Flight Question Keeping a constant deceleration load

[Hurricane]

Hello everyone :hello:
Now that I am quite acquainted with the works behind reentries, I decided that I want to do it "the advanced way": keeping a constant deceleration load instead of a constant descent rate. I also think that it will be more accurate than keeping a constant descent rate, as I find that keeping a constant VS makes deceleration loads slowly build up to about 3g up until about 1 km/s where it is safe to go feathered.

So, my question is: if I see that my deceleration load increases (e.g. from 10 to 20 m/s^2), do I have to pitch down, up? Anything much more elaborate than that?

~Thanks in advance, Oz. :tiphat:

 

[Urwumpe]

So, my question is: if I see that my deceleration load increases (e.g. from 10 to 20 m/s^2), do I have to pitch down, up? Anything much more elaborate than that?

No, you have to increase your vertical lift... how you do that, that depends on you, but most spacecraft don't like pitching down from an stalled angle of attack to maximum lift, since this exposes the vulnerable side of the spacecraft to the reentry heating. Better is reducing your bank angle for long term corrections, and use only minimal change in AOA for short term variations.

 

[Hurricane]

No, you have to increase your vertical lift... how you do that, that depends on you, but most spacecraft don't like pitching down from an stalled angle of attack to maximum lift, since this exposes the vulnerable side of the spacecraft to the reentry heating. Better is reducing your bank angle for long term corrections, and use only minimal change in AOA for short term variations.

So to reduce deceleration, I need more lift. That's doable, I suppose.
Now, how to get it to work with the DGIV computer... A bit hard.
OR I could just use the PRO105 program instead of the PRO104. I'll fiddle with it, thanks.

 

[Urwumpe]

So to reduce deceleration, I need more lift. That's doable, I suppose.
Now, how to get it to work with the DGIV computer... A bit hard.
OR I could just use the PRO105 program instead of the PRO104. I'll fiddle with it, thanks.

Read the fine manual. You can, if you read it carefully, find out that you can set the bank angle during reentry.

 

[Hurricane]

Read the fine manual. You can, if you read it carefully, find out that you can set the bank angle during reentry.

NOOO... I didn't know that. [/SARCASM]
I know that I can set the bank angle during reentry, but my biggest concern is that deceleration loads will get TOO HIGH, hence I'll have to pitch down. That means, smaller increments than what the DGIV autopilot supports.

 

[Urwumpe]

NOOO... I didn't know that. [/SARCASM]
I know that I can set the bank angle during reentry, but my biggest concern is that deceleration loads will get TOO HIGH, hence I'll have to pitch down. That means, smaller increments than what the DGIV autopilot supports.

No, you misunderstand the physics.

The deceleration loads and heating both depend on the dynamic pressure on your vehicle. Dynamic pressure is [math]P = \frac{1}{2} \rho v^2[/math]
Rho is the density of the atmosphere, it decreases with altitude. For reducing this deceleration, you need actually to loose speed faster, than density increases - thus you need to keep a high drag factor, but reduce your descent rate.

pitching down would actually INCREASE the deceleration - for a short time. You get more lift, thus more acceleration and descent rate will drop. If you reduce your bank angle (until you are horizontal) you achieve the same: You get more vertical lift and descent rate drops.

 

[Hurricane]

So say I want to keep a deceleration of 15m/s^2.
If I see it GROW, what do I do?
If I see it GET SMALLER, what do I do?

 

[Urwumpe]

So say I want to keep a deceleration of 15m/s^2.
If I see it GROW, what do I do?
If I see it GET SMALLER, what do I do?

Deceleration higher than target: Reduce bank
Deceleration lower than target: Increase bank

 

[Hurricane]

Deceleration higher than target: Reduce bank
Deceleration lower than target: Increase bank

And if bank is at zero, then:
Deceleration higher than target: reduce alpha
Deceleration lower than target: increase alpha

 

[Urwumpe]

And if bank is at zero, then:
Deceleration higher than target: reduce alpha
Deceleration lower than target: increase alpha

No. if bank is zero:

Deceleration higher than target: Danger, Will Robinson, Danger!
Deceleration lower than target: Bank towards the runway.

If you are at full vertical lift and still getting excessive heating, you are already too low. All you can then do is keep lift vertical and pray than it works out.

 

[Hurricane]

And if I'm already aligned with the runway, then I have to make 2 consequent bank turns, right?

 

[Urwumpe]

And if I'm already aligned with the runway, then I have to make 2 consequent bank turns, right?

Yes, the classic S-turn. Most spacecraft here should at least tolerate 5° difference in azimuth between your ground track and the landing site, Space Shuttle does 10.5°, Deltaglider like spacecraft can fly with similar variations.

sadly, there is no instrument in Orbiter itself to provide Delta-Az to you, only add-on MFDs.

 

[Tommy]

Maintaining a constant decelleration isn't feasible in real life. The amount of decceleration you can withstand without melting isn't constant throughout the re-entry. At the beginning, with a velocity of almost 8km/s, you can't support as much decelleration as you can when you are later in the re-entry and only going 3km/s.

Watch the G-forces during a re-entry. You'll see that early in the re-entry you will have max hull temps and still have 1G or less of decelleration. Sure, you could maintain that low decelleration rate throughout the re-entry - at least in Orbiter. In real life, that lower decelleration means a much longer re-entry. Temps may be lower - but they will still be quite high for a much longer period of time.

