What Are Our Motors "Missing"?

[bobkrech]

After reading 120 posts on this thread, I'd like to go back and answer the original question. "What are our motors "missing"?

Aboslutely nothing.

Nationally organized hobby rocketry has been around for half a century and we've never had the selection of motors that we have today. There are over 1,000 certified commercial motors. and when various delays are considered, several times more.

We can purchase affordable COTS motors ranging from micromax to full O impulse (40 KNs) without difficulty, and certified individuals have no problems purchasing them and launching them. Waivers are not difficult to obtain, and waivers for Class 3 Advance High Power Amateur launches are not that difficult to obtain for qualified individuals and groups.

The propellants available today are better, safer and more affordable than those developed in the 40's and 50's. Anyone believing the contrary needs to spend some time researching why we use what we use today, and why we don't do it the old way.

Bob

 

[Adrian A]

Bob, I agree with the spirit of your answer, and what you say is true, but I took the original question as: "Why aren't amateurs launching rockets that are going as high as similarly-sized professional sounding rockets (it must be the motors)" In that context, having micromax motors to choose from is great, but it doesn't really answer the question.

To get the altitude performance of an ARCAS, it helps a lot to have the 30-second+ burn duration like the ARCAS did, so that the rocket is above most of the atmosphere when it's at its highest speed. That aspect of motor performance is missing from today's commercial motors, but you can get around that using staging to delay the final burn. The person who has explored that corner of the envelope most successfully so far is Jim Jarvis, who sent an N-N 2-stage rocket over 100,000 feet. I think it's possible to push even higher with less propellant in 2 and 3 stage rockets using commercial motors.

For the Loki dart, we can already reproduce comparable performance with today's commercial motors, and almost certainly with research motors. I'm glad to hear that someone is working on that. On the larger end of things, I think it's also cool how the USC team was designing for real sounding rocket altitudes with the same amount of propellant that Derek Deville used, by using a more optimal airframe.

So I think getting to these really high altitudes is a system design problem, and it's a mistake to think that there's a single bottleneck with the motors, particularly the propellant formulation. The relatively short burn durations of motors available now, compared to professional sounding rockets, just means that the rocket designs need to go down other paths to reduce the low-altitude heating and drag losses, and staging and darts are feasible ways to do that that have only begun to be explored by amateur rocketeers.

 

[r1dermon]

Bob, i found an old post from the archives of yours, and i think it coincides with what Adrian has stated...

https://www.rocketryforumarchive.com/showpost.php?p=594942&postcount=17

the key factor is the burn time. hobbyists and amateurs have launched larger motors, and larger motor clusters than a 54% P motor, but that 54% P motor driving a 220lb 4.72" diameter, 9' tall rocket, sporting a P7000 motor with a 9 second burn. there's no real equivalent motor available. the closest thing, going by thrustcurve, would be the CTI O4900 with a nearly 8 second burn and 37,000 Ns.

 

[cjl]

Bob, i found an old post from the archives of yours, and i think it coincides with what Adrian has stated...

https://www.rocketryforumarchive.com/showpost.php?p=594942&postcount=17

the key factor is the burn time. hobbyists and amateurs have launched larger motors, and larger motor clusters than a 54% P motor, but that 54% P motor driving a 220lb 4.72" diameter, 9' tall rocket, sporting a P7000 motor with a 9 second burn. there's no real equivalent motor available. the closest thing, going by thrustcurve, would be the CTI O4900 with a nearly 8 second burn and 37,000 Ns.

The O4900 isn't really a good choice for altitude though, since it has a relatively poor mass fraction of around 54%, a specific impulse of only 214 seconds, and a large diameter for its impulse. For an altitude rocket, you want a higher mass fraction and as small of a diameter as possible to minimize drag and maximize delta V. I'm still convinced that one of the best altitude motors available at the moment is the N5800 C-star, with a 61% mass fraction, a 4 inch diameter, and a 228 second specific impulse. Yes, the burntime is a bit short, but the small diameter for its impulse and good mass fraction and Isp should allow for incredible performance (it would be really awesome for a boosted dart).

