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Skating the Edge of a Theoretical Curve...

Posted by: mikelong - Fri Sep 25, 2009 8:07 pm
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Skating the Edge of a Theoretical Curve... 
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Post Skating the Edge of a Theoretical Curve...   Posted on: Fri Sep 25, 2009 8:07 pm
Hello all... first time caller, long time listener...

I just thought I would take a moment to introduce myself. Up until about a year ago I was the lead aerodynamics engineer for TGV Rockets, the smaller, less talked about X-Prize contender based out of Oklahoma (Rocketplane being the other). About the time of Spaceship One, we scrapped the space tourism idea and shifted focus to a sub-orbital VTVL observation and micro-gravity platform, worked that up through PDR and CDR and then promptly ran out of steam... and by steam I mean money.

Since then, I've been working as an applications engineer for a small company producing simulation tools for fixed wing aircraft design. Unfortunately, I haven't been able to shake the rocket bug, so I'm doing some back burner simulations based on data that I've been skimming from the AA posts. Some highlights from this work can be found on my blog here:

http://specificimpulses.blogspot.com

My ultimate goal is to build a tool set that will make it much easier to do aerodynamic design and propulsion integration for new launch vehicles, specifically, filling GNC lookup tables and predicting engine power on effects. I think I've got a pretty good start on it, but there is still a lot more to do and not that much free time to devote to it.

I should mention that I'm not expecting any interest from Armadillo in what I'm doing. John and Matt came to visit TGV in the fall of 2005 and at the time John expressed his disdain for simulation, Computational Fluid Dynamics being especially offensive to him. While I can't help see the irony in his dislike of computer simulations, I respect his intellect and can't really mount a decent argument against his build/fly/break/rebuild-better development philosophy. However, the years I spent trying to close the return-to-launch-site-in-military-spec-all-weather-conditions problem with a VTVL design gave me what I believe is a good perspective on the challenges they will face as they begin to explore the flight envelope.

Ultimately though, it's not that important for AA to care one way or the other about what I'm doing. My goal is simply to show that it CAN be done and done accurately enough for clean sheet design, not just for tweaking out a few counts of drag here and there on an existing vehicle. Of course I would love for them to provide me with some occasional feedback, but if they don't I'll probably be lobbing data over the wall at them anyway.

In the end if it turns out that I was way off the mark, I'll eat my words and return to the aforementioned drag count tweaking. Either way, I intend to have fun with it and hopefully provide some interesting reading for the rocket community at large in the process.

Thanks for listening!

Mike


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Sat Sep 26, 2009 7:08 am
Hi, welcome to the forum.

Its a pity about TGV, money is the real rocket fuel after all.

The rocket bug is a hard one to shake, I am pretty sure everyone here will be looking forward to your contributions, even if the vehemently oppose them :)

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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Sat Sep 26, 2009 7:46 am
I do not share Mr. Carmack's reputed disdain of simulations. On the contrary! When I could do no hands-on experimentation, I did simulations in my head! I used to be good at it, too, not that it matters, now. In any event, I look forward to your contributions.


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Mon Sep 28, 2009 3:43 am
Thanks for the warm welcome guys!

I really do have some things planned that should be very interesting, and not just for the pretty pictures! My biggest obstacle right now is just time. That.. and the fact that like many working class engineers I do all of my programming in Fortran, so I've taken on the additional task of learning some Python so when I ultimately want to do something besides crunch numbers, like say... talk to a database or create a nice UI, I won't be pushing a cart with square wheels.

My ultimate goal is to have something akin to STK, but for sub-orbital missions where aerodynamics consume substantial delta-v and the engine plume interacts with the flow field in unpredictable ways, especially for vehicles with ganged engines in a stage. I've already proven to myself that I can write a fast and reasonably accurate trajectory integrator, along with a set of pre and post-processing tools for controlling the CFD solutions and collecting the force and moment data for use in the trajectory. Now I just have to re-write the whole thing from scratch, making it more modular and robust and tying everything together to run on a cluster without too much hand-holding. Sounds like a piece of cake right?

