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#100
by
Barley
on 08 Mar, 2020 03:03
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Combining the western and eastern ASDS ground tracks from the Draft Environmental Assessment, and interpolating (in red) also leaves me confused. F9 ASDS MECO is typically no more than 100km downrange adjacent Vero Beach, and FH core MECO perhaps 150km downrange adjacent Port St Lucie. So if S2 has already separated, what is the point of the booster plane change in the western ASDS trajectory at around 250km downrange? Would it not just be a waste of propellant?
Not a rocket scientist, but I suspect the issue is in your interpolation.
By my intuition a launch that would land in the western area would not follow the red path. I think it would launch more easterly, heading further out to sea and perform most of the turn using the first stage while it's still relatively slow. The first stage would be further out to sea and heading south-by-west towards the western landing area at MECO. More of a big sweeping curve, with most of the curve in the north, rather than straight lines connected by a sector of a circle.
You probably want to have an actual rocket scientist optimist the entire trajectory rather than piecing together trajectories optimized for different things.
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#101
by
OneSpeed
on 08 Mar, 2020 09:21
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Not a rocket scientist, but I suspect the issue is in your interpolation.
By my intuition a launch that would land in the western area would not follow the red path. I think it would launch more easterly, heading further out to sea and perform most of the turn using the first stage while it's still relatively slow. The first stage would be further out to sea and heading south-by-west towards the western landing area at MECO. More of a big sweeping curve, with most of the curve in the north, rather than straight lines connected by a sector of a circle.
You probably want to have an actual rocket scientist optimist the entire trajectory rather than piecing together trajectories optimized for different things.
I agree with your intuition. I've simulated some of this, and by conducting the plane change at lower (booster) velocities, it is much less expensive, especially if the plane change is combined over a large proportion of the boost phase. The point I was trying to make was that the draft EA and other proposed ground tracks show the plane change being performed entirely by the second stage, and that this would probably not be the most efficient solution for a retrograde polar insertion. SAOCOM is going to 615 km × 634 km, at 97.90°, so I'll be very interested to see the actual profile adopted.
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#102
by
gongora
on 08 Mar, 2020 13:08
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Not a rocket scientist, but I suspect the issue is in your interpolation.
By my intuition a launch that would land in the western area would not follow the red path. I think it would launch more easterly, heading further out to sea and perform most of the turn using the first stage while it's still relatively slow. The first stage would be further out to sea and heading south-by-west towards the western landing area at MECO. More of a big sweeping curve, with most of the curve in the north, rather than straight lines connected by a sector of a circle.
You probably want to have an actual rocket scientist optimist the entire trajectory rather than piecing together trajectories optimized for different things.
I agree with your intuition. I've simulated some of this, and by conducting the plane change at lower (booster) velocities, it is much less expensive, especially if the plane change is combined over a large proportion of the boost phase. The point I was trying to make was that the draft EA and other proposed ground tracks show the plane change being performed entirely by the second stage, and that this would probably not be the most efficient solution for a retrograde polar insertion. SAOCOM is going to 615 km × 634 km, at 97.90°, so I'll be very interested to see the actual profile adopted.
If your simulations suggest the western ASDS spot in the document simply wouldn't happen, then the easiest and most likely explanation would probably be a mistake in the document. These documents aren't perfect. Maybe that area is really just for fairing recovery.
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#103
by
Comga
on 08 Mar, 2020 20:36
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Not a rocket scientist, but I suspect the issue is in your interpolation.
By my intuition a launch that would land in the western area would not follow the red path. I think it would launch more easterly, heading further out to sea and perform most of the turn using the first stage while it's still relatively slow. The first stage would be further out to sea and heading south-by-west towards the western landing area at MECO. More of a big sweeping curve, with most of the curve in the north, rather than straight lines connected by a sector of a circle.
You probably want to have an actual rocket scientist optimist the entire trajectory rather than piecing together trajectories optimized for different things.
I agree with your intuition. I've simulated some of this, and by conducting the plane change at lower (booster) velocities, it is much less expensive, especially if the plane change is combined over a large proportion of the boost phase. The point I was trying to make was that the draft EA and other proposed ground tracks show the plane change being performed entirely by the second stage, and that this would probably not be the most efficient solution for a retrograde polar insertion. SAOCOM is going to 615 km × 634 km, at 97.90°, so I'll be very interested to see the actual profile adopted.
If your simulations suggest the western ASDS spot in the document simply wouldn't happen, then the easiest and most likely explanation would probably be a mistake in the document. These documents aren't perfect. Maybe that area is really just for fairing recovery.
