Attempt towards an amateur orbital rocket.

[plugger]

If I was designing up an orbital capable vehicle (especially from solids), I certainly wouldn't consider anything not monocoque unless there was a really pressing case for it (no pun intended). Ditto for a Karman Line shot, although there's much more margin for "discretionary dry mass" with the latter.

TP

I 100% agree Troy, but I don't believe Bob's statement of "you do have to construct the rockets without using airframes" is correct, technically or otherwise.

 

[charrington]

I thought we'd already established that solids to orbit (let alone interplanetary!) on consumer-scale motors was completely infeasible.

The notion that university teams are up to/have the budget for casting 1500 kg+ of propellant is ... interesting. Not to mention designing and testing those motors, developing controls, etc. etc. Maybe if a professor was managing the project and providing continuity, but it's really hard to sustain 5-10 year projects at universities.

Having experience in both sides of the university coin I have some input on this. I was in engineering at Cal Poly Pomona and worked on both the Liquid Rocket Lab and UMBRA (the solids program) during 2017-2020. I had a change of heart and moved to a biology related degree working in Neuroscience and Molecular Biology.

The biggest thing I noticed during this change is the structure of how research is conducted. Research in science, specifically the hard sciences (think biology, chemistry, physics, mathematics) is conducted over long periods of time. It has to be and that is the expectation. The Neuro research paper I am apart of right now is on its 6th year. For your paper to get published it needs to be peer reviewed by multiple experts in the related field and passed by the publisher. This involves sometimes years of edits and rewrites. It always involves a PhD overseeing the research along with multiple masters students and then some undergraduates who assist in the research. Sometimes, actually most of the time unless you are the lead researcher, you graduate before it is actually published so you miss out on being able to present your findings in national conferences. Most of this research when published leads to a new line of research that will dive deeper into the topic. Scientific research is a painstaking and lengthy process.

In engineering as many have mentioned before (besides USCRPL and a few others) it is a group of students who are most often inexperienced that have a professor who is asleep at the wheel or asking 18 year old students, who just finished their intro to aeronautics course, to turn oxygen and methane into thrust like some sort of Rocket Jesus. Granted, I don't think this is inherently a bad thing. I think getting hands on experience is important even if the project is most likely going to fail. A lot of professional engineering companies have had their fair share of failures.

Back to the point though. I think the biggest issue facing engineering rocketry programs in universities is the lack of further education. Most engineers I know in industry only received their Bachelors. This isn't bad because experience is learned on the job but for university research it is. The quality research an undergraduate can do is very limited and they ALL want to see their project launch before they graduate. Their job is to pass their classes and get a degree and go make money, not to conduct quality research in hopes of publishing it. That is for Masters and Doctoral level students. Most students that are completing a grad degree are discouraged from doing anything that has been done many times before (sounding rockets have been a thing since the 50s) or things that will cost an insane amount of money (orbital rockets would bankrupt even the wealthiest schools without serious government funding). Doctoral level students in aerospace related engineering are probably focusing on novel materials or methods to construct them. Most masters engineering students I know don't even have to do a research thesis and they usually just get the masters to get a bump in pay at the work who is probably paying for that degree.

Back to the fun stuff though. If I was gonna go to orbit I think I would need a copy of Super ROCSIM XTREME Pro. Its like ROCSIM Pro but for orbital launches. Then I can 'borrow' the decommissioned SRB at march air force base, ask Scott from Loki research to cast me something that will fit inside it (along with a sweet polished nozzle). Im thinking a SRB with loki cocktail would be pretty sweet. Then id borrow a launch tower from one of you, or maybe Ill just throw some rail buttons on it lol. Angle the tower a few degrees laterally and then let her rip!


Sorry for any spelling mistakes its just late and I'm tired lol!

 

[RGClark]

I 100% agree Troy, but I don't believe Bob's statement of "you do have to construct the rockets without using airframes" is correct, technically or otherwise.

Ok. If you’re making the argument that advanced amateurs can use straight-off-the-shelf motors with staging and without the added complexity of doing it without airframes can still reach the von Karman line, then I’ll accept your argument.

