02-25-2017, 06:44 PM | #11 | |
Join Date: Aug 2008
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
__________________
Buy My Stuff! Free Stuff: Dungeon Action! Totem Spirits My Blog: Above the Flatline. |
|
02-25-2017, 07:36 PM | #12 |
Join Date: Feb 2005
Location: Berkeley, CA
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
In practice, any spaceship that isn't huge can achieve however much drag you really want just by going deeper in the atmosphere, but it may not survive the process.
|
02-26-2017, 02:01 AM | #13 | |
Join Date: Aug 2008
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
It is also worth noting that the portion of the ship facing forward during aerobraking should generally be blunt to keep the shockwaveoff of the craft's frame. Otherwise, the craft will be subjected to substantially higher stresses and heat than otherwise. Laatly as a follow-up on drag coefficients, a reasonable number would probably fall between around 0.4 ish for an Apollo-style capsule down to as little as 0.04 for an extremely thin, perfectly smooth airfoil-shape. The latter is pretty unlikely to be achieved, though, if for no other reason, no craft will be that smooth. I could believe in the 0.1 ti 0.2 range readily enough, though.
__________________
Buy My Stuff! Free Stuff: Dungeon Action! Totem Spirits My Blog: Above the Flatline. |
|
02-26-2017, 02:28 AM | #14 |
Join Date: Feb 2005
Location: Berkeley, CA
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Coefficient of drag varies with mach number, I wouldn't take the subsonic numbers as indicative of performance in a mach 40 aerobrake.
|
02-26-2017, 08:09 AM | #15 |
Join Date: Aug 2004
Location: traveller
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Drag for aerobraking is most efficient from blunt rather than streamlined surfaces. A flat plate perpendicular to the airflow has a Cd ~ 2.0. SMAD III (p. 145) recommends 2.2 for a generic satellite.
|
02-26-2017, 09:48 AM | #16 |
Join Date: Aug 2008
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
What you reference in SMAD III in section 6.2.3 is perterbations due to atmospheric drag that cause orbital decay in satelites. Furthermore, it includes drag from extended solar arrays and does not even address aerobreaking. All of that said, it is pretty obvious that increasing any of Cd, A, or rho (although you can't affect rho with spacecraft design in any meaningful way) will increase the drag force felt by a reentering craft, but intentiinal atmospheric injection with extended solar panels seems like a good way to lose your solar panels. So again, the broad blunt plate for aerobreaking is primarily there to keep shockwaves off the airframe, allowing for lighter construction. The side benefit is slowing down faster, but deceleration will happen no matter what.
__________________
Buy My Stuff! Free Stuff: Dungeon Action! Totem Spirits My Blog: Above the Flatline. |
02-26-2017, 10:22 AM | #17 | |
Join Date: Aug 2004
Location: traveller
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
Edit to add: You should look at the history of the Magellan spacecraft. Last edited by thrash; 02-26-2017 at 10:26 AM. |
|
02-26-2017, 11:24 AM | #18 | |
Join Date: Aug 2008
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
I have yet to say air drag is not the chief force in aerobreaking. In fact, I specifically called it out as providing a method for finding the time to land, as the OP asked. I simply contend that the figures you are using are misleading. I am also pointing out that the predominant reason to have a blunt nose is to avoid excessive stresses from the bow shock. If you want a faster descent, you need more weight in DR, probably, to account for this. On a side note, it is possible to determine how much energy is dissipated as a function of time, which can then be used to find an energy pressure, through which you miiiiiight be able to sort out an actual DR, or at least a self-consistent one. Odds are, most GMs can just set a number and move on, but I know Dalton likes detail in these matters.
__________________
Buy My Stuff! Free Stuff: Dungeon Action! Totem Spirits My Blog: Above the Flatline. |
|
02-26-2017, 12:59 PM | #19 | |
Join Date: Aug 2004
Location: traveller
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
"The drag coefficient for satellites in the upper atmosphere is often approximately 2.2 (using a flat plate model). Spheres have Cd ~ 2.0 - 2.1." Vallado, Fundamentals of Astrodynamics and Aerodynamics, 3d ed., p. 549. "...Cd is the coefficient of drag ~ 2.2, ..." Squibb, Boden, and Larson (eds), Cost Efficient Space Mission Operations, 2d ed., p. 359. Your figures (0.4-0.04) are just way too low. The Apollo Cd values I've seen quoted are around 1.4-1.6. The "smooth airfoil" number is for a sub-critical angle of attack, which is specifically the way to minimize drag, not maximize it for braking effect. Once you exceed the critical angle, a wing surface acts more like a flat plate (though at a lower angle to the flow at that point). |
|
02-26-2017, 04:34 PM | #20 | ||||
Join Date: Aug 2008
|
Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
Quote:
Quote:
Quote:
Based on your replies, I'm going to assume you are just looking to provoke an argument, so I'm going to just stop responding to you from this point out. If anyone else has any interest in continuing what has devolved into a rather esoteric discussion on aerodynamics, I'm game, but otherwise, I hope I've helped Dalton with his question. Quote:
Really in the end, so much of that calculation would require the GM to just invent numbers that it's hardly even worth working out - especially for anyone using the Spaceships books, since those intentionally introduce broad generalizations to preserve people's sanity.
__________________
Buy My Stuff! Free Stuff: Dungeon Action! Totem Spirits My Blog: Above the Flatline. |
||||
Tags |
space, spaceships |
Thread Tools | |
Display Modes | |
|
|