Has this discussion evolved to being about any future SSO F9/FH launches from the Cape?
SAOCOM 1B has been stated to be RTLS
The trace on gongora's original post from Space News has to be the IIP, which pretty much ends at the ASDS location for "land forward" down-range recoveries with no boostback burn. But that's not relevant to this mission.
If an RTLS flight included plane change in the first stage flight the boostback burn would have to include the complexity of keeping the IIP off-shore. Could that be relevant to this mission?
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#104
by
Lars-J
on 09 Mar, 2020 06:26
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Not a rocket scientist, but I suspect the issue is in your interpolation.
By my intuition a launch that would land in the western area would not follow the red path. I think it would launch more easterly, heading further out to sea and perform most of the turn using the first stage while it's still relatively slow. The first stage would be further out to sea and heading south-by-west towards the western landing area at MECO. More of a big sweeping curve, with most of the curve in the north, rather than straight lines connected by a sector of a circle.
You probably want to have an actual rocket scientist optimist the entire trajectory rather than piecing together trajectories optimized for different things.
I agree with your intuition. I've simulated some of this, and by conducting the plane change at lower (booster) velocities, it is much less expensive, especially if the plane change is combined over a large proportion of the boost phase. The point I was trying to make was that the draft EA and other proposed ground tracks show the plane change being performed entirely by the second stage, and that this would probably not be the most efficient solution for a retrograde polar insertion. SAOCOM is going to 615 km × 634 km, at 97.90°, so I'll be very interested to see the actual profile adopted.
If your simulations suggest the western ASDS spot in the document simply wouldn't happen, then the easiest and most likely explanation would probably be a mistake in the document. These documents aren't perfect. Maybe that area is really just for fairing recovery.
Has this discussion evolved to being about any future SSO F9/FH launches from the Cape?
SAOCOM 1B has been stated to be RTLS
The trace on gongora's original post from Space News has to be the IIP, which pretty much ends at the ASDS location for "land forward" down-range recoveries with no boostback burn. But that's not relevant to this mission.
If an RTLS flight included plane change in the first stage flight the boostback burn would have to include the complexity of keeping the IIP off-shore. Could that be relevant to this mission?
I don’t see how a boost-phase dog-leg makes much sense - even if you start out heading further east. Why? Because the failure of a boost-back burn (even a partial one) could put the impact point in a very populated area.
No, I think any dog-leg maneuver would be left entirely to the upper stage.
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#105
by
OneSpeed
on 09 Mar, 2020 10:52
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I don’t see how a boost-phase dog-leg makes much sense - even if you start out heading further east. Why? Because the failure of a boost-back burn (even a partial one) could put the impact point in a very populated area.
No, I think any dog-leg maneuver would be left entirely to the upper stage.
Here are a couple of suggested SAOCOM-1B ground tracks to a -97.90° inclination. In both cases the failure of a boostback burn would send the booster towards the ocean, certainly not over Florida, and moving too slowly to reach Bermuda. By tracking further east, the total plane change requirement is greater, but it is performed at lower velocities and yaw, so it may well be less expensive. However, SAOCOM-1B is a very light payload, so there is plenty of ΔV available for this mission, and the second stage probably could perform all of the 1,200 or so m/s of plane change required.
Perhaps this discussion belongs in a different thread, because where I suspect a boost-phase dogleg might make more sense is a polar mission with a much heavier payload and an ASDS landing (Starlink?). Here the physics may well favour minimising (or even eliminating) the second stage plane change requirement.
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#106
by
kdhilliard
on 09 Mar, 2020 22:06
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...
I don’t see how a boost-phase dog-leg makes much sense - even if you start out heading further east. Why? Because the failure of a boost-back burn (even a partial one) could put the impact point in a very populated area.
...
Should we expect to see the plane change start at the very beginning of the S2 burn, and might the S2 actually overfly parts of Florida, even as its IIP stays at sea, always south of the penninsula?
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#107
by
Barley
on 10 Mar, 2020 06:01
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Should we expect to see the plane change start at the very beginning of the S2 burn.
Once you are free of constraints, such as Miami and the atmosphere, it's best to spread any needed inclination change over the entirety of all remaining burns. You do not want to "First correct the plane, then go for orbital velocity".
For small angles the components sin(Θ) and cos(Θ) are approximately Θ and 1-Θ^2. For small angles you get some sideways acceleration for almost no loss of downrange acceleration, so if Θ=0 at any point you're leaving performance on the table.
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#108
by
kdhilliard
on 10 Mar, 2020 06:50
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Should we expect to see the plane change start at the very beginning of the S2 burn.
