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Bug #15392

1.25M Heat Shield Moves Aerodynamic Center To Unstable Condition

Added by pieindaface almost 7 years ago. Updated over 4 years ago.

Status:
Closed
Severity:
High
Assignee:
-
Category:
Physics
Target version:
-
Start date:
05/28/2017
% Done:

100%

Version:
Platform:
Windows
Expansion:
Language:
English (US)
Mod Related:
No
Votes:
Arrow u r green
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Description

The Aerodynamic center is moved to an unstable condition when the heat shield is added to the Mk1 cockpit with added hardware(bays, fuel, etc.) below the cockpit and above the heat shield. The only stable condidtion for the heat sheild is when the heat shield is mounted directly to the MK1 Cockpit.

screenshot3.png (2.38 MB) screenshot3.png Mk1 pod with heatshield and chute. RCS to brake for reentry. Note CoL shows 'low', supposedly unstable. swjr-swis, 05/28/2017 04:19 PM
screenshot4.png (1020 KB) screenshot4.png Start of reentry, note SAS off, pod stays retrograde. swjr-swis, 05/28/2017 04:19 PM
screenshot5.png (1.55 MB) screenshot5.png End of reentry heating, note SAS off, pod has stayed retrograde. swjr-swis, 05/28/2017 04:19 PM
screenshot6.png (2.15 MB) screenshot6.png Mk1 pod with bay, heatshield and chute. RCS to brake for reentry. Note CoL shows even 'lower', supposedly more unstable. swjr-swis, 05/28/2017 04:19 PM
screenshot7.png (1.69 MB) screenshot7.png Start of reentry, note SAS off, pod stays retrograde. swjr-swis, 05/28/2017 04:19 PM
screenshot8.png (2.33 MB) screenshot8.png End of reentry heating, note SAS off, pod has stayed retrograde. swjr-swis, 05/28/2017 04:19 PM
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History

#1 Updated by swjr-swis almost 7 years ago

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1.3.0.1804 Win x64

I cannot confirm this assessment.

When tested in actual flight (reentry from LKO), the Mk1 Command Pod stays perfectly retrograde throughout the whole reentry whether with the heatshield alone, or with a 1.25m bay between the pod and heatshield. It does this even with SAS off, which proves the assembly is aerodynamically stable that way.

You are probably referring to the fact that the CoL indicator in the editor (VAB/SPH) is below the CoM indicator, and moves even lower away from the CoM when adding a bay/tank between shield and pod. In the retrograde reentry direction this would seem to put it 'in front' of the CoM, a condition that is generally said to be aerodynamically unstable.

However, the CoL indicator in the editor is only a very rough indication and only part of the equation that decides whether something will be 'stable'. The true test is in flight, and in flight the assembly remains perfectly stable in retrograde reentry (see added screenshots). Note that I tested this with the least favourable state - the bay was left empty; if anything is added to the bay, it moves the CoM down towards the heatshield and can only improve the aerodynamic stability during reentry.

Can you add a craft file that is actually unstable in flight?

#2 Updated by chris.fulton over 4 years ago

  • Status changed from Need More Info to Resolved
  • % Done changed from 0 to 100

#3 Updated by chris.fulton over 4 years ago

  • Status changed from Resolved to Closed

Closing in effort of database cleanup.

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