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g_boxwood
Anmeldungsdatum: 08.06.2006 Beiträge: 172
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Verfasst am: So 06 Jun 2010, 11:18 Titel: |
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I passed the LES over to Umberto to add some finishing touches and to do the e-bays for me: it is something he does extremely well so I think the LES is in very good hands.
Umberto will be adding a set of custom photo-etched details he drew and had made for the purpose:
_________________ Giacomo 'g_boxwood' Bosso
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Juerg Site Admin
Anmeldungsdatum: 27.02.2005 Beiträge: 4545 Wohnort: Oberengstringen
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Verfasst am: Mo 07 Jun 2010, 12:48 Titel: |
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Mmm, can't wait to see it, photo etch ist the way to go...
Cheers
Juerg _________________ http://www.SpacetecRocketry.com |
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g_boxwood
Anmeldungsdatum: 08.06.2006 Beiträge: 172
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Verfasst am: Do 10 Jun 2010, 17:15 Titel: |
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Flight Simulation
I'm now facing the challenge of simulating the first flight: I'm lookin for rough aestimates about what to expect.
Unfortunately, it seems that Rocksim 9 cannot find the CP when dealing with transitions only, i.e. no fins at all.
Basically I drew the entire airframe as a single stage rocket and I designed the capsule and the heatshield as conical and parabolic transistions respectively.
Fact is, no CP results from it.
I bought RSim 9 a few weeks ago, I previously owned the 5.021 version and I recall it did compute the CP from transitions only.
Am I missing something? Do I have to downgrade to version 5.021 to sim the model flying?
I also edited the motor data to account for the wasted thrust component along the sine: MMT are canted 35° with respect to the longitudinal axis and the resulting thrust loss is close to -18%.
I'm adding "_LES" motors to the engine database, i.e. CTI Pro38 3G I170_LES and AT I211W_LES.
Any hint would be greatly appreciated. _________________ Giacomo 'g_boxwood' Bosso
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Andreas Müller
Anmeldungsdatum: 28.02.2005 Beiträge: 2493 Wohnort: Altendorf
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Verfasst am: Do 10 Jun 2010, 17:35 Titel: |
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g_boxwood hat folgendes geschrieben: | I previously owned the 5.021 version and I recall it did compute the CP from transitions only. |
I wouldn't expect it to be very precise, e.g. Barrowman only works for small transition angles, something that is definitely not true here. Other synthetic methods have similar problems. But judge stability it may be good enough.
You can do Barrowman manually, as you only have a nosecone and two transitions, that shouldn't be difficult. And iPowerrocket wouldn't hesitate to do it (in fact I have an approximation to your LES as a test case for our software development, but the measures are most certainly different, so I cannot simply give you the result). Use with a grain of salt in any event. Add some extra stability margin to be on the safe side. |
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g_boxwood
Anmeldungsdatum: 08.06.2006 Beiträge: 172
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Verfasst am: Fr 11 Jun 2010, 8:26 Titel: |
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Andreas, I'll provide you with the exact measurements so you can test your software.
Fact is, I'm not checking stability issues: the design is overstable on its own.
I was simply simming it to get an aestimated altitude.
RSim simply spins it out of control upon launch as it doesn't compute any CP.
I added some fake fins but they're not enough to keep it stable even with the CG in the foremost position (tip of the nose cone).
The funny part is that version 5.021 simmed it without any problem, I'll have to get back to it or to wait for iPowerrocket simulations.
I'll get back to you with the data _________________ Giacomo 'g_boxwood' Bosso
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Andreas Müller
Anmeldungsdatum: 28.02.2005 Beiträge: 2493 Wohnort: Altendorf
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Verfasst am: Fr 11 Jun 2010, 9:11 Titel: |
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g_boxwood hat folgendes geschrieben: | I was simply simming it to get an aestimated altitude.
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But that's easy! You just take any rocket with the same diameter (can be a 3FNC) and take an artificially large C_d to account for the large base drag you expect, and off you go.
But: the next version of iPowerrocket will need to have a thrust reduction factor setting with each motor to compensate for canted motors and reduced thrust e.g. because of those silly cone retainers. |
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Juerg Site Admin
Anmeldungsdatum: 27.02.2005 Beiträge: 4545 Wohnort: Oberengstringen
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Verfasst am: Fr 11 Jun 2010, 9:36 Titel: |
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g_boxwood hat folgendes geschrieben: | The funny part is that version 5.021 simmed it without any problem, I'll have to get back to it or to wait for iPowerrocket simulations. |
The scientific value of the output still is questionable, with such "odd" designs"...
Cheers
Juerg _________________ http://www.SpacetecRocketry.com |
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g_boxwood
Anmeldungsdatum: 08.06.2006 Beiträge: 172
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Verfasst am: Fr 11 Jun 2010, 17:25 Titel: |
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I do agree, Juergh.
I wanted to check if it is Kaltbrunn-flyable and I do not really know how to do it in any other way.
All I can think is:
1. design is draggy = no coast phase + the higher the combined thrust, the higher the speed, the higher the drag (^2)
2. -18% thrust but combined thrust is around 800 N average nonetheless
3. final mass will be around 8 kilos
What do I expect? No more than 300 metres.
What it simmed out according to the 5.021 (if I recall correctly)? About 700 metres.
I'll check again but I'm more confident in our guess-timates... _________________ Giacomo 'g_boxwood' Bosso
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Juerg Site Admin
Anmeldungsdatum: 27.02.2005 Beiträge: 4545 Wohnort: Oberengstringen
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Verfasst am: Sa 12 Jun 2010, 12:14 Titel: |
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g_boxwood hat folgendes geschrieben: | I wanted to check if it is Kaltbrunn-flyable and I do not really know how to do it in any other way.
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The only unknown value is the drag. Why not find it experimentally?
You need a scale, some friction free suspension that allows the LES to be mounted on top or on the side (2m away) of a car and a straight road.
Measure the drag at a certain speed and from there it is simple to calculate the Cw value.
As far as thrust is concerned, you need to take into consideration that the motors are inclined, only a fraction (~70%) of the thrust is actually available.
So you may want to sim with a correspondingly smaller motor.
Cheers
Juerg _________________ http://www.SpacetecRocketry.com |
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