No insulation is perfect - it doesn't prevent heat transfer - only slows it down. Even the TPS on the shuttle has a limit as to how long it can protect the craft before the heat soaks through. So even thought the temps on the outside of the hull are lower, the longer time allows more heat through the insulation.

For a vessel to withstand a constant decceleration rate, it would need more insulation or some form of active cooling - either of which would increase the mass quite a bit. The higher mass would render the vessel much less efficient - and in spaceflight efficiency is second only to safety.

 

[Quick_Nick]

sadly, there is no instrument in Orbiter itself to provide Delta-Az to you, only add-on MFDs.

Surface HUD with a base target selected on MapMFD?

 

[Hurricane]

So, Tommy, what method will be best for reentry?
I mean, what method will allow me to best predict my final landing spot?

 

[n72.75]

So, Tommy, what method will be best for reentry?
I mean, what method will allow me to best predict my final landing spot?

A LOT of practice.

 

[Tommy]

So, Tommy, what method will be best for reentry?
I mean, what method will allow me to best predict my final landing spot?

The short answer is add-ons! There are three tools that can help. One is ReEntryMFD - but I'm not sure that works in Orbiter 2010P1. Another is GlidescopeMFD. This one is a bit hard to deal with - but it's good for showing you the Delta-Az. The third is AerobrakeMFD. This one is the easiest to use - hit the <MOD> button, then the <PRJ> button, and it will show you a small map and a green line showing your projected course assuming you keep the same AoA and bank.

For any vessel/orbit combination, there is a minimum and a maximum distance from the base that you need to de-orbit from. BaseSyncMFD's DEO option will help you tell when to de-orbit. Once you hit the atmo, hold the 40 degree AoA. This will provide more lift than you want, so use bank to control the descent rate (more bank = faster descent). Watch the Delta-Az in glidescope and perform roll reversals when it exceeds 5 degrees. You can use glidescope to watch the range - or use Aerobrake to keep an eye on the landing point (will miss to the side, but helps judge range.)

There are several dozen threads and tutorials for BaseSync and Aerobrake (and re-entry in general) which will help you set the DEO in basesync, etc, so I won't repeat that info here. The forum's search feature is very handy.

 

[Hurricane]

The short answer is add-ons! There are three tools that can help. One is ReEntryMFD - but I'm not sure that works in Orbiter 2010P1. Another is GlidescopeMFD. This one is a bit hard to deal with - but it's good for showing you the Delta-Az. The third is AerobrakeMFD. This one is the easiest to use - hit the <MOD> button, then the <PRJ> button, and it will show you a small map and a green line showing your projected course assuming you keep the same AoA and bank.

For any vessel/orbit combination, there is a minimum and a maximum distance from the base that you need to de-orbit from. BaseSyncMFD's DEO option will help you tell when to de-orbit. Once you hit the atmo, hold the 40 degree AoA. This will provide more lift than you want, so use bank to control the descent rate (more bank = faster descent). Watch the Delta-Az in glidescope and perform roll reversals when it exceeds 5 degrees. You can use glidescope to watch the range - or use Aerobrake to keep an eye on the landing point (will miss to the side, but helps judge range.)

There are several dozen threads and tutorials for BaseSync and Aerobrake (and re-entry in general) which will help you set the DEO in basesync, etc, so I won't repeat that info here. The forum's search feature is very handy.

A *very* short answer indeed!
Anyway, I will practice it using AB and BS MFD, and see what comes out.
(Back to routine it is, so now I can)

 

[Tommy]

Well, a "long" answer would have taken up most of a page! I'll point out that Aerobrake has an AoA hold autopilot - but it's not the best. But if you are flying the "stock" shuttle it's better than nothing. You may also want to try AttitudeMFD, it also can hold an AoA and may work better.

 

[ADSWNJ]

A *very* short answer indeed!

Short maybe, but spot on the money. (Or the glideslope, etc!). BaseSync is great for deorbit burns, though you still have to tell it how much anticipation you want, what interface altitude you want and how steep you want to come in. The best thing is to get onto your deorbit orbit, save the simulation and run a bunch of tests from that consistent start-point to see what works for you.

On the reentry, Aerobrake's prediction displays are outstanding, and highly worth exploring in detail. The predicted ground-track and range give you a highly accurate indicator of where you will crash into the earth (or land if you are fortunate or skilled!).

The more I look at Glideslope, the more impressed I am. It has an excellent set of displays, and for the roll reversals and Delta Az data, it's invaluable. (Simply said - you want to maintain close on 40 degrees AoA regardless of roll, so the airflow strikes the right part of the shuttle. Rolling up to 90 degrees takes the wing lift vector and "dumps" it sideways rather than vertically, allowing the shuttle to descend more rapidly. You'll need to reverse the roll every now and then, as the Delta Az gets to say 10 degrees off track, so you smoothly and quite quickly reverse it and you will start tracking back towards the zero Delta Az again).

Finally - the GPCMFD is pretty interesting too. I like the clarity of the pitch, roll and yaw numbers, and the vertical speed and acceleration numbers. One day (hopefully with Kwan's help), I would love to integrate that into Glideslope with some other preference changes! We'll see!