 

[Adrian A]

Bob, i found an old post from the archives of yours, and i think it coincides with what Adrian has stated...

https://www.rocketryforumarchive.com/showpost.php?p=594942&postcount=17

the key factor is the burn time. hobbyists and amateurs have launched larger motors, and larger motor clusters than a 54% P motor, but that 54% P motor driving a 220lb 4.72" diameter, 9' tall rocket, sporting a P7000 motor with a 9 second burn. there's no real equivalent motor available. the closest thing, going by thrustcurve, would be the CTI O4900 with a nearly 8 second burn and 37,000 Ns.

I looked at that info that Bob provided, and I was struck by a comparison between the motor for the Black Brant VI that's a 54% P, and Derek Deville's Q flight. They both had about the same burn duration, 8 seconds vs. 9 seconds. The biggest differences are that Qu8k was 8" in diameter, vs. the Brant 4.7," and it was 4.2 meters, rather than 2.8, and weighed 320 lbs, rather than 220. It had almost twice the impulse and it went about half as high as the Black Brant.

The O4900 is a .16 meter diameter motor, compared to .12 for the Black Brandt, but it's only 1 meter long, so if its propellant were repackaged into the BB's diameter, it would be 1.8 meters long. If the length were increased by the total impulse ratio, keeping BB's diameter, it would be 3.06 meters long. So it looks like the Black Brant's performance was at least in part due to packaging higher impulse per unit volume than CTI did with its blue streak propellant in that the O4900.

 

[r1dermon]

perhaps CTI can come out with some moon burning O motors for a super long 98mm case. what type of drawbacks are there with lengthening a casing? clearly the govt gets enough efficiency to launch P motors well over 100km into space, using a 4.5" airframe...

are there any builders in TRA making P motors out of less than 6" casings? 54mm M motors? 75mm N motors? each time i look at these govt sounding rockets, they fall within what has been done impulse-wise with amateur rocketry, but they're always cramming a P motor into a 4" case (or equivalent...like an N into a 75mm...etc...). from aerotech or CTI's perspective, what would be the drawback design wise? AT makes (or made...no idea) an N1000 with like a 13 second burn. super duper long burn motors have been done before...it's a matter of cramming super long burn, with high ISP, into a longer than normal case, and narrower case...certainly would be cool to see a 98mm moon burning O motor. haha.

 

[Adrian A]

The O4900 isn't really a good choice for altitude though, since it has a relatively poor mass fraction of around 54%, a specific impulse of only 214 seconds, and a large diameter for its impulse. For an altitude rocket, you want a higher mass fraction and as small of a diameter as possible to minimize drag and maximize delta V. I'm still convinced that one of the best altitude motors available at the moment is the N5800 C-star, with a 61% mass fraction, a 4 inch diameter, and a 228 second specific impulse. Yes, the burntime is a bit short, but the small diameter for its impulse and good mass fraction and Isp should allow for incredible performance (it would be really awesome for a boosted dart).

Even better for a boosted dart would be the infamous single-use P that Tim tried to fly at LDRS30. What was it, a P25,000 or something? I can't seem to find the thread for it.

 

[Adrian A]

perhaps CTI can come out with some moon burning O motors for a super long 98mm case. what type of drawbacks are there with lengthening a casing? clearly the govt gets enough efficiency to launch P motors well over 100km into space, using a 4.5" airframe...

are there any builders in TRA making P motors out of less than 6" casings? 54mm M motors? 75mm N motors? each time i look at these govt sounding rockets, they fall within what has been done impulse-wise with amateur rocketry, but they're always cramming a P motor into a 4" case (or equivalent...like an N into a 75mm...etc...). from aerotech or CTI's perspective, what would be the drawback design wise? AT makes (or made...no idea) an N1000 with like a 13 second burn. super duper long burn motors have been done before...it's a matter of cramming super long burn, with high ISP, into a longer than normal case, and narrower case...certainly would be cool to see a 98mm moon burning O motor. haha.

Loki has a 75mm small N that's either certified or about to be. It shredded a rocket on a demo flight at LDRS.

