Unexpectedly high RASAero estimate for a two-stage rocket.

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Change the empty weight of the second stage from 3 lbs to 6 lbs, and then to 10 lbs, make equivalent changes to the first stage empty weight, and believe me, you will see a radical change in the altitude of the rocket. Rockets of this class fly like sounding rockets, the higher the propellant fraction, the greater the burnout velocity, and the higher the coast. These rocket are also very sensitive to changes in CD, more drag in the lower atmosphere, lower velocity when you get to the thinner upper atmosphere, the lower the coast distance.

You really need to nail down accurate empty weight estimates for the first and second stages. As I posted, take a look at the Loki Dart booster and other sounding rockets. But remember, almost all high power/amateur rocketeers have not demonstrated those levels of empty weights/propellant fractions.


Chuck Rogers

Sounding rockets or orbital rockets don’t use airframes or body tubes. These take up a significant fraction of the dry mass of amateur rockets. Without this, you just have the weight of the fins for a stage not intended to be recovered, which for space rockets is almost always the case. The fins make a small proportion of the dry weight of the rocket.

Bob Clark

S-Series_(rocket_family)
 
Thanks for that. By the way, why are airframes(body tubes) used in high power rocketry? Is this a holdover from the Estes model rocket days where you put a little Estes motor inside the cardboard tube that served as the body of the rocket?

When I was first reading about orbital rockets I was surprised to learn that when you’re looking at rockets meant to fly to space you are looking at the actual propellant tanks or motor casings as the outside surface of the rocket. There is no airframe around the propellant tanks or motors. The same is true for suborbital rockets. This is important of course because for rockets flying to space saving weight is paramount.

For high power rockets attempting to maximize altitude the airframes don’t appear to have any purpose. You could just as well attach the fins to the motor casing using a fin can. With sufficient care, so as not to degrade the casing strength, you could also directly weld the fins to the casing and dispense with the fin can.

Bob Clark

There are some people who make flying motor cases/sub-minimum diameter rockets; there's been a few recently if you troll back through the HPR forum section. I can't speak for anyone else, but I use motor mounts because (a) I want to be able to fly that casing in several different rockets and (b) the performance delta is not that important to me. If I were flying commercial orbital where a little optimization makes a big difference in cost and capability, then I'd put more effort into it. The chances of that happening are vanishingly small.
 
Bob:

Here's the Qu8k rocket, a rocket that was built and flown to 120,000 ft.

Link to a report on the rocket here:

https://ddeville.com/derek/Qu8k.html

It uses the motor case as the airframe, with a fin canister added over the motor case. (And RASAero II of course handles fin canisters.) In your earlier simulations, you had no fin canisters. Thus you missed the weight and drag of the fin canisters.

The problem is you assume that almost everything above the forward motor bulkhead isn't needed. Unless you are flying from a Government Range, or a private Range where there is full control of the impact area (zero personnel in the area except those associated with the launch), then you have to have a recovery system. You also need a flight data system, if you want to get data on the flight.

Rockets like the Loki Dart and other sounding rockets were/are flown from Government Ranges, so they could be flown without recovery systems, except for a recovery system for the payload, if they wanted to recover the payload.

As others have stated, there have been rockets flown with everything, including the recovery system, stuffed in the nose cone. But this is tricky in terms of the parachute deployment. And the parachute and the flight data system may not fit in the 4:1, 5:1 nose cone.

And the booster needs a recovery system also. And it needs launch shoes or rail guides for launch.

The rocket isn't the motor case, with a nose cone stuck on the top. Real rockets are stuffed with real items, which take up volume and add weight.


Chuck Rogers
Rogers Aeroscience

Qu8k Rocket.jpg
 
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In my OpenRocket sim I noticed I didn’t include any fin cant, necessary to induce spin on the rocket for stability. I was surprised, once again, that including this increased altitude significantly. I haven’t checked yet if this option is included with RASAero so don’t know if this will increase altitude with the RASAero sim.

In the new sim, I also realized that the large fins on the upper stage were pulling the CP forward. So
I reduced the upper stage fins size to allow the CP to move rearward, increasing the distance between the CG and CP.

The modified OpenRocket sim is attached.

