Hybrid Rocket Build Thread

[diyaerospace]

Hello everyone,

I am currently in the process of building a HPR hybrid rocket motor and am posting to get opinions, advice, and help.

1. The motor has a theoretical average thrust of 800N

2. Propellant is paraffin wax with carbon black additive.

3. Oxidizer is N2O

I have already casted the fuel grains and epoxied them into the paper liner. For a casing I am using a 56mm fiberglass casing from the RCS store. The nozzle will be machined out of graphite and a nozzle carrier will be an aluminum ring that is bolted to the casing. The forward closure contains the oxidizer injector and spacer. I will post CAD photos later.
I am following Half Cat Rocketry's "Pressure Vessel Design" pdf to calculate the number of bolts needed for the forward and aft closure.

I am using these tutorials to calculate nozzle dimensions, chamber length, etc



Right now I am looking for advice on making a n2o safe valve. My current plan is to use a motorized ball valve (https://www.halfcatrocketry.com/sabv) but I am also looking into pyro valves. Do these solutions seem reasonable?

Although I would like to fly this I am more interested in getting the motor working on a static test stand.

Any replies would be appreciated, I have built an E size hybrid rocket before using 16gram n2o cartridges.

Thanks,
Walter

 

[G_T]

Drilled fiberglass isn't going to be all that strong for holding bolt/pins. All the fibers are violated in the area of the drilling, and quite probabaly you have microfracturing extending into the matrix a little ways. Essentially it will start out somewhat broken.

With composites, generally if you want a hole somewhere, you mold in the hole.

I know that's not what you wanted to hear.

What you'd need to do is make a test piece and find out how much force is needed to cause unacceptible damage to the casing for a hole. Figure the force generated at MEOP, and apply a margin of safety. Depending on design, be conservative on that margin. Rocket motors are not kind to underestimation. Then devide that resulting force by how much a bolt can take. That's your minimum number of bolts.

Then, come up with a layout such that each hole is at least 2x hole diameter from any other hole or edge - for metal cases. For drilled fiberglass, I'd go farther than that.

For a static test motor, at least initially, why use fiberglass?

Or did you really mean a phenolic liner inside a structural tube of some sort?

Also, why epoxy wax to anything? Melt it and cast it where you want it to be.

Good luck!

Gerald

 

[rocket_troy]

Also, why epoxy wax to anything? Melt it and cast it where you want it to be.

Good luck!

Gerald

Yeah, epoxying wax doesn't make a great deal of sense. If you are concerned about wax adhesion, you can (1) add some hot melt glue into the wax (which will also strengthen and toughen the matrix) and/or (2) line the bonding interface with a coating of hot melt glue prior to casting. However, I agree with Gerald that it's generally not necessary - at least on a smaller scale - so long as you're not casting the wax too hot which can lead to excessive shrinkage during cooling.

TP

 

[diyaerospace]

Thanks for the replies,

Drilled fiberglass isn't going to be all that strong for holding bolt/pins. All the fibers are violated in the area of the drilling, and quite probabaly you have microfracturing extending into the matrix a little ways. Essentially it will start out somewhat broken.

The specific casing I bought is this one,

54mm X 24.994

Filament-wound fiberglass casing for 54mm motors. Dimensions: 2.125' O.D. 1.875' I.D. 24.994' long Both ends machined 2.000' I.D. X 0.77' & 1.76' long for bonding nozzle & bulkhead Color: Black Weight: 618 grams

www.rocketmotorparts.com

Here are all the safety factors,
Bolt shear: 4.7
Bolt tear out: 2.792 (Assuming shear strength of 30,000 psi)
Tensile stress: 1.72 (Assuming a yield strength of 7,000 psi)
Bearing stress 2.8875 (Assuming bearing yield strength of 30,000 psi)

Here is some more context,
Do 2.125 inch
Di 1.96(ends of the tube have a shorter wall thickness)
T = 0.0825
D bolt (major) = 0.112
D bolt (minor) = 0.0825
smallest edge distance 0.224 inches (I will make this greater after Gerald's response)
Number of bolts: 16

Does this seem okay for a fiberglass tube? Visually the number of bolts seem pretty large, do you think this could cause the casing to fail along the bolt line?

