Zeppelin's amplifiers.

Some are, others roar.

 

[waltr]

Link to Heros Sounding rocket project
https://hyend.de/index.php/previous-projects/heros/
This one seems very impressive.

 

[speed]

Hybrids are also loud. They sound like a wet fart blasted through Led Zeppelin's amplifiers.

Yes, they do!

 

[Johnly]

Nitrous Oxide is safe until it isn't. It has a high activation energy threshold, but when that is exceed in the bulk storage tank it transitions violently releasing its energy, as it is a mono-propellant.

As far as large N2O hybrid safety, the Scaled Composites accident:

https://www.americaspace.com/2013/0...t-that-killed-three-during-spaceshiptwo-test/https://www.space.com/4123-explosion-kills-mojave-air-space-port.html
John

 

[G_T]

N2O allows pretty simple hybrids and comparatively simple GSE to make fairly reliable little rocket motors.

But once your plumbing approaches 1" diameter, all bets are off. At that size, a nitrous blast wave will potentially propagate. The little plumbing of the hobby sized motors won't do that.

The density of N2O isn't great, and is greatly dependent on temperature. Solvable by chilling. The colder, the denser - and the less likely to detonate. Two wins...

But it becomes very much harder to light. And the pressure drops a lot. Two losses...

Also solvable, but at the cost of more mass (losing the density improvement) and more complexity (making it expensive).

And compared to other reasonable liquid oxidizers, the ISP is never going to be all that great anyway.

N2O also burns hot. Something like H2O2 (HTP) has better density, better ISP, and burns a lot cooler. Of course it has its own issues. Any usable oxidizer for a rocket motor has issues.

If you want fairly safe in lots of ways, simple and cheap, but low performance, there are sugar motors.

If you want very high performance, go liquids with lots of required support equipment and a whole pile of issue$.

Hybrids are halfway to a liquid motor. You get some of the issues that a liquid motor would have, and some parts of the engineering start to look that way. They are an interesting alternative to solids.

I think that last sentence sums it up.

Gerald

PS - Pics from my EX hybrid, THRP-1, at Potter this past year.

 

[Elj4y]

Virtues of N2O-HDPE:
Solid performance over a wide range of oxidizer-fuel ratios ie. 5:1 to 10:1 allowing less complicated injector design and core geometries.
Burns clean over a wide range of O:F ratios and minimal environmental impact
Easy to fabricate and manufacture both the motor and the fuel grain
Oxidizer is self pressurizing allowing simple blowdown configurations
Oxidizer can be multi phase across injector allowing for easier ignition and flame holding
Oxidizer is a monopropellant which is virtuous for the above but also a potential hazard
Oxidizer is easily stored and transported - no dewar or cryogenics or acids or acutely toxic fumes or hazmat kit
+ the virtues hybrids have in general eg. easily to terminate, possible to throttle, inherent separation of oxidizer and fuel, can utilise the fuel grain as an insulator, can easily incorporate reactive metals into the fuel and others.

Cons:
Raw Performance
N2O requires respect (like all rocket grade oxidizers). It is a monopropellant that is sensitised with the combination of organic contamination and pressure.
Not really suited for chilled liquid injection, so achieving high densities (whilst possible) will generally require much more development to nail down.
Hybrids in general have a tendency to underachieve to their theoretical c*
Don't pack as well as solids (volumetric loadings will nearly always be lower)
More complex than solids and more development needed to make them
More ground support equipment needed than solids
Not as reliable as solids due to the extra complexity
More (ambient) temperature affected than solids
+ others

TP

Thank you! This has definitely given me a lot to work with.
Would you be okay with me contacting you privately for an interview? Assessment requires us to interview at least one source and you clearly have a lot of field experience with these.
Interview can be email or over Zoom, whatever works best for you. I'd also like to ask about your involvement with the Australian rocketry community if that's okay.

 

[Elj4y]

Hybrids sacrifice the simplicity of a solid rocket motor for a small theoretical increase in specific impulse that doesn't usually pan out in the real world. A hybrid also has the same design problems as a solid motor with determining what grain geometry to use, with the added complexity of having to plumb your liquid oxidizer towards whatever surfaces you want to burn. The cost/benefit of hybrids really doesn't pan out in the real world.

I think it's pretty telling that the only major commerical user of a hybrid motor I can think of is Virgin Galactic with their long, LONG-delayed SpaceShipTwo.

I didn't realise there were any commercial projects using hybrids, but with all the info I've learned here, I'm starting to see why VG's project is getting delayed so much.

Would you be okay if I quoted you directly for the article?

 

[rocket_troy]

I didn't realise there were any commercial projects using hybrids, but with all the info I've learned here, I'm starting to see why VG's project is getting delayed so much.

Would you be okay if I quoted you directly for the article?

You're in Brisbane Australia and you don't know of Gilmour

TP

 

[Elj4y]

You're in Brisbane Australia and you don't know of Gilmour

TP

I'm not much of a rocketeer, really only learned about these motors through my housemate and thought it was an interesting topic to cover for article writing assessment

 

[rocket_troy]

I'm not much of a rocketeer, really only learned about these motors through my housemate and thought it was an interesting topic to cover for article writing assessment

Fair'nuff. I just imagined they'd be saturating everything with pixel or video feed with their latest successes claims throughout Qld.

TP

 

[Elj4y]

Fair'nuff. I just imagined they'd be saturating everything with pixel or video feed with their latest successes claims throughout Qld.

TP

I looked them up and the latest news release was November, by the looks. It's an interesting lead to follow up on when their launch date draws closer though, so thanks for the heads up!

Are you okay if I quote some of the info you've stated in this thread for my assessment?

 

[rocket_troy]

I looked them up and the latest news release was November, by the looks. It's an interesting lead to follow up on when their launch date draws closer though, so thanks for the heads up!

Are you okay if I quote some of the info you've stated in this thread for my assessment?

No probs. I also think Gerald had some good points worth quoting FWIW.

Good luck with it!

Troy

oh... ps: one more piece of advice: if there ever was an Olympic event for audaciously hubristic unachievable claims, the NewSpace industry would need to corner the tip truck market for the medal haul - don't quote me on that though

 

[rocket_troy]

Thank you! This has definitely given me a lot to work with.
Would you be okay with me contacting you privately for an interview? Assessment requires us to interview at least one source and you clearly have a lot of field experience with these.
Interview can be email or over Zoom, whatever works best for you. I'd also like to ask about your involvement with the Australian rocketry community if that's okay.

No probs. Email or zoom is fine with me.

Cheers,

TP