Bob Clark


O8000_to_N1100_ver_3.jpg






View attachment O8000 to N1000, ver3.ork
 
On a reddit discussion on the issue, someone raised the example of the FourCarbYen rocket by Jim Jarvis as an example of the highest altitude you could get with commercial motors, ca. 120,000 feet:

[video=youtube;mWOicBydGzc]https://www.youtube.com/watch?v=mWOicBydGzc[/video]

However, I noted quite heavy airframes were used:

Screen_Shot_2018_05_01_at_12_55_05_PM.png



The weight of the two airframes on the rocket was 32.7 pounds out of 90.9 pounds. So I wondered how high we could get without the airframes as these are not used for suborbital or orbital rockets.

The motors were the N5800 and N1100 motors. Their total weight was only 58 pounds. I did an OpenRocket sim using these motors with no body tube, i.e., airframe weight. I didn't emulate the fins or nose cone since I only wanted to get an idea of the altitude possible:

Modified_Four_Carb_Yen.jpg




Bob Clark

View attachment Modified FourCarbYen - N5800 to N1100.ork
 
In my OpenRocket sim I noticed I didn’t include any fin cant, necessary to induce spin on the rocket for stability. I was surprised, once again, that including this increased altitude significantly. I haven’t checked yet if this option is included with RASAero so don’t know if this will increase altitude with the RASAero sim.

RASAero doesn't include fin cant. The effect of fin cant can be included by running the rocket on RASAero using the Run Test feature (described on Pages 68-74 in the RASAero II Users Manual) with an angle of attack equal to the coning angle of the spinning rocket. 3.5 deg is a good typical angle to use. The ratio of the CD at 3.5 deg angle of attack relative to the CD at 0 deg angle of attack, on average over the Mach number range of the rocket, is then used to increase the frontal area of the rocket by that ratio by scaling the rocket up. If the CD at 3.5 deg angle of attack relative to 0 deg angle of is 10% higher, then the rocket is scaled up so the frontal area of the rocket is 10% higher. For these runs use zero wind, as there is no attempt being made to model the dynamics of the spinning rocket, just the increase in drag from the coning angle. The increase in drag from the small fin cant itself is minor.

This technique was used in a tech article I publishing in High Power Rocketry Magazine for a 94,000 ft altitude rocket, with the article available on the RASAero web site here (OuR Project R Rocket):

https://www.rasaero.com/dl_technical_reports.htm


Chuck Rogers
Rogers Aeroscience
 
On a reddit discussion on the issue, someone raised the example of the FourCarbYen by Jim Jarvis rocket as an example of the highest altitude you could get with commercial motors, ca. 120,000 feet:

[video=youtube;mWOicBydGzc]https://www.youtube.com/watch?v=mWOicBydGzc[/video]

However, I noted quite heavy airframes were used:

Screen_Shot_2018_05_01_at_12_55_05_PM.png



The weight of the two airframes on the rocket was 32.7 pounds out of 90.9 pounds. So I wondered how high we could get without the airframes as these are not used for suborbital or orbital rockets.

The motors were the N5800 and N1100 motors. Their total weight was only 58 pounds. I did an OpenRocket sim using these motors with no body tube, i.e., airframe weight. I didn't emulate the fins or nose cone since I only wanted to get an idea of the altitude possible:

Modified_Four_Carb_Yen.jpg




Bob Clark

The nose cone, which you keep overlooking, is the biggest contributor to the area rule.

The fins, which you keep overlooking, are the second largest contributor to the area rule.
 
FWIW, I tweaked your OR sim and optimized upper stage mass and ignition delay, and got an altitude of 392K feet. So, if you believe OR and your impossibly low airframe masses, you could get above 100 km with these motors. Of course I don't for a second believe that one could build something at this weight that would hold up to this motor combination, much less be recoverable, so I'm not sure what this proves, if anything. (BTW, I was using the filename you originally used which suggests you were using an N1000 for the upper stage, but all of your sims are with the N5800 for the upper stage. The N1000 would be a much easier motor to survive but only sims to 262K feet.)

View attachment 343854

View attachment 343853

OpenRocket seems to give higher altitude with fin canting. What altitude do you get with fins canted 2 to 3 degrees?