Also, why epoxy wax to anything? Melt it and cast it where you want it to be.

I casted the grains into propellant casting tubes from the rcs store. When inserting them into the liner I epoxied each face to each other to prevent the flame from slipping in between the grains and burning the liner. Was this unnecessary?

Thanks,
Walter

 

[G_T]

Wax is used as a mold release for epoxy. The epoxy won't stick to the wax. What you end up with is a thin hard washer (hard relative to the wax) that is not attached to the wax. It will have a slower regression rate than the wax (almost everything is slower than wax). It will stick up a bit from the wax during the burn, contributing to turbulence which is a good thing. But otherwise contributing nothing good.

If you need to glue wax, consider softening the faces and sticking them together.

In your case, with a hybrid, if the gap is pretty thin than I wouldn't think gluing the faces would be required. The wax will generate a reducing atmosphere between the grains during the burn. It will be essentially pure wax vapor, once the motor is burning. The grains are cooler away from the core, so the vapor will tend to condense there. It might even start to stitch the grains together. But in any event, the wax vapor will protect the liner just fine.

If you don't use a post-combustion chamber, the wax will also provide considerable film cooling for the nozzle - at the cost of combustion efficiency.

Next time you cast a wax grain for a hybrid, consider not using casting tubes at all. Cast directly into the liner. The wax grain is part of your insulation, and in a hybrid motor unlike in a solid propellant motor, a paper casting tube IS fuel! Just not as good a fuel as the wax.

Troy's suggestion to blend in some hot glue material is a good one. Wax has lousy mechanical properties by itself. Whatever you use if you do it, get a supply and weigh it out like any other ingredient. I would not assume the next batch will be the same chemically, and I caution you that substituting a different brand/type will achieve slightly different results. When making motors, consistency is important.

Gerald

 

[diyaerospace]

Wax is used as a mold release for epoxy. The epoxy won't stick to the wax. What you end up with is a thin hard washer (hard relative to the wax) that is not attached to the wax. It will have a slower regression rate than the wax (almost everything is slower than wax). It will stick up a bit from the wax during the burn, contributing to turbulence which is a good thing. But otherwise contributing nothing good.

If you need to glue wax, consider softening the faces and sticking them together.

In your case, with a hybrid, if the gap is pretty thin than I wouldn't think gluing the faces would be required. The wax will generate a reducing atmosphere between the grains during the burn. It will be essentially pure wax vapor, once the motor is burning. The grains are cooler away from the core, so the vapor will tend to condense there. It might even start to stitch the grains together. But in any event, the wax vapor will protect the liner just fine.

If you don't use a post-combustion chamber, the wax will also provide considerable film cooling for the nozzle - at the cost of combustion efficiency.

Next time you cast a wax grain for a hybrid, consider not using casting tubes at all. Cast directly into the liner. The wax grain is part of your insulation, and in a hybrid motor unlike in a solid propellant motor, a paper casting tube IS fuel! Just not as good a fuel as the wax.

Troy's suggestion to blend in some hot glue material is a good one. Wax has lousy mechanical properties by itself. Whatever you use if you do it, get a supply and weigh it out like any other ingredient. I would not assume the next batch will be the same chemically, and I caution you that substituting a different brand/type will achieve slightly different results. When making motors, consistency is important.

Gerald

Noted

 

[G_T]

On the link to the fiberglass case, note the word "bonding".

Gerald

 

[diyaerospace]

On the link to the fiberglass case, note the word "bonding".

Gerald

Would it be possible to make an aluminum retaining piece that is bonded to the fiberglass and uses radial bolts to hold the forward and aft closures in place? This allows for the casing to be reused by removing the forward or aft closure to swap out the fuel grains and liner. It also eliminates the need to drill into the fiberglass.

I am relatively doubtful that and aluminum and fiberglass bond will hold up to 510 psi.

What do you all think?