 

[cjl]

Even better for a boosted dart would be the infamous single-use P that Tim tried to fly at LDRS30. What was it, a P25,000 or something? I can't seem to find the thread for it.

I think you're confusing two separate motors. The P that Tim tried to fly was a fairly long burning motor (P10k perhaps? P8k?). There was also the 130ish mm O25000 Vmax. However, I'm not convinced that either would best the N5800 C-star for a boosted dart. For a boosted dart, you want maximum delta V, which will depend pretty much exclusively on mass ratio and specific impulse. The N5800 is quite good in both of those areas. The P probably bests it on mass ratio, since that is one of the main benefits of single use, composite-cased motors, but it had a very large diameter and a fairly long burn, so the drag loss would be enormous.

 

[Adrian A]

I think you're confusing two separate motors. The P that Tim tried to fly was a fairly long burning motor (P10k perhaps? P8k?). There was also the 130ish mm O25000 Vmax. However, I'm not convinced that either would best the N5800 C-star for a boosted dart.

Thanks for setting me straight. I was thinking of the O25000 Vmax for the boosted dart. With the 1.2 second burn duration, you don't need to care much about the booster's drag, but the mass fraction is still important.

Ah, here's one thread on it:

https://www.rocketryforum.com/showthread.php?t=24126

 

[Adrian A]

For a boosted dart, you want maximum delta V, which will depend pretty much exclusively on mass ratio and specific impulse. The N5800 is quite good in both of those areas. The P probably bests it on mass ratio, since that is one of the main benefits of single use, composite-cased motors, but it had a very large diameter and a fairly long burn, so the drag loss would be enormous.

I'm dreaming about how to put the O25,000 to good use. A booster for your N5800 design would be a good use, to get your rocket above most of the atmosphere so you don't burn your fins off or shed your nosecone (again :eyeroll. How high does your N5800 design go if it ignites at 40kft? or 60kft? Just guessing, I bet you could get over 200kft combined.

 

[AlphaHybrids]

I think one thing that is important to bring up is that the casing material we use. We can't achieve higher mass fractions with aluminum due to the need for 1) thicker casings or 2) more insulation for thinner casings. If you wanted to buy a new casing each time, you could up the mass fraction a bit. The ARCAS used a very well designed steel casing. Steel doesn't lose strength at higher temperatures as much as Aluminum does and lets you get very high mass fractions with thinner liners. But, the choice to only use aluminum in our casings is a very wise one.

Edward

 

[bobkrech]

Bob, I agree with the spirit of your answer, and what you say is true, but I took the original question as: "Why aren't amateurs launching rockets that are going as high as similarly-sized professional sounding rockets (it must be the motors)" In that context, having micromax motors to choose from is great, but it doesn't really answer the question.

To get the altitude performance of an ARCAS, it helps a lot to have the 30-second+ burn duration like the ARCAS did, so that the rocket is above most of the atmosphere when it's at its highest speed. That aspect of motor performance is missing from today's commercial motors, but you can get around that using staging to delay the final burn. The person who has explored that corner of the envelope most successfully so far is Jim Jarvis, who sent an N-N 2-stage rocket over 100,000 feet. I think it's possible to push even higher with less propellant in 2 and 3 stage rockets using commercial motors.

For the Loki dart, we can already reproduce comparable performance with today's commercial motors, and almost certainly with research motors. I'm glad to hear that someone is working on that. On the larger end of things, I think it's also cool how the USC team was designing for real sounding rocket altitudes with the same amount of propellant that Derek Deville used, by using a more optimal airframe.

So I think getting to these really high altitudes is a system design problem, and it's a mistake to think that there's a single bottleneck with the motors, particularly the propellant formulation. The relatively short burn durations of motors available now, compared to professional sounding rockets, just means that the rocket designs need to go down other paths to reduce the low-altitude heating and drag losses, and staging and darts are feasible ways to do that that have only begun to be explored by amateur rocketeers.

Adrian

You are the winner.

To meet a specific reuirement you need a system design, not simply a motor design. Minimizing losses is far more important than maximizing power.