Bob Clark
 
Thanks for that. By the way, why are airframes(body tubes) used in high power rocketry? Is this a holdover from the Estes model rocket days where you put a little Estes motor inside the cardboard tube that served as the body of the rocket?

When I was first reading about orbital rockets I was surprised to learn that when you’re looking at rockets meant to fly to space you are looking at the actual propellant tanks or motor casings as the outside surface of the rocket. There is no airframe around the propellant tanks or motors. The same is true for suborbital rockets. This is important of course because for rockets flying to space saving weight is paramount.

For high power rockets attempting to maximize altitude the airframes don’t appear to have any purpose. You could just as well attach the fins to the motor casing using a fin can. With sufficient care, so as not to degrade the casing strength, you could also directly weld the fins to the casing and dispense with the fin can.

Bob Clark

What would you attach your fins to Bob? You either have a Duncan that slides over the motor casing, or you are welding fins to the motor case. I would worry about distorting/weakening the case.
 
OpenRocket seems to give higher altitude with fin canting.
It made very little difference to my altitude. Spinning the rocket is good for reducing dispersion, but if the rocket is flying vertical it should make little difference to the maximum altitude.

I suspect that the odd aspects of your design file are just exposing bugs in the simulation software. It's important not to confuse bugs in software with real rocket behavior.
 
It's important not to confuse bugs in software with real rocket behavior.

It's important not confuse the limitations of the mathematical models you are manipulating with bugs.

If you take a model outside the bounds of its assumptions you'll get numbers, but probably not ones that reflect physical reality.
 
On a reddit discussion on the issue, someone raised the example of the FourCarbYen by Jim Jarvis rocket as an example of the highest altitude you could get with commercial motors, ca. 120,000 feet:

[video=youtube;mWOicBydGzc]https://www.youtube.com/watch?v=mWOicBydGzc[/video]

However, I noted quite heavy airframes were used:

Screen_Shot_2018_05_01_at_12_55_05_PM.png



The weight of the two airframes on the rocket was 32.7 pounds out of 90.9 pounds. So I wondered how high we could get without the airframes as these are not used for suborbital or orbital rockets.

The motors were the N5800 and N1100 motors. Their total weight was only 58 pounds. I did an OpenRocket sim using these motors with no body tube, i.e., airframe weight. I didn't emulate the fins or nose cone since I only wanted to get an idea of the altitude possible:

Modified_Four_Carb_Yen.jpg




Bob Clark
I find it a bit annoying that you keep pointing out orbital rockets don't use airframes to justify ridiculous simulations.

Orbital rockets don't use commercial hobby motors either, they use steel wall motors at higher pressures with metal nozzles, and often they don't use fins they have gimballed motors or other active controls, and they're components are designed to be multipurpose which allows the tanks to double as structural members. And they still have some airframe/skins/structural components to hold all the tanks together.

So when people point out your wrong to assume you don't need an airframe, or a proper nose cone, recovery gear, proper fins, you should listen.
 
Comparing commercial orbital vehicles to ours will show the difference.

Many are steel, which can take higher pressure and heat.

The real kicker with orbital vehicles versus our stuff is mass fraction. The ratio of propellant mass to total mass. Most of our motors forgetting the rocket vehicle are around 50%. Orbital vehicles tend to be much higher...I did some looking into this at one time, and as I recall the orbital vehicles were closer to 85-90%. This is a massive difference. This is why an amateur is likely to never accomplish an orbital shot. At least not in the foreseeable future.

Ask yourself how many nations can successfully launch a satellite? I do not have an exact number, but I doubt it is more than 15. There are a lot of reasons for that. Money being a large portion of this.
 
What would you attach your fins to Bob? You either have a Duncan that slides over the motor casing, or you are welding fins to the motor case. I would worry about distorting/weakening the case.

Using a fin can would be the simplest option for amateurs. However, using a flange at the base of the fin to give a larger bonding area, they can be welded or riveted to the motor case. If welded you would have to have it be done by someone skilled with aluminum welding.


Bob Clark
 
Using a fin can would be the simplest option for amateurs. However, using a flange at the base of the fin to give a larger bonding area, they can be welded or riveted to the motor case. If welded you would have to have it be done by someone skilled with aluminum welding.