Walter

 

[rocket_troy]

Depends a lot on the details. How is it bonded and to the inside or out? What surface prep, what area of bonded interface per magnitude of stress.
Eg: you ideally want to bond your alloy bulkhead retainer to the outside of the FG cylinder so the radial strain on the cylinder actually pushes the bonded interface together, not pull them apart as could be the case for an inside mount.
Or, you could bond a sleeve of alloy inside the FG cylinder and drill your frankenbolt holes through both layers ie. the alloy sleeve only really should need support the longitudinal stress (which is about 1/2 hoop) as you will need a seal on the inner side of the bolt/screw holes anyway.
Saying all that, if using a composite material for N2O tank casings, it's recommended to implement some kind of inert(ish) liner that will provide a barrier between the N2O and the epoxy of the composite. More details can be found in the Scaled composites report from the fatal tragedy back in 07: https://www.ibb.ch/publication/N2O/N2OSafetyGuidelines.pdf

TP

 

[diyaerospace]

How is it bonded and to the inside or out? What surface prep, what area of bonded interface per magnitude of stress.

Thanks for the reply.

The RCS casing I have is designed to have the bulkheads bonded on the inside of the tube. I do have the option to change the design to bond the aluminum to the outside though.

As for surface prep I plan on using acetone, high grit sand paper, and lint free wipes.

The area of each bulkhead is 12.56 inches. Given the MEOP is 510 psi the force on each bulkhead is 6408.8 pounds.
I can bond an aluminum sleeve over the entire casing but that defeats the purpose of using a fiberglass casing. Realistically I could bond around 63.6 inches^2 of material for each bulkhead.

Does all this check out?


On a separate note my design two O rings to seal with graphite and a cardboard liner (nozzle). Aluminum and a cardboard liner (forward closure). I believe some Aerotech motors use O rings that seal with cardboard to some extent but it still seems sketchy.
Should I be worried about this?

Thanks,
Walter

 

[G_T]

If you can get the bond right you should be able to get 2000psi shear strength to the aluminum I would think. Getting it right with a rocket motor isn't necessarily easy.

If you get it really right - professionally right - you can get several times this strength.

Permabond ET5422 Structural Epoxy Ideal for Aluminum Bonding!

New Permabond ET5422 structural epoxy! This new, blue, extremely high strength two-part epoxy aluminum bonding adhesive.

www.permabond.com

You need to choose the geometry of the joint wisely, so that pressure induced stresses do not nucleate joint failure. You also have to do a very good job of aluminum surface prep. There are many ways to do that, all the way down to caveman method of carefully wet sanding while flooded with the epoxy you intend to use, and NEVER letting the surface get exposure to oxygen in the process. That's after a routine cleaning step of acetone or other degreaser several times (grease/oil destroys epoxy bonding generally) followed by denatured alcohol a couple times to remove any residue from the acetone or other degreaser. Or you can get nastier. Also - look up alodine. That's a common surface prep method. Treat the required chemicals with care.

In any event, for this sort of bond you'll want a rough surface (more surface area to bond, and some of the shear loading on the FRP is transferred to compressive/tensile loading at the aluminum side), and a good but not too tight fit. If you starve an epoxy joint by going too tight then you won't have a bond. In some applications you can occasionally find recommended bond thickness information.

You'll also need to do a good job prepping the fiberglass.

BTW, NEVER touch the surfaces to be bonded with your hands if you can avoid it. If you've touched it, don't bond it without going through the full cleaning regimen. Allow extra time and spend extra effort cleaning the fiberglass casing because it is microporous.

Even composite fabrics going into a structural layup should never be touched. Always wear gloves.

Gerald

 

[rocket_troy]

Yes, follow Gerald's advice on the prep and that should cover that. Again though, just be aware of the radial strain on the casing for example: For a 4" diameter casing with a 2mm wall FG-composite @750psi, you can expect the diameter of the casing to grow by 0.152mm due to radial strain. That's assuming a modulus of elasticity of 72 GPa (nominated for straight EGlass). That might not seem like much, but if your bulkhead isn't subject to any appreciable hoop stress, that bond is now not only subject to shear stress, it's also subject to tensile stress normal to the surface.
Of course, if there's fastener retention on that particular section such as countersunk or cap head screws, then much of that is moot.