Not to mention the fact that I have a regular job and a family to look after.. :)


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Tue Feb 09, 2010 7:24 pm
In case anyone is interested, I've posted a predicted trajectory for an 'approximate' Armadillo Six-Pack at specificimpulses.blogspot.com . I say 'approximate' because I made very quick and dirty estimates of all of the vehicle specs and probably missed the mark by quite a bit. Honestly, I didn't have the time to trawl the old AA posts looking for better numbers.

Here are the highlights:

====================================
T-plus at engine cut-off = 138.89 sec
Altitude at engine cut-off = 96824.6 ft
Mach number at engine cut-off = 2.46
T-plus at apogee = 194.37 sec
Altitude at apogee = 149988.8 ft
Max q = 252.3 psf : T-plus = 87.22 sec : 32480.7 ft
Take-off weight = 7366.7 lbm
Take-off thrust/weight = 1.25
Total dV delivered by engines = 12893.6 fps
Total dV lost to gravity = 6227.5 fps
Total dV lost to aero drag = 6666.5 fps
====================================

If anyone sees any glaring issues with the inputs or has better data for the vehicle then let me know and I'll update the trajectory. It looks like the Six-Pack isn't the direction that they are going now, but I had a flow solver grid ready for it so it was the easiest to get working.


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Thu Feb 11, 2010 10:26 am
At one point it was 85lb per 3' sphere, and about 100lbs per Pixel engine with valves, gimbals etc. which would obviously make your dry mass significantly too low. Obviously the differentially throttled engines are much lighter and the spheres were eventually going to be depending on the pressure, under a half the mass. Maybe ~110lbs per module? But including the polycarbonate sphere, the payload is a bit more too. Really, it would depend when they built it and what they wanted it to weigh and how badly, but as you said they not building it - so it's a bit up in the air. I don't really absorb the bigger less frequent updates very well, so I've probably forgotten something important anyway.

Thinking about a spectrum of alternate histories where this got built doesn’t seem to have made things much clearer either :?

About my only criticism so far is that I really need to convert to metric to appreciate this :-)

To the extent that you can isolate it from other things, I'm far more interested in the Cd vs Mach number curve anyway. Very interesting, lower Cd than I might have feared and also rather less smooth than I'd have thought!

I suppose another of the interesting things to look at would be the aero-interactions with the control system and the downward dynamics. It's a real shame these things take such a huge amount of CPU time.

Also, pretty renderings.

Nick


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Thu Feb 11, 2010 5:41 pm
Thanks for the great comments!

I knew I was being very generous on a lot of points, but I wanted to make sure that it performed well enough to cover a decent range of Mach numbers and engine pressure ratios. Don't take the Cd numbers too seriously either. I didn't do a rigorous frontal area calculation for the model, so the Cd could be skewed one way or the other. I was consistent with my use of the area, so the absolute forces are correct. The jaggedness of the drag plot has several factors, the most significant being that I probably needed to use a smaller mach number increment in the transonic region. I also need to write more feedback into the routine that controls the CFD solution, since there are probably cases in there that did not converge before I pulled the Cd out.

This was really just a trial run!

My plans for the future are to fix the issues above and then to start tackling the control moments, but that means at least an order of magnitude more time in the solver doing alpha sweeps for each trajectory point... and I only have 16 processors.. :(

As far as the units go, I'm fine SI (the unit system, not the magazine, although the magazine is OK too), but alas most industry in the US is stubbornly resistant to change in that department. Go Slugs!


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Sat Feb 13, 2010 3:29 pm
Just had a go looking at other thrust profiles with some approximation of your Cd data. I’m additionally assuming constant ISP at 92% vacuum and Cd not changing with thrust (!). That gives about the same altitude from sea level as you get. Having the fuel consumption rate wind down by 40% during the run gives 55 km up from 46 km. Being able to optimally reduce it up to 50% within that profile gets to 67 km. The same thing, but with 50% more initial thrust allows ~89km. Additionally, starting from somewhere like Clinton Sherman instead of sea level is enough grace to potentially reach 100 km(definitely falling of the end of the Cd data there). That’s not really much of a surprise given the big mass ratio. Haven’t yet seen the thrust profiles for all these, but it’s to be expected that I’ll find a huge mistake tomorrow and recant excessively.