Bob Clark

Over the years several amateurs have attached their fins directly to the motor case. They have used screws, welding, aluminum soldering, and high performance adhesives. These truly minimum diameter rockets are nothing new, even if they are different from what most of us do. They have typically been flown at BALLS, FAR, or RRS launches. Some have been documented on TRF.
People also make their own motors using composite cases.
To me, these advanced projects are a fascinating survey of what can be done, proving it’s stupid to dismiss “amateurs”, but most of us will always continue to use more traditional construction techniques because we want to fly frequently for recreation. It’s those differences between us that make this such a great hobby.
 
I can absolutely, 100% guarantee you that this nose cone behavior is nonphysical. In reality, you'll want a much longer, narrower nose cone to minimize supersonic drag. It's also notable that for your rocket, max q will occur at a quite high speed at a low altitude, and the Q at this max Q will be exceedingly high.

I modeled the same rocket in RASAero, changing the nose cone to short, squat shapes and this decreased the altitude. So I think you're right, OpenRocket needs some tweaking in the nose cone drag calculations.


Bob Clark
 
Over the years several amateurs have attached their fins directly to the motor case. They have used screws, welding, aluminum soldering, and high performance adhesives. These truly minimum diameter rockets are nothing new, even if they are different from what most of us do. They have typically been flown at BALLS, FAR, or RRS launches. Some have been documented on TRF.
People also make their own motors using composite cases.
To me, these advanced projects are a fascinating survey of what can be done, proving it’s stupid to dismiss “amateurs”, but most of us will always continue to use more traditional construction techniques because we want to fly frequently for recreation. It’s those differences between us that make this such a great hobby.

Thanks for that. There is another advanced welding technique I just remembered called "friction-stir welding". This is now often used in aerospace as it maintains the original strength of the metals. It works by not melting the metals to be joined to liquid, but by heating them to plasticity:

[video=youtube;EaguF5K9I-Q]https://www.youtube.com/watch?v=EaguF5K9I-Q[/video]


Two universities, Princeton and USC, are planning to make an attempt at the 100 km Karman line later this month. I think it is the work in high power rocketry by independent amateurs that led universities teams to also make flights to high altitude.

Then if and when university teams succeed in making the next step of a flight to space, the independent amateurs with their HPR flights can be regarded as the progenitors of these flights to space. In that case, they should be regarded as making an important contribution to science beyond that of just their work with their hobby.

Bob Clark
 
Comparing commercial orbital vehicles to ours will show the difference.

Many are steel, which can take higher pressure and heat.

The real kicker with orbital vehicles versus our stuff is mass fraction. The ratio of propellant mass to total mass. Most of our motors forgetting the rocket vehicle are around 50%. Orbital vehicles tend to be much higher...I did some looking into this at one time, and as I recall the orbital vehicles were closer to 85-90%. This is a massive difference. This is why an amateur is likely to never accomplish an orbital shot. At least not in the foreseeable future.

Ask yourself how many nations can successfully launch a satellite? I do not have an exact number, but I doubt it is more than 15. There are a lot of reasons for that. Money being a large portion of this.

The example of the FourCarbYen rocket shows just not having an airframe can greatly reduce the dry weight. For instance in that example, the airframes weigh more than the dry weight of the motors. Then the Princeton rocket sims, not having airframes, show it can reach the Karman line as a two-stage using commercial motors.

The USC attempt at the Karman line will be single stage. But it will use a much larger homemade motor. It will also use carbon fiber casing. This will save half off the weight over an aluminum casing.

That's for reaching suborbital space. Carbon fiber casings can give propellant fractions in the range of 80% and above. If you do a rocket equation calculation then you can calculate by using three or four stages you can actually reach orbital space.

Bob Clark
 
Carbon fiber casings can give propellant fractions in the range of 80% and above. If you do a rocket equation calculation then you can calculate by using three or four stages you can actually reach orbital space.

It turns out that if you are going into orbital space at orbital speeds the people you're orbiting over generally want orbital steering to avoid catastrophic orbital debris fields. Well characterized propellant families and known structural techniques are complicated but not especially complex.
 
