Flight Question Is it Okay to maintain -10 pitch during the launch

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
I've been trying to launch a satellite into an orbit of 260 km x 260 km using the Titan II. During the first stage I reach 30 deg (slowly pitching down from the start) in about 150 seconds. ApA at this moment is around 280-300 km. Taking this into account I've got to pitch down during the second stage to about -10 deg. After this I try to adjust everything in such a way that my ApA gets to 260 km (at this moment I pitch up to almost 0 deg to stop ApA from dropping any lower) at the same time trying to decrease my VS to 0. Maintaining zero VS I'm waiting while my PeA reaches 260 (or close to) and shut down the booster.

So, is this Okay or is it not how they do it out in real life? Do they pitch to negative values? Or maybe they do it in a completely different way?

Thanks.
 
Last edited:

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
It's a waste of delta-V. Real launches have an optimised trajectory so that isn't necessary.

I suppose we can't do it manually in Orbiter, can we?

(Or is it a waste of time and effort to try to acquire such a skill (at least with one launch vehicle)?)

Thanks.
 

Jarvitä

New member
Joined
Aug 5, 2008
Messages
2,030
Reaction score
3
Points
0
Location
Serface, Earth
I suppose we can't do it manually in Orbiter, can we?

(Or is it a waste of time and effort to try to acquire such a skill (at least with one launch vehicle)?)

Thanks.

It can be done, it just requires a bit of practice. Try it with the stock delta-glider first, horizontal launch is easier to manage so you end up in a circular orbit with a single burn and without needing to pitch excessively by the end. Or, you could watch an autopiloted vessel launch to orbit and observe the principles from that.
 

Kaito

Orbiquiz Coordinator
Joined
Jul 24, 2008
Messages
857
Reaction score
0
Points
0
If your APA is too high when you are at a nice reference point (in your case, 30 degrees in 150 seconds), try pitching down faster, say 30 degrees in 125, or 100 seconds.

Have fun, let us know how you're doing! :)
 

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
It can be done, it just requires a bit of practice. Try it with the stock delta-glider first, horizontal launch is easier to manage so you end up in a circular orbit with a single burn and without needing to pitch excessively by the end. Or, you could watch an autopiloted vessel launch to orbit and observe the principles from that.

I have watched a couple. It seems that they use the same method I do.
Here is an extract from Project P-7, for example:

[sic] Piece 5. Control of vertical speed. This piece uses, because control algorithm is
imperfect. When vertical speed is small, control system make it zero and stabilize
this value.


What kind of a launch vehicle with an autopilot actually follows the 'real rules'?

Thanks.
 
Last edited:

Cras

Spring of Life!
Donator
Joined
Apr 13, 2011
Messages
2,215
Reaction score
0
Points
36
Location
Los Angeles
Website
www.youtube.com
The Energia launch vehicle has a nice autopilot. I have had issues with it under D3D9 client (RC36), but it appears to launch in a very realistic fashion. I have no knowledge on how the real thing got into orbit, but it looks good from my end.
 

Tommy

Well-known member
Joined
Aug 14, 2008
Messages
2,019
Reaction score
86
Points
48
Location
Here and now
It's a matter of pitching down sooner. This way you will increase your velocity faster - and have less vertical speed so you have a lower ApA. Ideally, your ApA should never exceed your target altitude - if it does you will "waste" fuel lowering it back down.

When using a vertical launch vehicle, I drop the pitch to 60 degrees almost immediately, and try to keep my VS under 300 m/s by pitching down as needed.
 

Notebook

Addon Developer
Addon Developer
News Reporter
Donator
Joined
Nov 20, 2007
Messages
11,813
Reaction score
640
Points
188
A quote from this document: Section 4.

http://aerade.cranfield.ac.uk/ara/arc/cp/0848.pdf

The vehicle is launched vertically and climbs for 20 seconds. In the
two standard trajectories the vehicle turns gra&ally at 0.8 degrees per
second to achieve ultimate angles with the launcher horizontal of 36 degrees and 74 degrees respectively. The speed of sound is achieved in these trajectoriesafter about 57 seconds and 58 seconds respectively.
4.2 Assessment

That was for the UK Blue Streak rocket:
http://www.spaceuk.org/bstreak/bstreak.htm

I've seen a pitch over of 0.7 degrees per second, and a hold attitude of 20 degrees to the horizon before. I think the 74 degree hold is when Blue Streak was the first stage of the Europa rocket, not sure about that though.

EDIT : Just for info, that 30/20 degree hold was for the single stage Blue Streak, originally designed as a missile, so its a ballistic path.
Not designed to get into orbit, so I'm guessing those figures were intended to keep it in the Woomera test range, and maybe get an idea of its actual performance?

Some grpahs of data that might be useful.

http://s89.photobucket.com/albums/k207/Notebook_04/Blue Streak/?action=view&current=img001-1.jpg
N.
 