Bob Clark

 

[RGClark]

I remember the Project Prometheus from 12 years ago stating on TRF that an amateur satellite rocket was feasible. Twelve years have passed and there has been no announcement of success.
https://www.rocketryforum.com/threads/team-prometheus-n-prize-mission.2744/

The page is no longer active. It appears to have been competing for the Google Lunar X-Prize for which a private team would send a robot rover to the Moon and send back high-def video. The prize went unclaimed. I think that prize was too ambitious. Instead, what should be proposed is a prize for an amateur team getting just a cubesat size mass to orbit.

But, as they they say, before you can run, you have to learn how to walk, and before you can walk, you have to learn how to crawl.

I like the approach the USCRPL team took. They first learned how to do flights to 100,000 feet using solid motors. THEN they proceeded to do a launch to the von Karman line. Now they can aim for doing a flight to low Earth orbit.

Bob Clark

 

[FredA]

I like the approach the USCRPL team took. They first learned how to do flights to 100,000 feet using solid motors. THEN they proceeded to do a launch to the von Karman line. Now they can aim for doing a flight to low Earth orbit.

Pretty much the only approach.....
Waiting for YOU to start the journey to gain a basis for your rambles.

 

[jsdemar]

I like the approach the USCRPL team took. They first learned how to do flights to 100,000 feet using solid motors. THEN they proceeded to do a launch to the von Karman line. Now they can aim for doing a flight to low Earth orbit.

They are still 50:50 on test firing their 8" composite-cased motor. The continuity of effort and information from one year to the next is much better than the average university rocket efforts, but it's not perfect. At this point, they couldn't repeat a 300Kft+ flight. Thoughts of LEO are not even dreamed of... they're too smart and experienced to believe in such nonsense.

 

[Adrian A]

Hi Adrian,
Don't take this as gospel, but I would be sceptical about amateurs achieving both that mass fraction and Isp for the 1st stage in particular. As I mentioned previously in the thread, it should be easier to achieve both as you move up in stage count though.
I'm also a touch sceptical about your mass allowance for control systems earlier in the thread, but I can't claim any expertise with that and I'd be delighted to be convinced otherwise
Also, there needs to be an allowance for stage coupling even for 1st-order analysis IMHO.

Anyway, hats off for the contribution,

TP

I'm glad someone looked into the sheet. I'm also skeptical about the control mass, but I think it could at least conceivably be that low, and it would need to be for this scale of rocket to get the delta-V required on 4 stages.

For the mass fraction, if the very best practices demonstrated by amateurs, including composite motor case were implemented, and if the structural margins are lower than what commercial vendors are doing, I wonder how high the mass fraction could be for a small motor. I'm imagining a carbon filament-wound case with optimal angles wrapped over a thin (maybe zero) liner and end-caps of hand-laid carbon at least at the forward end. And with a nozzle designed out to the diameter of the rocket for higher vacuum Isp.

 

[bad_idea]

The page is no longer active. It appears to have been competing for the Google Lunar X-Prize for which a private team would send a robot rover to the Moon and send back high-def video.

No, they appear to have been competing for the N-Prize, as the title of the thread suggested. Team Prometheus appeared on the list of competing teams on the Wikipedia page but not the official team page. EDIT: they did have a forum on the N-Prize site: Prometheus

The N-Prize requirements were outlandish to say the least. The late Dr. Dear seems to have been something of an optimist.

 

[rocket_troy]

For the mass fraction, if the very best practices demonstrated by amateurs, including composite motor case were implemented, and if the structural margins are lower than what commercial vendors are doing, I wonder how high the mass fraction could be for a small motor. I'm imagining a carbon filament-wound case with optimal angles wrapped over a thin (maybe zero) liner and end-caps of hand-laid carbon at least at the forward end. And with a nozzle designed out to the diameter of the rocket for higher vacuum Isp.

For the 1st stage, IMHO, it would be a choice of either; choose the 250sec OR choose the mass fraction. You could probably hit the 250sec specific impulse with a hot composite propellant (no guarantees), however, you'd be relying on the nozzle quite heavily to achieve that and at seal level, that would require substantial chamber pressures ie. >1000psi or even perhaps >1500 which will obviously require a fair bit of structural containment & extra care with insulation. In fact, extra care with everything inside the walls.
Also, for an *amateur* to hit those chamber pressures with a home-brew all-composite casing is no trivial task. Certainly possible, but you need to know what you're doing especially regarding end retention and casing strain, unless you have access to a multi-axis winder and some nice software to work it.