Once you are free of constraints, such as Miami and the atmosphere, it's best to spread any needed inclination change over the entirety of all remaining burns. You do not want to "First correct the plane, then go for orbital velocity".
For small angles the components sin(Θ) and cos(Θ) are approximately Θ and 1-Θ^2. For small angles you get some sideways acceleration for almost no loss of downrange acceleration, so if Θ=0 at any point you're leaving performance on the table.
cos(Θ) ~ 1-
0.5Θ^2, right? Though the quadratic nature of the cosine error still holds. (Even doubly so!
)
But I'd have thought that as soon as you are free of constraints you would want to yaw as quickly as possible to stop accelerating out of plane, plus that small Θ to compensate for the earlier acceleration. I'm unclear what the resulting ground track would look like, and it still seems possible to me that it might involve overflight of Miami.
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#109
by
OneSpeed
on 10 Mar, 2020 10:22
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Once you are free of constraints, such as Miami and the atmosphere, it's best to spread any needed inclination change over the entirety of all remaining burns. You do not want to "First correct the plane, then go for orbital velocity".
For small angles the components sin(Θ) and cos(Θ) are approximately Θ and 1-Θ^2. For small angles you get some sideways acceleration for almost no loss of downrange acceleration, so if Θ=0 at any point you're leaving performance on the table.
It's true that for small yaw angles, the loss is small, but the cost of plane change at any point is also proportional to velocity.
E.g. if you are travelling at a constant 100m/s a 30° plane change only requires the addition of 100 x sin(30°) = 50m/s horizontally.
At 1000m/s it requires 500m/s, and at 7,800m/s it requires 3,900m/s.
Now try dividing it into three 10° step changes. At 100m/s 10° requires 17.4m/s, at 1000m/s it's 174m/s and at 7,800m/s it's 1,354m/s. That's a total of about 1,546m/s, much more than if the plane change was all at 100m/s, but much less than if it was all at orbital velocity (like IXPE's massive 28.5° plane change over the equator).
Of course these are rather coarse approximations, just to make a point. But if I was to properly integrate the plane changes in a simulation this trend would still be apparent. In the same way that an aircraft at 80 knots requires only 12° of bank for a two minute turn, a jet at 400 knots requires 48° of bank for the same turn rate. And if you are still in the atmosphere, even with a rocket, you could use aerodynamic forces generated by some yaw to assist your turn. In short, the faster you are going, the more expensive is the plane change.
Edit: added suggested ASDS ground tracks.
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#110
by
edkyle99
on 10 Mar, 2020 13:38
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A lesson from the past:
Thor 283 and Able Star stage AB006 launched from LC 17B with Transit 3A and GRAB 2/SolRad 2/Greb 2 on November 20, 1960 on a southbound trajectory toward a high inclination orbit. Things went awry when the main engine shut down early at T+151 seconds, only a few seconds short of its planned cutoff time. The second stage presumably separated and fired, but Range Safety sent a destruct command at T+325 seconds.
This was one of several early high inclination Cape Canaveral launches with ground tracks that crossed the eastern tip of Cuba. The practice created an international embarrassment during this launch. Debris from the second stage fell on eastern Cuba, with one fragment reportedly falling on a farm near Holguin and killing a cow. Castro called it part of a U.S. plot against Cuba. Protesters paraded six cows in front of the U.S. Embassy in Havana, paying tribute to the cow as a "victim of an imperialist rocket". The U.S. Government paid Cuba $2 million compensation for the incident, after demanding return of pieces of the then-secret rocket. There's an unconfirmed story that after the incident crews painted a cow on the LC 17 blockhouse wall next to the usual Thor images.
This wasn't even a sun synchronous launch attempt. There were at least five sun sync launches from the Cape, including Delta's 28, 32, 36, 37, and 67 with TIROS 9, 10, ESSA 1, 2, 9, during 1965-69. The Delta vehicles would perform three or so dog-legs during their ascent and would overfly Cuba and Panama. The biggest dog-leg - 33 degrees to the right - would be during the first stage burn, starting about 80 seconds after liftoff after initially flying on a 115 deg azimuth from the Cape.
- Ed Kyle
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#111
by
Barley
on 10 Mar, 2020 14:15
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E.g. if you are travelling at a constant 100m/s a 30° plane change only requires the addition of 100 x sin(30°) = 50m/s horizontally.
At 1000m/s it requires 500m/s, and at 7,800m/s it requires 3,900m/s.