These are some key design issues.

1.) With a well designed motor, thrust is essentially mass flow multiplied by Isp. If you need more thrust, you must burn more propellant per second.

• There are propellant mass limitations in a poorly designed motor

2.) Drag is proportional to cross-sectional area times velocity squared time atmospheric pressure.

• Increase the booster diameter by n, increase the drag by n^2
• Increase the velocity by m, increase the drag by m^2
• Long slender rockets have much less drag

3.) Aerodynamic heating is proportional to the cube of the velocity times the atmospheric density.

• If you double your velocity at low altitude, you increase the aerodynamic by a factor of 8! Triple it and you increase the heating by a factor of 27! Qyadruple it and you increase the heating by a facor of 64!
• If you go much beyond Mach 3 at low altitude, your standard hobby rocket design will fall apart due to thermal failure

4.) For a 10" diameter rocket, the window to 100 km is Mach 5+ at 40kft.

• Patriot PAC3 - 17' high, Isp 240, warhead but no recovery system
• GOFAST - 22' high, Isp 220, recovery system
• SS2S - 27' high, Isp 130, dual impulse burn with no staging, 82% propellant fraction
• The way you get to "space" is to do a complete system design, not simply a motor design

Now it's Test time.

1.) What are the design constraints of a 4.5" 30 second end burning motor?

Bob

 

[bobkrech]

Bob, i found an old post from the archives of yours, and i think it coincides with what Adrian has stated...

https://www.rocketryforumarchive.com/showpost.php?p=594942&postcount=17

the key factor is the burn time. hobbyists and amateurs have launched larger motors, and larger motor clusters than a 54% P motor, but that 54% P motor driving a 220lb 4.72" diameter, 9' tall rocket, sporting a P7000 motor with a 9 second burn. there's no real equivalent motor available. the closest thing, going by thrustcurve, would be the CTI O4900 with a nearly 8 second burn and 37,000 Ns.

And the Black Brant III is a 4th example of a 10" rocket that will exceed 100 km.

It's really system design, not burn time. A 30 second burn Arcus and a 2 second burn Loki both obtain the same apogee, but do it by totally opposite methods.

Bob

 

[r1dermon]

And the Black Brant is a 4th example of a 10" rocket that will exceed 100 km.

It's really system design, not burn time. A 30 second burn Arcus and a 2 second burn Loki both obtain the same apogee, but do it by totally opposite methods.

Bob

i agree...

 

[Alby]

[*]If you double your velocity at low altitude, you increase the aerodynamic by a factor of 8! Triple it and you increase the heating by a factor of 27! Qyadruple it and you increase the heating by a facor of 64!
[*]If you go much beyond Mach 3 at low altitude, your standard hobby rocket design will fall apart due to thermal failure

This is likely the most important factor, which to me, translates into,
Go slow when in low altitudes. Speed up as you get higher in altitude.

I assume that is why you heard the space shuttle communications
saying, "Go at Throttle Up" when it was around 50,000/ft.

 

[bobkrech]

I think one thing that is important to bring up is that the casing material we use. We can't achieve higher mass fractions with aluminum due to the need for 1) thicker casings or 2) more insulation for thinner casings. If you wanted to buy a new casing each time, you could up the mass fraction a bit. The ARCAS used a very well designed steel casing. Steel doesn't lose strength at higher temperatures as much as Aluminum does and lets you get very high mass fractions with thinner liners. But, the choice to only use aluminum in our casings is a very wise one.

Edward

Edward

You're my second winner.

You did your homework and realized the thermal issue surrounding an aluminum motor casing which is not used in professional, high performance motors.

In addition to s steel casing, insulation is key to a successful 30 second burn. There are better and more modern solutions than the Arcus asbestos insulation, but they can't be discussed. :dark:

Bob

 

[bobkrech]

The O4900 isn't really a good choice for altitude though, since it has a relatively poor mass fraction of around 54%, a specific impulse of only 214 seconds, and a large diameter for its impulse. For an altitude rocket, you want a higher mass fraction and as small of a diameter as possible to minimize drag and maximize delta V. I'm still convinced that one of the best altitude motors available at the moment is the N5800 C-star, with a 61% mass fraction, a 4 inch diameter, and a 228 second specific impulse. Yes, the burntime is a bit short, but the small diameter for its impulse and good mass fraction and Isp should allow for incredible performance (it would be really awesome for a boosted dart).