TP

 

[PhilC]

Having experience in both sides of the university coin I have some input on this. I was in engineering at Cal Poly Pomona and worked on both the Liquid Rocket Lab and UMBRA (the solids program) during 2017-2020. I had a change of heart and moved to a biology related degree working in Neuroscience and Molecular Biology.

The biggest thing I noticed during this change is the structure of how research is conducted. Research in science, specifically the hard sciences (think biology, chemistry, physics, mathematics) is conducted over long periods of time. It has to be and that is the expectation. The Neuro research paper I am apart of right now is on its 6th year. For your paper to get published it needs to be peer reviewed by multiple experts in the related field and passed by the publisher. This involves sometimes years of edits and rewrites. It always involves a PhD overseeing the research along with multiple masters students and then some undergraduates who assist in the research. Sometimes, actually most of the time unless you are the lead researcher, you graduate before it is actually published so you miss out on being able to present your findings in national conferences. Most of this research when published leads to a new line of research that will dive deeper into the topic. Scientific research is a painstaking and lengthy process.

In engineering as many have mentioned before (besides USCRPL and a few others) it is a group of students who are most often inexperienced that have a professor who is asleep at the wheel or asking 18 year old students, who just finished their intro to aeronautics course, to turn oxygen and methane into thrust like some sort of Rocket Jesus. Granted, I don't think this is inherently a bad thing. I think getting hands on experience is important even if the project is most likely going to fail. A lot of professional engineering companies have had their fair share of failures.

Back to the point though. I think the biggest issue facing engineering rocketry programs in universities is the lack of further education. Most engineers I know in industry only received their Bachelors. This isn't bad because experience is learned on the job but for university research it is. The quality research an undergraduate can do is very limited and they ALL want to see their project launch before they graduate. Their job is to pass their classes and get a degree and go make money, not to conduct quality research in hopes of publishing it. That is for Masters and Doctoral level students. Most students that are completing a grad degree are discouraged from doing anything that has been done many times before (sounding rockets have been a thing since the 50s) or things that will cost an insane amount of money (orbital rockets would bankrupt even the wealthiest schools without serious government funding). Doctoral level students in aerospace related engineering are probably focusing on novel materials or methods to construct them. Most masters engineering students I know don't even have to do a research thesis and they usually just get the masters to get a bump in pay at the work who is probably paying for that degree.

Back to the fun stuff though. If I was gonna go to orbit I think I would need a copy of Super ROCSIM XTREME Pro. Its like ROCSIM Pro but for orbital launches. Then I can 'borrow' the decommissioned SRB at march air force base, ask Scott from Loki research to cast me something that will fit inside it (along with a sweet polished nozzle). Im thinking a SRB with loki cocktail would be pretty sweet. Then id borrow a launch tower from one of you, or maybe Ill just throw some rail buttons on it lol. Angle the tower a few degrees laterally and then let her rip!


Sorry for any spelling mistakes its just late and I'm tired lol!

As a research supervisor at a UK university I can relate to your comments. Bachelors/masters students can gain some useful practical expereince by simply participating in a rocketry society. They'll never send anything ito space, but they'll graduate knowing how to use software and workshop tools to design and build something that flies. Professors and PhD students can provide the deep and long term research to produce serious contributions to the aerospace world. These contributions tend to be quite narrow in scope due to the nature of doctoral research and the limited funding available. Only large and well-funded departments could consider building a serious space vehicle. I'd love to work in such a department but I haven't found one yet.

 

[wclaybaugh2]

For the 1st stage, IMHO, it would be a choice of either; choose the 250sec OR choose the mass fraction. You could probably hit the 250sec specific impulse with a hot composite propellant (no guarantees), however, you'd be relying on the nozzle quite heavily to achieve that and at seal level, that would require substantial chamber pressures ie. >1000psi or even perhaps >1500 which will obviously require a fair bit of structural containment & extra care with insulation. In fact, extra care with everything inside the walls.
Also, for an *amateur* to hit those chamber pressures with a home-brew all-composite casing is no trivial task. Certainly possible, but you need to know what you're doing especially regarding end retention and casing strain, unless you have access to a multi-axis winder and some nice software to work it.