Regarding the seal: yeah, o-ring on cardboard and graphite is iffy (iffy with a myriad of ifs). Not something I'd feel completely comfortable with for a duration of more than a few seconds and even then it'd want to be in a low turbulent zone with good quality cardboard. Maybe dunk the ends of the cardboard in some diluted waterglass for a bit more security, but I would personally be looking at perhaps an additional gasket beyond that point for peace of mind.

TP

 

[G_T]

Like Troy, I'm not comfortable with cardboard and getting a seal.

To my mind it matters which end you are talking about. The end by the nozzle should have a slightly reducing atmosphere by the cardboard so for just a short burn it is probably fine. After all the goal is to consume the oxidizer by the time it gets to the nozzle! Otherwise it will consume your nozzle too... (yes, it's not really that simple)

The end by the injectors however has a strongly oxidizing atmosphere. Picture blowing oxygen on burning cardboard... IMHO not good at all...

Gerald

 

[diyaerospace]

Thanks for the advice,

I will post some CAD pics to illustrate exactly what I am trying to do. Starting with the forward closure.
One pic I sent shows the closure without the liner.
O rings in white,
Bolts in light blue,
Aluminum in dark blue,
Graphite in dark grey,
Liner in grey.

The purpose of the graphite is to provide a heat resistant spacer than can survive in the high oxygen environment, the aluminum exists as a piece to attach the injector (Mcmaster Carr spray nozzle https://www.mcmaster.com/3282K163/) and n2o line.

I plan to attach the aluminum and graphite section with epoxy or rtv(which is better?). I will design the parts with a "stair step" pattern to provide more adhesion area and to keep everything lined up.

Seeing as the O rings won't be able to seal with the cardboard liner could I make an aluminum liner extension that extends to seal with the O rings? It will be glued to the paper liner and use a wax grain as a "coupler".

Just trying to throw some ideas around and any more will be welcome.

Thanks,
Walter

 

[diyaerospace]

Here is the nozzle CAD.

How should I attach the nozzle to the propellant grain? Is a static seal enough?
Gerald mentioned heating up the ends of the wax grains to connect them to each other, will this work to connect the grain to the nozzle and spacer?

Anyway thanks for all the help, defiantly learning a lot!

 

[rocket_troy]

Graphite isn't an excellent choice for a thermal spacer. Whilst it might not be the most thermally conductive material, it does possess a high heat capacity so what heat it does conduct/soak is absorbed like a battery ready to be dumped into interfacing componentry following on.
Also, graphite certainly isn't impervious to oxidation at temperature. In fact, it's actually quite susceptible to it - in the presence of hot free oxygen radicals - generally resulting in pitting of the surface. Yes, it's generally uncommon to see that in typical APCP solids *nozzle* applications and many N2O hybrids (that run rich), although I have seen it in N2O hybrids and certainly in propellants employing straight oxygen as the oxidizer.
Maybe consider something like a disc of EPDM for that application or even perhaps a chunky potting of RTV silicone- or something along those lines.

How confident (structurally) are you in the single row of screws at the forward end? Apologies if I missed that discussion earlier. I guess we're talking about the combustion chamber here (not the tank?) so with (injector) pressure drops and all, it might be okay. Certainly the aft closure isn't structurally a concern unless you plan to hydrostatically test the chamber to generous proof margins.

With a double radial o-ring seal, there might be enough redundancy there to cope with a cardboard liner although still lots of provisos with how exposed that section is and how long the burn duration is etc.

TP

 

[G_T]

With a hybrid you generally don't have to worry all that much about the liner getting exposed to gasses in front of the nozzle. This is certainly true with wax fuel. Wax vapor will condense on the liner if it is cool enough. If not, then liquid wax will quite possibly film over it. That's assuming the gap is small. And many hybrids use a post-combustion chamber in front of the nozzle anyway. For small motors a phenolic liner would probably work there, with wax being the fuel.