Winner #3.

A formal engineering background will find a correct solution every time.

Bob

 

[Adrian A]

I assume that is why you heard the space shuttle communications
saying, "Go at Throttle Up" when it was around 50,000/ft.

Exactly. The density (and thus drag force and heating) roughly are cut in half every 16,000 feet. So at 50kft, you're down to about 15% air density

 

[r1dermon]

i agree...

i'd like to actually elaborate on this...

one thing to consider in any system is your design constraints. i think this thread is trying to find applicability of a super high altitude rocket project with *hobby* rocketry. as someone has already pointed out, one of the design constraints with hobby rocketry is our aluminum casings. and non-metallic airframes. i think when specifically speaking about a project such as this within the realm of hobby rocketry, you're going to have to factor for weaker materials used in construction. i think in mitigating the stresses placed on your vehicle, a longer burning design will prove more desirable for a rocket fitting within hobby rocket build constraints. a Loki dart goes mach 5+ by the time it reaches 30k AGL. using a 3" N9000 equivalent. 77% of the total mass of the vehicle is contained in the booster.

just thinking outloud.

 

[bobkrech]

This is likely the most important factor, which to me, translates into,
Go slow when in low altitudes. Speed up as you get higher in altitude.

I assume that is why you heard the space shuttle communications
saying, "Go at Throttle Up" when it was around 50,000/ft.

Not exactly.

That has to do with Max-Q or peak dynamic pressure which is a structural issue.


Getting to orbit is several orders of magnitude more difficult than putting a rocket into a ballistic trajectory into space.

• Mach 5 versus Mach 25
• Kinetic energy is proportional to velocity squared so you need 25 times more energy per unit mass to get an object into orbit than into space.
• Peak heating can be 125 times greater, but usually only important if you want to recover it from orbit
• You can only get a much smaller mass fraction into orbit than into space
  • Only 5% of the lift-off weight of the shuttle gets into orbit versus as much as 50% of a rocket going up to space (100 km)

Bob

 

[Adrian A]

i'd like to actually elaborate on this...

one thing to consider in any system is your design constraints. i think this thread is trying to find applicability of a super high altitude rocket project with *hobby* rocketry. as someone has already pointed out, one of the design constraints with hobby rocketry is our aluminum casings. and non-metallic airframes. i think when specifically speaking about a project such as this within the realm of hobby rocketry, you're going to have to factor for weaker materials used in construction. i think in mitigating the stresses placed on your vehicle, a longer burning design will prove more desirable for a rocket fitting within hobby rocket build constraints. a Loki dart goes mach 5+ by the time it reaches 30k AGL. using a 3" N9000 equivalent. 77% of the total mass of the vehicle is contained in the booster.

just thinking outloud.

I wouldn't consider aluminum fins and airframe verboten from an amateur perspective (seek qu8k), but I agree that longer burning motors make the thermal design for composite rockets a lot easier.

I was thinking recently about the benefits of longer burning motors, and how it changes with motor size. With small rockets, the main benefit is keeping the drag losses down by keeping the V^2 part of the drag equation small. A 29mm F10 has gone higher than all the 24mm F flights because it kept the velocity subsonic. Its 8 second burn takes about half of the time to apogee. For somewhat larger rockets, you'd need something like a 15-second burning I to use the same strategy, so the benefits for the moderately longer burns that are currently available aren't so dramatic. But when going to even larger rockets, increasing the burn time brings another advantage into play that isn't very significant with the smaller rockets, and that's the atmospheric density reduction. If two rockets get to Mach 2.5 at burnout, a rocket doing it at 40,000 feet ASL will add about 60,000 feet onto its altitude, vs. the same rocket that does it at 10,000 feet ASL that will only go another 17,000 feet or so.