TP

Troy:

I imagine you know this but because Isp is lineral and mass fraction is exponential (or logarithmic, depending on the form of the rocket equation) it is always the case that mass fraction is more important than Isp. Low pressure and very thin walls will always produce the highest performing solid motor.

Bill

 

[Adrian A]

Troy:

I imagine you know this but because Isp is lineral and mass fraction is exponential (or logarithmic, depending on the form of the rocket equation) it is always the case that mass fraction is more important than Isp. Low pressure and very thin walls will always produce the highest performing solid motor.

Bill

When you're staging, the Isp of upper stages is the biggest contributor to total mass fraction for each of the lower stages.

 

[charrington]

As a research supervisor at a UK university I can relate to your comments. Bachelors/masters students can gain some useful practical expereince by simply participating in a rocketry society. They'll never send anything ito space, but they'll graduate knowing how to use software and workshop tools to design and build something that flies. Professors and PhD students can provide the deep and long term research to produce serious contributions to the aerospace world. These contributions tend to be quite narrow in scope due to the nature of doctoral research and the limited funding available. Only large and well-funded departments could consider building a serious space vehicle. I'd love to work in such a department but I haven't found one yet.

I don't really know of one that would be willing to do it. Maybe CalTech through JPL? I don't know what kind of funding doctoral research for engineering in aerospace gets to begin with but I cant imagine its anywhere near orbital work. I'm not even sure a university would allow that to be the focus of the research anyway unless it was utilizing a new method/technology. Orbital launches have also been a thing since the mid 20th century at this point too. Governments spend billions of dollars to get these things going so it would have to be a very serious university with a very serious set of doctorates that are willing to spend 8+ years on developing a new method of getting to orbit.

Not trying to derail the convo at hand, I just find it interesting to discuss the viability of a university attempting a launch of such magnitude. I do think that it may be possible with a group of very experienced amateur's, a few professionals, and quite a few millions of dollars. The next problem would obviously getting the go ahead from the GOV to actually launch this thing. I imagine its not as simple as getting just the country of origin's approval either. I dont think the US, China, Russia, etc. would be too pleased if the LV damaged another satellite.

 

[wclaybaugh2]

When you're staging, the Isp of upper stages is the biggest contributor to total mass fraction for each of the lower stages.

Adrian:

I’m not quite sure I follow this.

Given that there is an ideal delta-v split among stages for reaching orbit, it looks to me that higher Isp in the n th stage (and the consequent lower propellant fraction given a common solid propellant among all stages) means there must be an increase in the mass of that stage and all others below it compared to a stage with minimum dry mass because of low chamber pressure and thus lower Isp.

It appears to me that one wants the highest achievable mass ratio (and thus lower Isp in this case) in the n th stage and then keeps adding lower stages until the available Isp allows reaching LEO.

Have I got something wrong here?

Bill

 

[PhilC]

I don't really know of one that would be willing to do it. Maybe CalTech through JPL? I don't know what kind of funding doctoral research for engineering in aerospace gets to begin with but I cant imagine its anywhere near orbital work. I'm not even sure a university would allow that to be the focus of the research anyway unless it was utilizing a new method/technology. Orbital launches have also been a thing since the mid 20th century at this point too. Governments spend billions of dollars to get these things going so it would have to be a very serious university with a very serious set of doctorates that are willing to spend 8+ years on developing a new method of getting to orbit.

Not trying to derail the convo at hand, I just find it interesting to discuss the viability of a university attempting a launch of such magnitude. I do think that it may be possible with a group of very experienced amateur's, a few professionals, and quite a few millions of dollars. The next problem would obviously getting the go ahead from the GOV to actually launch this thing. I imagine its not as simple as getting just the country of origin's approval either. I dont think the US, China, Russia, etc. would be too pleased if the LV damaged another satellite.

It was just wishful thinking as I slide gracefully into retirement.

 

[Adrian A]

Adrian:

I’m not quite sure I follow this.