What about ditching the cardboard liner, the O-rings, and spin casting a liner into the tube with the liner bonding into the bulkhead?

What I'm doing in this short little video is essentially the same as that operation. I'm coating a layer of rubber on the inside and using spin to keep the distribution pretty uniform. Levelling required first, of course, unless you want a tapered thickness distribution. Something that can flow some, and cures slowly, works best with this technique IMHO.

Something like EPDM would make a good liner. Barring that, some other rubber choices could work (R45, any variant, with curative). Potentially you could add fillers to improve the behavior. NOT the same fillers you'd use if it were your fuel! Phenolic microspheres would be one candidate. There are others. Research...

And of course phenolic tubes work as liners. The higher the phenolic content, the better they work.

Even EPDM sheet, bonded in place, should work. You can get the sheet in thin pieces easily enough. If there is an option, you want the peroxide cured version IMHO.

Designing a complex thing is easy. Designing a simple thing can be harder. Think about each individual item you are putting into your design, and ask yourself if you need it, or if it is there because you are used to seeing it on a reloadable aluminum motor. Also ask yourself if your motor is intended to be reusable or single-use.

In the early stages of motor work, you'll likely find that after a burn you want to change something. So then the reusable motor doesn't get reused.

And if your motor is high performance, then generally you are shaving the margins a little and reuse becomes less wise. A successful burn may trigger a failure in the next burn, for a reason you didn't detect beforehand.

And of course for high performance flights, then the whole thing might get design evolution before the next burn. Or it fails...

Sometimes reusable is the expensive approach, not the cheap approach. Figure which approach makes sense for what you want to accomplish. Alternative approaches may exist to make a single use chamber and have it be a lot simpler to make and therefore cheaper.

If I go composites for my next hybrid motor, and I'm considering doing that, I may well make much of it or all of it single use. But my biggest reason would be to use materials and methods that won't survive the burn well enough to survive a second burn, or are not ideal for long-term repeated exposure to nitrous, but are lighter and simpler and cheaper. You can see my current motor in the THRP-1 thread and I expect why I want it simpler will be pretty obvious!

Gerald

 

[G_T]

BTW, saturate a cardboard tube with Krytox grease (how to turn a cheap liner into an expensive one) and it will quite possibly be fine for burns of the duration you are likely to be making. But only saturate AFTER you have cast in the fuel! Or you will have no hope of ever getting a bond.

Just in case you didn't know, you want to be using Krytox grease where exposure to hot oxidizer is expected or possible. That would include the injector end of the combustion chamber. Krytox is fully fluorinated so it is safe for oxygen service. It is also toxic and expensive. Don't handle; wear gloves.

Gerald

 

[rocket_troy]

I don't think Krytox is toxic? At least I hope it's not. I've always handled the stuff quite nonchalantly. Sure, the fumes of any decomposed fluoropolymer will be toxic, but I was under the impression the grease itself is almost safe enough to ingest.

TP

 

[rocket_troy]

Even EPDM sheet, bonded in place, should work. You can get the sheet in thin pieces easily enough. If there is an option, you want the peroxide cured version IMHO.

Even EPDM in thicker pieces should be available. Contact your local gasket supplier/maker. They often stock such materials in sheet to punch out flange and rectangular (section) gaskets.

TP

 

[G_T]

Thanks Troy; I stand corrected!

I just went through three randomly chosen SDS and MSDS spanning several years and the all agree with Krytox being pretty benign. I thought I had an MSDS on my computer that indicated otherwise, but perhaps that is another variation on a fluorinated grease.

-----

Just search for EPDM sheet on Amazon to get an idea of what is available.

-----

Thanks!
Gerald

 

[diyaerospace]

Thanks for the replies,

I have decided to make a bunch of design changes after everyones input.

I think I will make the combustion chamber single use. This will simplify the design to glueing the nozzle and forward closure in place? What adhesives will bond well with graphite?