Now try dividing it into three 10° step changes. At 100m/s 10° requires 17.4m/s, at 1000m/s it's 174m/s and at 7,800m/s it's 1,354m/s. That's a total of about 1,546m/s, much more than if the plane change was all at 100m/s, but much less than if it was all at orbital velocity (like IXPE's massive 28.5° plane change over the equator).
Between the first and second of your corrections you go from 100m/s 20 degrees out of plane to 1000m/s 20 degrees out of plane. This means you are accelerating at 20 degrees away from the target plane. This is accelerating in the wrong direction. Don't do that.
A non-optimal but much better alternative is to accelerate in the target plane. Then the traverse component of velocity will remain 33.3m/s.
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#112
by
Comga
on 11 Mar, 2020 04:04
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For this discussion, we have to keep separate the concepts of ground track and Instantaneous Impact Point.
Satellites fly over Miami and every other city all the time, but they don't have IIPs when in orbit.
Similarly, the eastern of OneSpeed's sonic boom areas seems to be the IIP for the first stage if no burns occur after staging.
The stage is still far north, east of Vero Beach at that time.
For an RTLS, the IIP will "pull back" during the boostback burn, back down the path it took off the coast, possibly to the point of the dogleg maneuver.
My guess is that the landing burn will move the first stage laterally by the same amount it traversed before the turn to the SSE.
If the rocket did initiate the plane change before staging, the path would go closer to land, but the IIP, particularly for the second stage, would gently curve around Florida.
It wouldn't necessarily be a problem if the second stage overflew Miami, so long as it was high with it's IIP far to the south.
It would just be more complex to have the boostback burn make the first stage IIP complete a "figure 8", sweeping out to sea before pulling back to just offshore of the Cape.
It's physically possible.
Not that we have any evidence for that being the plan for the SAOCOM 1B launch.
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#113
by
Lars-J
on 11 Mar, 2020 06:43
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For an RTLS, the IIP will "pull back" during the boostback burn, back down the path it took off the coast, possibly to the point of the dogleg maneuver.
My guess is that the landing burn will move the first stage laterally by the same amount it traversed before the turn to the SSE.
No, the landing burn (and braking burn) is almost entirely vertical, and it happens close to the ground. The landing burn cannot provide any dog leg horizontal shift.
Don’t make the mistake of thinking that the outbound trajectory (under constant thrust) is the same as the inbound (mostly gliding). They are very different. The boost-back burn sets the direction back to the landing site, in a straight line. (When viewed from above) The remaining burns do not alter the ballistic impact point by any significant margin.
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#114
by
kdhilliard
on 11 Mar, 2020 07:31
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Here is a screengrab from the
SpaceX CRS-20 Mission Control Audio webcast at stage separation. My best estimate is that the IIP (terminus of the blue arc) is 380 km downrange from the launch site. That sort of range would put SAOCOM 1B's IIP well south of Miami (and a bit south of Biscayne Bay) at stage separation.
What I'd love to see is the ground track plot for such an RTLS mission which runs down the coast 20 km offshore until stage separation when the upper stage then takes up the necessary course for its desired orbit. Might it come inland even a bit north of West Palm Beach?
Edit: Stripped leading www from YouTube link to suppress embedding.
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#115
by
Zed_Noir
on 11 Mar, 2020 08:51
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Any info on fairing recovery for the SAOCOM-1B mission?
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#116
by
RocketLover0119
on 11 Mar, 2020 11:01
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Any info on fairing recovery for the SAOCOM-1B mission?
Hmmm... good question
If I had to guess I would say yes, but that would put Ms. tree and Ms. Chief in very interesting positions.....
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#117
by
zubenelgenubi
on 18 Mar, 2020 23:38
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Cross-post re: early Merlin 1D shut down on the first stage,
Starlink v1.0 Flight 5 today; possibility of launch delay:
https://twitter.com/elonmusk/status/1240262636547100672
Elon: Yeah. There was also an early engine shutdown on ascent, but it didn’t affect orbit insertion. Shows value of having 9 engines! Thorough investigation needed before next mission.
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#118
by
Raul
on 19 Mar, 2020 12:33
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SAOCOM-1B Launch Hazard Areas based on issued NOTMAR, valid for Mar 30 23:18 UTC, alternatively Mar 31.
LZ1 landing for the booster. Extra drop area (red) in case of boostback failure.
Stage2 dogleg maneuver to polar orbit azimuth. Possible fairing recovery in southern (orange) area.
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#119
by
Thorny
on 19 Mar, 2020 13:41
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LZ1 landing for the booster. Extra drop area (red) in case of boostback failure.
Curious... how can the boostback failure impact zone be so far off from the ground track?
)