Given that there is an ideal delta-v split among stages for reaching orbit, it looks to me that higher Isp in the n th stage (and the consequent lower propellant fraction given a common solid propellant among all stages) means there must be an increase in the mass of that stage and all others below it compared to a stage with minimum dry mass because of low chamber pressure and thus lower Isp.

It appears to me that one wants the highest achievable mass ratio (and thus lower Isp in this case) in the n th stage and then keeps adding lower stages until the available Isp allows reaching LEO.

Have I got something wrong here?

Bill

Lower chamber pressure doesn't result in lower Isp for a vacuum engine. Instead it is just is proportional to the gas exit velocity, which is related to the gas temperature and molecular weight. The reason why spacecraft use a relatively high chamber pressure even in a vacuum is to get the same thrust from a smaller throat area, which allows the nozzle and everything else to get smaller, which improves the mass fraction and packaging convenience even if the Isp difference is negligible.

For a rocket in atmosphere, having a lower chamber pressure does hurt the Isp.

https://space.stackexchange.com/que...-impact-on-isp-for-different-types-of-engines
You should think of propellant mass in stages above the currently-burning stage is just like any other dry mass in the currently-burning stage. If you can reduce the propellant mass of the third stage by improving its Isp while keeping everything else equal, it acts just like reducing the casing mass of the 2nd stage during the 1st and second stage burns, in addition to improving delta-V you get out of the third stage. This compounding benefit of efficiency in the upper stages is why so many orbital rockets go to the trouble of using a LOX/H2 upper stage like the high-Isp (~450 seconds) Centaur upper stage, when most of the impulse is provided by a high-thrust, lower-Isp (~340 seconds), LOX-kerosene-burning first stage like the Atlas and Delta series of rockets.

The ideal mass ratio between stages depends on implementation choices like how you trade aero drag loss vs. gravity loss, and how efficiently the different stages package together.

 

[wclaybaugh2]

Lower chamber pressure doesn't result in lower Isp for a vacuum engine. Instead it is just is proportional to the gas exit velocity, which is related to the gas temperature and molecular weight. The reason why spacecraft use a relatively high chamber pressure even in a vacuum is to get the same thrust from a smaller throat area, which allows the nozzle and everything else to get smaller, which improves the mass fraction and packaging convenience even if the Isp difference is negligible.

For a rocket in atmosphere, having a lower chamber pressure does hurt the Isp.

https://space.stackexchange.com/que...-impact-on-isp-for-different-types-of-engines
You should think of propellant mass in stages above the currently-burning stage is just like any other dry mass in the currently-burning stage. If you can reduce the propellant mass of the third stage by improving its Isp while keeping everything else equal, it acts just like reducing the casing mass of the 2nd stage during the 1st and second stage burns, in addition to improving delta-V you get out of the third stage. This compounding benefit of efficiency in the upper stages is why so many orbital rockets go to the trouble of using a LOX/H2 upper stage like the high-Isp (~450 seconds) Centaur upper stage, when most of the impulse is provided by a high-thrust, lower-Isp (~340 seconds), LOX-kerosene-burning first stage like the Atlas and Delta series of rockets.

The ideal mass ratio between stages depends on implementation choices like how you trade aero drag loss vs. gravity loss, and how efficiently the different stages package together.

Adrian:

My problem here is that the two variables in the rocket equation have differing effects on stage mass. I recognize that one can use higher Isp to get lower propellant (and thus stage) mass but--in the subject case--that higher chamber pressure results in a non-linear increase in dry mass. Intuitively, it looks to me like trading mass ratio for Isp can never result in lower stage mass in a solid rocket (only).

I do get that in vacuum the Isp is not much effected by chamber pressure, but I have always understood that to argue for the lowest possible chamber pressure for vacuum stages so as to minimize dry mass.

I'll try studying on this some more....

Bill

 

[cerving]

Has anyone ever achieved orbit with solid fuel only? Not just the first stage and/or the second stage where aero compensation is feasible, but 100% solid?

 

[StreuB1]

Has anyone ever achieved orbit with solid fuel only? Not just the first stage and/or the second stage where aero compensation is feasible, but 100% solid?