I will leave the nozzle unbonded to the liner to prevent the liner from taking any of the pressure.

I will bond a phenolic or EPDM insulator to the forward closure instead of graphite.

I will use a UC style injector to simplify the design.

My hope is to make the N2o tank reusable. TBH I have not put a huge amount of though into the nitrous oxide tank but I am thinking of using aluminum tubing with machined aluminum bulkheads connected with radial bolts. The aft bulkhead will connect with the combustion chamber. The forward bulkhead will contain a pressure transducer to determine if the tank pressure is safe to begin the test.
Of course all of this will be cleaned with acetone and lint free wipes to eliminate any hydrocarbon contaminants.

Nitrous loading will be done remotely.

Thanks,
Walter

 

[rocket_troy]

Don't know about bonding "well" to graphite, but epoxy will provide some level of bond. In the 90s we use to make all our single use APCP motors with straight graphite nozzles that were basically potted in epoxy for seal and retention. Can't recall any failures of the nozzle at least. The bond will be single use ie. once the heat soak makes its way to the bond, that's it for the bond, which conveniently makes it easier to remove the graphite and use it again for another single use motor.
Caveat: that bond wasn't 100% shear loading on the graphite - the aft section of the graphite was reduced diameter so there was a tapered step taking most of the load via compression so it's not a great example.

Reading what you said about the tank, it appears I had my wires crossed before when I was discussing nitrous compatibility - I thought we were discussing the tank, not the CC.
What kind of volumes are you thinking of for the tank?

TP

 

[diyaerospace]

What kind of volumes are you thinking of for the tank?

Roughly 1.3 kg/1166 cc.

 

[rocket_troy]

Roughly 1.3 kg/1166 cc.

Consider something like this: https://www.alibaba.com/product-det...771.html?spm=a2700.details.0.0.793a41e2fYf1In
They used to be available (in a higher grade alloy) for free from used gas analysis applications. Some smaller sizes are probably still available around the traps.
These Chinese ones aren't the perfect alloy for this application, but they're flight weight and I've hydrostatically tested them to over 1100psi IIRC. There's enough meat at the top to drill & tap 1/8" pipe threads and the main threaded connection is a very nice... ah... 1" UNF if I vaguely recall.
These Chinese suppliers were willing to supply 1 or 2 pieces as samples 12 months ago, so I'm assuming they still might be.

TP

 

[diyaerospace]

Thanks for the suggestions,

Here are some more updates.
The nozzle will be a phenolic/glass nozzle from RCS. This will be bonded into the fiberglass with rocket poxy?

54mm Nozzle, 0.455

Molded glass/phenolic nozzle for 54mm diameter motors. Dimensions: 1.985' O.D. 0.455' diameter throat 1.250' diameter exit Weight = 85 grams Click here for drawing

www.rocketmotorparts.com

As mentioned previously the nozzle will not seal with the liner.

The forward closure will be machined from aluminum with a phenolic insulator. My hope is for the phenolic section to slide a half inch into the liner and be bonded in place. The aluminum section will be bonded with the fiberglass either with rocket poxy or with ET5442 aluminum bonding epoxy.
The forward (aluminum) section of the forward closure will interface with the ox tank by means of radial bolts.

As for the injector I will be using a 3/16 UC injector from Contrail with a CNVT to improve atomization of the oxidizer.

Medium Injector 3/16 line 1/8 NPT

This Medium Injector 3/16 line 1/8 NPT is a push to connect fitting is designed for use in the Contrail Rockets motor system. The fill line is matched to the use of this fitting and the use of the …

contrailrockets.com

# 4 CNVA 1/8 NPT

# 4 Contrail Nickel Vortex Adapter (CNVA) 1/8 NPT for uses with 5/16 inch fill line, push to connect injector fitting and graphite nozzle. Use of this part requires that you have a CNVA injector ba…

contrailrockets.com

Thanks for all the help,
Walter

 

[G_T]

Have you done the full modeling to verify injector size vs nozzle throat vs grain dimensions vs sufficient web thickness vs tank volume and over supposed temp range? It's usually during that process that dimensions fall out. Then the design is tweaked to make the dimensions naturally fit things you can get or make, while still being in the ballpark of your original design requirements.