If you do a rocket equation calculation then you can calculate by using three or four stages you can actually reach orbital space.
It's clear that multistage solid rockets can put payloads in orbit. There are many examples of this having been done going back to the early 60s.

What's far less obvious is that this is now "easy" as your signature claims. In my opinion it's still out of the reach of a group without a lot of technical expertise and funding resources, and nothing in this thread has caused me to think otherwise.
 
Thanks for that. There is another advanced welding technique I just remembered called "friction-stir welding". This is now often used in aerospace as it maintains the original strength of the metals. It works by not melting the metals to be joined to liquid, but by heating them to plasticity:

[video=youtube;EaguF5K9I-Q]https://www.youtube.com/watch?v=EaguF5K9I-Q[/video]


Two universities, Princeton and USC, are planning to make an attempt at the 100 km Karman line later this month. I think it is the work in high power rocketry by independent amateurs that led universities teams to also make flights to high altitude.

Then if and when university teams succeed in making the next step of a flight to space, the independent amateurs with their HPR flights can be regarded as the progenitors of these flights to space. In that case, they should be regarded as making an important contribution to science beyond that of just their work with their hobby.

Bob Clark

Bob, how is this an important contribution to science? I am not trying to diminish the accomplishment, but it is not significant from a scientific standpoint. The Germans broke the Karman line with the V2. Last time I checked that was almost 75 years ago. It has been done many times since. The technology is not really new.
 
The example of the FourCarbYen rocket shows just not having an airframe can greatly reduce the dry weight. For instance in that example, the airframes weigh more than the dry weight of the motors. Then the Princeton rocket sims, not having airframes, show it can reach the Karman line as a two-stage using commercial motors.

The USC attempt at the Karman line will be single stage. But it will use a much larger homemade motor. It will also use carbon fiber casing. This will save half off the weight over an aluminum casing.

That's for reaching suborbital space. Carbon fiber casings can give propellant fractions in the range of 80% and above. If you do a rocket equation calculation then you can calculate by using three or four stages you can actually reach orbital space.

Bob Clark

Bob, I am not saying it is impossible. Everything needs to be perfect. Suborbital is easy compared to orbital. Guidance, significantly higher velocities etc. it is not easy, and you saying it is possible does not mean it is going to happen. Frankly, if it is so doable why don’t you do it? Talk is cheap.
 
...
The weight of the two airframes on the rocket was 32.7 pounds out of 90.9 pounds. So I wondered how high we could get without the airframes as these are not used for suborbital or orbital rockets.

The motors were the N5800 and N1100 motors. Their total weight was only 58 pounds. I did an OpenRocket sim using these motors with no body tube, i.e., airframe weight. I didn't emulate the fins or nose cone since I only wanted to get an idea of the altitude possible:

Modified_Four_Carb_Yen.jpg


The two motors N5800 and N1100 cost about $2,000 together, and weigh about 27.6 kg together. I found a 4-fin aluminum fin can that costs about $200 at about 1 kg weight so two would be about $400 at 2 kg weight. A carbon fiber one would be about half the weight and twice the cost. The nosecone can be had for about $40, at a quarter-kilo weight.

So all up you have a suborbital space rocket to exceed the 100 km von Karman line at a ca. $3,000 cost and ca. 30 kg weight.


Bob Clark
 
Bob, how is this an important contribution to science? I am not trying to diminish the accomplishment, but it is not significant from a scientific standpoint. The Germans broke the Karman line with the V2. Last time I checked that was almost 75 years ago. It has been done many times since. The technology is not really new.

The going rate for suborbital rockets is in the range of 1 to 2 million dollars per launch:

The commercial suborbital sounding rocket market: a role for RLVs?
by John M. Jurist Monday, October 13, 2008
The current total US market for high altitude sounding rockets with payloads in the 50 to 200 kilogram range and apogees in excess of 100 kilometers is roughly 100 launches annually. At an average of one million dollars charged per launch, one might conclude that a real market exists for RLVs filling this niche.
https://www.thespacereview.com/article/1228/1

The Virginia Tech suborbital space rocket costs about $11,000 total:

0f5ee2a6eca6e9dcd23b44c10cce759a_original.png

0f5ee2a6eca6e9dcd23b44c10cce759a_original.PNG

Funding Needed: $5,838
0f5ee2a6eca6e9dcd23b44c10cce759a_original.PNG

https://www.kickstarter.com/projects/2095728333/olvt-hokie-06-rocket

And the Princeton rocket costs about $20,000:

https://rockets.princeton.edu/spaceshot

Note that once you have a suborbital rocket, then it can be used as the first stage of a multi-stage orbital rocket. This is in fact how the first U.S. orbital satellite Explorer I was launched on the Juno I rocket.