 

[AlphaHybrids]

When I first started doing HPR, a K/L motor was a big thing at a launch. Now people barely turn their heads for an M motor. I still see many more recovery issues than propulsion ones, but N/O/P motors are still very few and far between.

It seems like this discussion that people think these kinds of motors (especially research) are a piece of cake and you could just throw them together and they will work. They aren't. Each batch of ingredients is different, each process is a little different. It takes a lot of research and quality control to get the motors working well. More than the average flier is willing to do. 90% of the fliers I see put a 38mm motor in a 4" rocket and fly it. That's awesome, I have my own rockets where I just enjoy sending them on a ride. It is part of what makes it fun.

I think it will be quite a long time before we see O/P/Q motors as commonplace. The engineering, time, money and commitment are more than the average person will put into it. Derek did an amazing job with Qu8k. A top notch job that had much thought and engineering put into it. It took a lot to make it that simple - don't let the simplicity fool you.

Edward

 

[Adrian A]

Yes, major congratulations to Derek on his phenomenal flight. His video did (and does) a great job of representing rocketry to the outside world. It's been featured on Space.com and I even saw it on the Lockheed Martin home page. Few people outside of professional aerospace have had such a successful flight at such high altitude, and there are plenty of people who have tried and failed to do what he did.

 

[SSenesy]

I've been lurking in this thread since it started, and I'd just like to thank everyone who's posted. You've done more to advance my knowledge of flight and propulsion dynamics than any other source since I started in HPR 5 years ago. I may never get to fly at the level being discussed here, but it's fascinating to learn.

Keep it up!

 

[jsdemar]

Edward

You're my second winner.

You did your homework and realized the thermal issue surrounding an aluminum motor casing which is not used in professional, high performance motors.

In addition to s steel casing, insulation is key to a successful 30 second burn. There are better and more modern solutions than the Arcus asbestos insulation, but they can't be discussed. :dark:

Bob

Read the whole thread, we already made these points early on.

 

[jsdemar]

I think it will be quite a long time before we see O/P/Q motors as commonplace. The engineering, time, money and commitment are more than the average person will put into it. Derek did an amazing job with Qu8k. A top notch job that had much thought and engineering put into it. It took a lot to make it that simple - don't let the simplicity fool you.

It was simpler because it was single use. But, it wasn't as simple as it could have been, given the fact they rushed into the project specifically to win the Carmack Micro Prize. The engineering is based on previous designs, and comes from paid development work that's not quite amateur level.

What is even more amazing to me is the number of people who are making their own large motors with very little money and no professional experience. The success rate is lower because the learning curve takes longer compared to getting paid to do the development work. The other factor is that most do not have a formal engineering background and work through the process by hear-say and trial-and-error. Nothing needed for high-performance amateur rocketry is new or secret; one just needs to look at the prior art, understand the engineering process, and reach into your bank account.

 

[daveyfire]

Read the whole thread, we already made these points early on.

Yeah I'm sad that quals just ended and so I got my life back, but I don't get to be one of Bob's winners

 

[jsdemar]

Yeah I'm sad that quals just ended and so I got my life back, but I don't get to be one of Bob's winners

"The forum where everything is made up and the points don't matter".

Welcome back to the living.

 

[bobkrech]

Yeah I'm sad that quals just ended and so I got my life back, but I don't get to be one of Bob's winners

Dave

Surviving your quals make you a winner too.

You, John, Mark and Fred also explained how the Arcus and other sounding rockets worked, but my point is that it's not simply the motor that gets you into space (100 km), it's the total system design. This thread has illustrated that small professional rockets with burn time ranging from 2 to 30 seconds can all exceed 100 km apogee so it's not just the motor. It also illustrates that there's nothing secret about how to make an Arcus as several technical reports document the process quite well.

It also illustrates that while amateurs make some fantastic hobby rockets, aerospace professional using professional techniques and larger budgets can do better, however it should be noted that many aerospace professionals are slso hobbyists and are almost always involved in the larger amateur rocketry efforts.

Bob