I would make or get a fiberglass ring and bond it behind the nozzle as a carrier. You'll have more bond area that way, and the bond will be away from initial heat exposure. You will have to make sure you align the nozzle or you will get thrust vectoring which is a vary bad thing in a flight motor! Tolerable in a test motor though.

I've been mighty tempted to pick up one of those phenolic nozzles just to see if I could turn it into a nozzle throat in a phenolic carrier.

Gerald

 

[diyaerospace]

Have you done the full modeling to verify injector size vs nozzle throat vs grain dimensions vs sufficient web thickness vs tank volume and over supposed temp range

Yep, given a 4-4.5 mm regression rate and an injector flow rate of 257 g/s. (I am still trying to figure out the effects of adding a CNVA to the flow rates of Contrail's injectors. I can find the flow rate of their orafice injectors by looking at motor certification data. The question of adding a CNVA seems to come down to changed Cd(0.57) and injector area.)

I used the equations in the hybrid rocket video tutorial (found in original post) and NASA CEA to calculate the motor dimensions.

Side note: I am looking for a better fuel than paraffin to burn as the wax is relatively brittle and has a high regression rate meaning a higher ox flow is required. The length of the grain will have to be pretty short to to maintain the ox to fuel ratio which limits the burn time of the motor.

I am thinking of ABS because it allows me to skip the casting process.

Thanks,
Walter

 

[G_T]

ABS should be a good fuel and is one I may consider with my next hybrid design.

The length of the grain has nothing directly to do with burn time. Regression rate by itself doesn't have anything to do with required oxidizer mass flow rate. Web thickness minus remainder for insulation, divided by maximum regression rate, determines maximum permissible burn time. Length of grain then determines total available impulse via mass of fuel, under assumption of trying to stay close to a desired O:F ratio. If the resulting grain lengh is too short to ensure combustion occurs predominantly before the nozzle (insufficient mixing of oxidizer and fuel or just not enough time), then a post-combustion chamber may be added to allow residence time for combustion to more nearly complete. Thrust is predominantly determined by oxidizer flow rate - in simplified terms the area of the injectors. Simplified because the Cd of an injector goes down as its diameter increases, though of course that is also oversimplified. The greater the desired initial thrust, the longer the grain will have to be to achieve the required fuel rate. Too high of an oxidizer flow rate for the initial grain port area will cause problems. So essentially the desired thrust through restrictions on port area, and the desired burn time presenting restrictions on minimum permissible fuel web, end up determining the minimum practical combustion chamber inner diameter for the desired burn profile to be achievable. Somewhere in there is also a decision on how much pressure drop you choose across the injectors. Not enough, the motor is likely to CATO. Almost not enough and the motor experiences considerable combustion instability. Too much and the ISP suffers particularly at ground level. Once a pressure drop is chosen then this essentially sets the desired combustion chamber pressure (if you know your initial oxidizer conditions). So then more thrust means more injector area which means more nozzle throat area. The ratio between those two areas becomes something resembling a constant in the design process, much like Kn can be treated for solid propellant motor design.

Sorry I rambled. There are a lot of interrelations here. Hopefully you are familiar with the things I mentioned above.

Gerald

 

[G_T]

What I'm stating here is just an opinion, and not meant as a criticism.

In particular, I applaud and want to encourage you on your attempt to make your own hybrid motor of your own design.

The problem with having simulators available is they provide a shortcut to getting to a design or an evaluation of a design, without necessarily understanding the underlying interrelationships in the item being simulated. If the item is sufficiently compilcated as to be essentially not understandable, then that's the best we can do.

However IMHO it is well worth understanding what is going on, at least at a solid conceptual level even devoid of any math, before entering upon simulations. Then you could even design a motor somewhat in the ballpark with back of the envilope computations using simple math. The simulations would then be for verification and fine tuning.

Otherwise the process can easily become an exercise in GIGO.

Gerald