Minotaur less HAPS/Super-HAPS

Pegasus, 3-stages only, less monoprop 4th stage or HAPS.

 

[bad_idea]

Also several Japanese launchers, Lambda 4S, Mu-5, SS-520-5.

Edit: There's quite a list of all solid orbital launchers on Wikipedia.

 

[OverTheTop]

Also several Japanese launchers, Lambda 4S, Mu-5, SS-520-5.

Edit: There's quite a list of all solid orbital launchers on Wikipedia.

Thanks for posting the list. Good find .

I remembered ISRO had one but the rest of the list is surprisingly long, and surprising!

 

[wclaybaugh2]

My problem here is that the two variables in the rocket equation have differing effects on stage mass. I recognize that one can use higher Isp to get lower propellant (and thus stage) mass but--in the subject case--that higher chamber pressure results in a non-linear increase in dry mass. Intuitively, it looks to me like trading mass ratio for Isp can never result in lower stage mass in a solid rocket (only).

I do get that in vacuum the Isp is not much effected by chamber pressure, but I have always understood that to argue for the lowest possible chamber pressure for vacuum stages so as to minimize dry mass.

I'll try studying on this some more....

Bill

Adrian:

I am finding that for a stage exhausting to vacuum with a nozzle exit fixed at the stage outside diameter, increasing chamber pressure can produce a lower overall mass due to the increase in Isp for turbopump liquid stages only. For pressure fed liquids and for solid motors the benefit of slightly lower propellant mass due to the higher Isp is overwhelmed by the increase in structural mass caused by the higher pressure.

Bill

 

[RGClark]

Member, Neutronium95 noted that Aerotech M2050 has a 290+s vacuum Isp as modeled by OpenMotor:
https://www.rocketryforum.com/threads/orbital-space-is-25-times-harder-than-suborbital.175851/
It turns out it uses a propellant formulation called "Propellant X" by Aerotech. If you search on Thrustcurve.org you see three motors by Aerotech use Propellant X:
https://www.thrustcurve.org/motors/search.html?availability=all&text=propellant+x
The three motors are the K1103, the M2050, and the O5280. So I tried modeling an orbital rocket using these three motors. For an orbital rocket though getting high propellant fraction is critical so for each stage I lightweighted the casing to get a 0.8 propellant fraction. The modified thrustcurves are attached. The thrust data, propellant mass, diameter, and length are the same. The only number changed is the total mass since I reduced the dry mass to get the 0.8 propellant fraction. You have to copy these to the Thrustcurves folder in the OpenRocket folder.

Note again OpenRocket and RasAero can't model orbital trajectories. The best you can do is a sim that shows the rocket can reach a vertical velocity equaling the tangential, i.e., horizontal speed for orbit and at the same time reach the required altitude for space.

I found the three stages wouldn't do it. I added then side boosters consisting of 4 copies of the O5280. This reached the desired speed and altitude. The OpenRocket sim is attached. I used the recent OpenRocket version 22.02 since that was easier to model boosters. Note the boosters are actually, a separate 4th stage since they fire before the 3rd stage, not in parallel.

Bob Clark

 

[Adrian A]

Adrian:

I am finding that for a stage exhausting to vacuum with a nozzle exit fixed at the stage outside diameter, increasing chamber pressure can produce a lower overall mass due to the increase in Isp for turbopump liquid stages only. For pressure fed liquids and for solid motors the benefit of slightly lower propellant mass due to the higher Isp is overwhelmed by the increase in structural mass caused by the higher pressure.

Bill

If you hold the nozzle exit diameter fixed as you reduce pressure, I agree you could reduce dry mass. But if you need a particular thrust value for vacuum thruster and hold the thrust constant, then decreasing chamber pressure can increase dry mass because the rocket has to get physically larger for the same expansion ratio (and same Isp), because the thrust is directly related to throat area and pressure. For this application, a lower thrust would require longer burn times, which is o.k. up to a point, but the longer it takes to get to orbital speed, the more delta-V is spent on gravity losses.

 

[rocket_troy]

For pressure fed liquids and for solid motors the benefit of slightly lower propellant mass due to the higher Isp is overwhelmed by the increase in structural mass caused by the higher pressure.