Now since the first stage of an orbital rocket makes up the bulk of the cost of the rocket, because of its much larger size than the other stages, if the cost of this stage can be cut by a factor of 100, the cost of the full launcher quite likely can be also.

The result: flights to orbital space at 1/100th the cost than previously, at least for small payloads.
Note that with modern micro-miniaturization, important observations can be made with satellites at such small scales.


Bob Clark

 
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So all up you have a suborbital space rocket to exceed the 100 km von Karman line at a ca. $3,000 cost and ca. 30 kg weight.

No, for $3000 you have a rocket that will destroy itself before it reaches 1% of the target altitude. A simulation program does not handle strength of materials, fin flutter, or a multitude of other dynamic limitations.

Your optimistic cost estimate - and cartoon-like design - does not include onboard electronics for staging or tracking, or a place to carry them.
 
The two motors N5800 and N1100 cost about $2,000 together, and weigh about 27.6 kg together. I found a 4-fin aluminum fin can that costs about $200 at about 1 kg weight so two would be about $400 at 2 kg weight. A carbon fiber one would be about half the weight and twice the cost. The nosecone can be had for about $40, at a quarter-kilo weight.

So all up you have a suborbital space rocket to exceed the 100 km von Karman line at a ca. $3,000 cost and ca. 30 kg weight.


Bob Clark
I'm starting a Kickstarter tomorrow to turn LiquidFyre Rocketry into a commercial suborbital / orbital launch provider. Should only cost about half what I've already invested in development of my camera system and be millions of times more profitable. I probably can even keep working out of my basement... If I knew it was so easy I would have gone that route years ago instead of wasting my time working hard as an engineer. Thanks Bob!
 
The going rate for suborbital rockets is in the range of 1 to 2 million dollars per launch:

The commercial suborbital sounding rocket market: a role for RLVs?
by John M. Jurist Monday, October 13, 2008
The current total US market for high altitude sounding rockets with payloads in the 50 to 200 kilogram range and apogees in excess of 100 kilometers is roughly 100 launches annually. At an average of one million dollars charged per launch, one might conclude that a real market exists for RLVs filling this niche.
https://www.thespacereview.com/article/1228/1

The Virginia Tech suborbital space rocket costs about $11,000 total:
<snip>

Now since the first stage of an orbital rocket makes up the bulk of the cost of the rocket, because of its much larger size than the other stages, if the cost of this stage can be cut by a factor of 100, the cost of the full launcher quite likely can be also.

The result: flights to orbital space at 1/100th the cost than previously, at least for small payloads.
Note that with modern micro-miniaturization, important observations can be made with satellites at such small scales.


Bob Clark


There are two really obvious problems which I see here.

The first is that you talk of carbon fibre motor cases and no air frames and 1mm fins to get the performance to back up your idea, so basically a motor with fins. But then talk about using it to launch a multi stage rocket with a payload, so it will have no where near the performance you are using as a baseline, even before the necessary increase in weight to allow for the heavier load on top of the rocket.

Second, you pull these sorts of costs out. Where is the R&D and labour budget? Of course prices look cheap when you just cost up a very basic BOM, but who is going to turn that into a payload lifting machine for not just free, but at their cost once development is allowed for?



It seems strange for a mathematician to apply a linear difficulty scaling for a task that clearly is not.
 
Another thing that is consistently ignored is that suborbital flights can still be valuable with passive stability where for an orbital payload guidance and orientation/throttle control is necessary to achieve a specific orbit.

On a slightly related note, I have decided not to go through with the LiquidFyre Space Systems Kickstarter after a few moments of consideration revealed massive and glaring fatal flaws in the logic and calculations underlying the business plan.
 
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