Last edited:

Radu094

Donator
Donator
Beta Tester
Joined
Sep 5, 2008
Messages
36
Reaction score
0
Points
6
If you launch from a body w/out atmosphere, the optimum pitch profile implies tan(pitch) to decrease linearly over time, such that pitch=0 and VerticalSpeed=0 at MECO (ie. you are at Pe). Anything else is suboptimal.

When you add the atmosphere things get complicated. You want to climb above the thick layers, and you want to climb fast, before you accelerate too much and spend fuel just to push your vessel through thick gas. This implies a large vertical speed imediately after launch, a speed that you might not be able to null by gravity alone (ie. negative pitch will be required to attain desired orbit). It's all very dependent on atmosphere profile and aerodynamic properties of your vehicle, but in general the sweet spot of pitch profiles lies somewhere with negative pitch on MECO.

IIRC the Shuttle used negative pitch too
 

Moach

Crazy dude with a rocket
Addon Developer
Joined
Aug 6, 2008
Messages
1,581
Reaction score
62
Points
63
Location
Vancouver, BC
(...)
IIRC the Shuttle used negative pitch too

it did, but do mind also it had a severely off-pitch thrust vector, thus that angle is not as extreme as it'd initially appear


Apollo also went some degrees below at the last stretch of the launch, AFAIK.... it seems it's not that far fetched from an optimal trajectory if you have an atmosphere to deal with :hmm:
 

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
So far I have achieved this:

I pitch down slowly from the launch so that my ApA is about 240-250 km when I reach 30 degrees and the second stage begins. After this I go down to about -3 degrees because this particular pitch allows my ApA to remain constant. At this time my ApA is about 260-265 km so it has increased a bit while I was pitching down. My VACC is now negative, but my ACC is way too big and it leads (if no actions are taken) to my reaching the orbital speed before my VS is 0 which leads, in its turn, to a complete disaster (ApA and PeA going up uncontrollably...). Taking this into account I have to reduce my thrust to ensure that my VS becomes 0 before I hit the orbital speed. Then I just wait making small adjustments until my PeA reaches an optimal value.
Is this any better?

Thanks.
 

C3PO

Addon Developer
Addon Developer
Donator
Joined
Feb 11, 2008
Messages
2,604
Reaction score
17
Points
53
Not only do you have to consider drag, but also the reduction in efficiency due to ambient pressure at lower altitudes. This could mean that an initial "high" Vvel could be more efficient for some launches.
 

Radu094

Donator
Donator
Beta Tester
Joined
Sep 5, 2008
Messages
36
Reaction score
0
Points
6
So far I have achieved this:
... to my reaching the orbital speed before my VS is 0 which leads ... Taking this into account I have to reduce my thrust to ensure that my VS becomes 0 before I hit the orbital speed....

Thanks.

Reducing thrust to meet MECO objectives is wasteful as you are essentially fighting off gravity loss for no gain what so ever.(ie. part of your engines burn so that you do not fall)

If the launch profile wasn't sufficient to meet the expected MECO (VS=0 , V= Vcirc ,etc) I usually call on a contingency MECO: if my current altitude is sufficiently high (100...120km), and I have a desired apogee, and I have enough[*] orbital velocity I will prefer to coast up to apoapsis and then circularize the orbit there. Remember the most efficient burns are at apoapsis and periapsis.


Again, this is essentially what the shuttle was doing in most of it's profiles, using the OMS-1 & OMS-2 to raise apogee and perigee.

[*] enough orbital velocity = when I know I can circularize my orbit at apoapsis within a max. few minutes of burn.
eg:
Reaching apoapsis at 1000m/sec orbital velocity means I will pretty much descend back into the atmosphere before I have a chance to circularize it. bad ideea.

Reaching apoapsis at 7000m/sec.. easy job to raise periapsis to a respectable height. this will work.
 

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
As far as I can understand the strategy at the time being for me to implement is to pitch down sooner, so that when the second stage begins I've got to be much lower than my intended ApA.
Am I right?

Thanks.
 

Radu094

Donator
Donator
Beta Tester
Joined
Sep 5, 2008
Messages
36
Reaction score
0
Points
6
Yes. 300km ApA seems a bit much for starting the second stage. I have no idea what the actual Titan profile was, but as a rule of thumb aim you ApA to be 10-15% below your intended MECO altitude at staging. Then on the second stage aim to keep that ApA constant (adjust it to your intended MECO alt). Then fail the first few 10s of attempts; repeat until partially successful.
 

SuprunP

New member
Joined
Nov 28, 2011
Messages
76
Reaction score
0
Points
0
To sum up my several last attempts:

1) The beginning of the second stage: 30 deg, ApA~180k (I aim for 260k);
2) pitching to ~0 deg;
3) as I approach the orbital speed my ApA begins to raise rather steeply (I reckon this is because my VS is still positive);
4) when ApA hits ~260 km I shut down the booster;
5) when altitude reaches ApA I circularize the orbit.

I'm not sure if this method saved me a lot of dV though. After circularizing the orbit via my previous method 'MAIN PROP' showed ~600-700, now it showed ~ 1.10k. Although flying this way felt kind of 'right' (I'm not sure about this either... :rolleyes:)
 
Top