Bill,
can you elaborate on that analysis; in particular the assumptions used for the structure ie. the numbers produced from bleeding edge carbon composites are going to provide more favourable numbers for greater chamber pressures than something like 6061 T6 structure.

again, I'm assuming we're talking strictly about 1st stages here.

TP

 

[wclaybaugh2]

Bill,
can you elaborate on that analysis; in particular the assumptions used for the structure ie. the numbers produced from bleeding edge carbon composites are going to provide more favourable numbers for greater chamber pressures than something like 6061 T6 structure.

again, I'm assuming we're talking strictly about 1st stages here.

TP

Troy:

A friend used to run cost analysis at NASA. He has proprietary regression equations for the relationship between motor chamber pressure and stage weight for the three classes of space propulsion systems that have extensive historical datasets.

You are correct that the level of technology changes the masses but the broad conclusion remains the same: for final stages operating in vacuum total mass is minimized at the lowest practical pressure for solids and pressure fed liquids. That is why the latter all operate at a few hundred psia. The “minimum” pressure for solid last stages does seem to have drifted up a bit over the last several decades; I don’t *know* this but my first guess would be that minimum gage issues are driving chamber pressure when one gets to composite solid upper stages.

Bill

 

[rocket_troy]

Troy:

A friend used to run cost analysis at NASA. He has proprietary regression equations for the relationship between motor chamber pressure and stage weight for the three classes of space propulsion systems that have extensive historical datasets.

You are correct that the level of technology changes the masses but the broad conclusion remains the same: for final stages operating in vacuum total mass is minimized at the lowest practical pressure for solids and pressure fed liquids. That why the latter all operate at a few hundred psia. The “minimum” pressure for solid last stages does seem to have drifted up a bit over the last several decades; I don’t *know* this but my first guess would be that minimum gage issues are driving chamber pressure when one gets to composite solid upper stages.

Bill

Okay, for final stages indeed, it's a no-brainer up to practical limitations. My guess for the minimum Pc drift would be more related to combustion efficiencies. Maybe Al % content (which historically likes high chamber pressures) has crept up over that period?

TP

 

[Neutronium95]

Member, Neutronium95 noted that Aerotech M2050 has a 290+s vacuum Isp as modeled by OpenMotor:

I did not say that. I ran some quick and dirty sims in Openmotor with a different design and came up with around a 20% difference between sea level and vacuum ISP, and applied that to several different commercial motors. The M2050 has a very high sea level ISP of 245s, and if given a larger nozzle may have a 290s vacuum isp, but that is a very rough guess, and I would not bet any money on it.

Again, you have disregarded the mass and complexity of every single system of the rocket outside of the motors. Your fantasies have little to no basis in reality.

I am certain that a sane analysis of a rocket using solid rocket motors with a usable payload of around a 3U cubesat would end up converging on something very similar to the SS-520, which is far beyond the capabilities of any amateur group. Nobody that can manufacture literal tons of APCP is an amateur.

 

[wclaybaugh2]

Okay, for final stages indeed, it's a no-brainer up to practical limitations. My guess for the minimum Pc drift would be more related to combustion efficiencies. Maybe Al % content (which historically likes high chamber pressures) has crept up over that period?

TP

Troy:

Could be, the historical data doesn’t allow any conclusions.

But let me circle around: if minimizing the mass of an upper stage solid wants the lowest practicable chamber pressure, then does it not follow that maximizing Isp in upper stages (excepting turbopumped liquids) can not lead to lower overall vehicle mass?

Bill

 

[rocket_troy]

But let me circle around: if minimizing the mass of an upper stage solid wants the lowest practicable chamber pressure, then does it not follow that maximizing Isp in upper stages (excepting turbopumped liquids) can not lead to lower overall vehicle mass?

Bill

Bill,
Well, (ignoring the extra mass from extra expansion ratio for now), I guess it depends on what your c* efficiency expectations are. If you're happy to compromise that and you don't hit other problems like combustion stability issues by operating at very low chamber pressures, then I guess there could be a trade there for upper stages, just like there would be for the 1st stage, but in reality there are practical limitations to how low you can go for the chamber. So, it depends on what's governing your lower limit, especially for the final stage.

TP