Anyway, all is well. Just need to increase the imaginary tank pressure and drive it to Las Cruces. Good thing that Cd data was completely accurate, I couldn't cope with any more approximations.
mikelong wrote:
since there are probably cases in there that did not converge before I pulled the Cd out.
Damn you good sir! :wink:
mikelong wrote:
doing alpha sweeps for each trajectory point
I suppose there's no great need to do that all in one go. Hope it goes well.


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Mon Feb 15, 2010 6:38 pm
It sounds like you've been having a lot of fun with those numbers, Nick! There are definitely a lot of things I'd like to experiment with once I get the solver automated completely and fill the Cd vs Mach vs EPR curves. Putting in a blow-down thrust profile is definitely needed. I've actually been keeping a laundry list of features that I am planning to add as time permits, so I'll probably post that over on the blog soon. I'm also going to switch my efforts to the Scorpius module since I can make mods to the aeroshell for that geometry a lot easier than the Sixpack (and it's closer to what they are actually flying right now)

If you want the Cd curves in a more usable format then e-mail me and I'd be happy to send you the look-up tables in a text file.

mikelong1971@gmail.com


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Post Re: Skating the Edge of a Theoretical Curve...   Posted on: Tue Feb 16, 2010 7:10 pm
mikelong wrote:
It sounds like you've been having a lot of fun with those numbers, Nick!
You may have caught me having a J F Sebastian "I make my friends" moment. :)
mikelong wrote:
There are definitely a lot of things I'd like to experiment with once I get the solver automated completely and fill the Cd vs Mach vs EPR curves. Putting in a blow-down thrust profile is definitely needed. I've actually been keeping a laundry list of features that I am planning to add as time permits, so I'll probably post that over on the blog soon.

One of the things that might be important, but harder to guess at, is how the vacuum ISP varies with propellant consumption. Injectors generally work better at higher pressure, but the greatest portion of the envelope expansion I'm seeing comes from operating for fairly long periods at reduced thrust. Fortunately some of that is happening at low ambient pressure, but even a 10% hit on ISP would alter the reality of what can be achieved just through modulation fairly radically.

I actually just tried to put some nod to a linear reduction in vacuum ISP based on that, to see what happens. Took a few minutes, when I could have been thinking about it for ages, realizing all the bad things that would inevitably result.

The thrust curve started breaking up into about six different discrete lines. The lines also overlap in time. That looked like a bug. The upshot is that there isn't a bug, but I average six thrust values to every one drawn. The optimizer followed the rules and PWM'd the engine high and low to modulate the thrust without as big a penalty. I'd noticed the possibility of that game if it were possible to turn the engine off(causing some angst), but for some reason it wasn't nearly as obvious when you're just able to reduce thrust but not stop it. It still doesn't seem quite obvious as it should, and more ridiculously, anticipating it must have been a reason I didn't try it when the program was first written.

Another approach would be to add another variable to the state, so that there's a limit to how fast valves can be slew. That wrecks the 'elegance' of the approach, but would fix up most of the noise in the thrust curves. Yet another possibility would be to allow fast slewing but limit the total amount of slew involved. Either one could also make things slower in every way. Still have to explicitly put in the atmospheric losses.

Anyway - Manual optimization has got things going for it.

mikelong wrote:
I'm also going to switch my efforts to the Scorpius module since I can make mods to the aeroshell for that geometry a lot easier than the Sixpack (and it's closer to what they are actually flying right now)
In light of the shifting ISP due to changing combustion efficiency and in atmospheric pressure working overexpanded, would be interesting to see what you think the total delivered impulse for the LLC was :-)
mikelong wrote:
I'd be happy to send you the look-up tables in a text file.
Thanks.


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