SpaceX at Pad 39A

[smartcooky]

Pad 39A has quite a history of significant launches;

► first launch of a Saturn V rocket (the uncrewed Apollo 4)
► first launch of a crewed Saturn V (Apollo 8 - first humans to leave earth orbit)
► launching Apollo 11 (first men on the moon)
► last Saturn V mission to Skylab
► second Space Shuttle launch (STS2)
► Last Space Shuttle launch (Atlantis to the ISS)

Now, another historic moment; the first SpaceX Falcon-9 launch to deliver the CRS10 resupply capsule to the ISS AND soft land the first stage back at The Cape.

I don't know about others here, but for mine, this is right out of the sci-fi I used to read as a teenager in the 1960's and 1970's... spaceships landing tail down on a column of flames.

and its good to see 39A back in business.

[bknight]

Just finished breakfast and saw a video of the landing from a drone.  Impressive as always and a new era in launches from 39A.

[Glom]

The short landing burn is bizarre.

[Dalhousie]

At least they didn't blow it up.

[smartcooky]

Some questions for the rocket scientists here.

At main engine cutoff (T+2:25), the spacecraft is accelerating through 6000 km/h at 65km altitude, with staging taking place a few seconds later. I imagine it would be at least that distance downrange, and previous to this, they have gone for a downrange landing on a ship, so they could simply allow the Stage to carry on falling into the atmosphere, but for this mission Stage 1 has to change direction through 180° to head back to The Cape, so they have to have a "boostback" burn to accomplish this.

So, my first question is; does bringing the Stage back to The Cape impact on the payload size. Surely they have to carry extra fuel (and therefore extra weight) for the "boostback" burn.

My second question is this: Is the purpose of the "entry burn" to slow the re-entry speed down due to the lack of any kind of heat shield? 

[Peter B]

Some questions for the rocket scientists here.

At main engine cutoff (T+2:25), the spacecraft is accelerating through 6000 km/h at 65km altitude, with staging taking place a few seconds later. I imagine it would be at least that distance downrange, and previous to this, they have gone for a downrange landing on a ship, so they could simply allow the Stage to carry on falling into the atmosphere, but for this mission Stage 1 has to change direction through 180° to head back to The Cape, so they have to have a "boostback" burn to accomplish this.

So, my first question is; does bringing the Stage back to The Cape impact on the payload size. Surely they have to carry extra fuel (and therefore extra weight) for the "boostback" burn.

I believe it does. But it's probably best to look at this the opposite way around: the size of the payload determines whether they can load the fuel to do the boostback burn, or whether they go without it and land out on the barge in the ocean.

I figure that landing on the barge is a more expensive and complicated logistical exercise than landing back at the launchpad (or some other nearby location on land), so I assume they'd go for a "land landing" rather than a barge landing whenever the payload is light enough to permit it.

My second question is this: Is the purpose of the "entry burn" to slow the re-entry speed down due to the lack of any kind of heat shield?

I don't know the maths of this, but I assume so. I also figure they have to balance the desire to slow the stage down as much as possible before re-entry against the extra fuel this requires (just think of those lunar lander simulation games - the later you fire up the engine the less fuel you use to land, only in the case of landing on the Earth you also need to take into account the atmosphere). Hence the unnervingly late ignition for the touchdown burn.

[Glom]

Yes, the whole point is that there is a payload penalty, mitigated if using the barge, with the extra cost and risk that entails.

[smartcooky]

Thanks for the replies

I'm having some trouble understanding exactly how this all works. For mine it seems that Stage 1 is heading downrange very fast (6000 kph) and reversing that velocity would take a lot of energy. There are plenty of general descriptions on the interwebz of what happens, but actual details seem to be very sparse, so what I will do is try to explain how I think it might work...

1. At staging, Stage 1 does not have enough velocity to make orbit, so it carries on a sub-orbital (ballistic?)  trajectory as gravity begins to drag it back to earth.

2. Four grid fins are deployed near the top of the Stage which "weathercocks" the vehicle so that it will tend to travel tail first when it begins to be affected by atmosphere.

3. To this point, the Earth's gravity is being used  to turn/change the direction of the vehicle.

4. When the vehicle is falling at an angle of about 45°, the "boostback burn" takes place. This has two effects
    a. slowing the vehicle's vertical speed, and
    b. killing its forward velocity to get it heading back toward the Cape.

5. As the vehicle begins to re-enter the atmosphere, it is still traveling too fast and because it has no heat-shielding, the "entry burn" is used to slow the vehicle down further to around 1000 m/s

6. The vehicle begins to speed up a little after the entry burn, but the increasing density of the atmosphere slows it down to sub-sonic speed.

7. By this time, the vehicle is almost directly over the LZ, free-falling tail down at around 1000 km/h. Its attitude and trajectory are being trimmed and corrected using the grid fins. The landing burn kicks in at around 4000m altitude, rapidly decelerating the vehicle to a soft landing about 8 minutes after lift off.

Corrections, additions, amendments and comments welcome.

[Peter B]

Thanks for the replies

I'm having some trouble understanding exactly how this all works. For mine it seems that Stage 1 is heading downrange very fast (6000 kph) and reversing that velocity would take a lot of energy. There are plenty of general descriptions on the interwebz of what happens, but actual details seem to be very sparse, so what I will do is try to explain how I think it might work...

1. At staging, Stage 1 does not have enough velocity to make orbit, so it carries on a sub-orbital (ballistic?)  trajectory as gravity begins to drag it back to earth.

2. Four grid fins are deployed near the top of the Stage which "weathercocks" the vehicle so that it will tend to travel tail first when it begins to be affected by atmosphere.

3. To this point, the Earth's gravity is being used  to turn/change the direction of the vehicle.

4. When the vehicle is falling at an angle of about 45°, the "boostback burn" takes place. This has two effects
    a. slowing the vehicle's vertical speed, and
    b. killing its forward velocity to get it heading back toward the Cape.

5. As the vehicle begins to re-enter the atmosphere, it is still traveling too fast and because it has no heat-shielding, the "entry burn" is used to slow the vehicle down further to around 1000 m/s

6. The vehicle begins to speed up a little after the entry burn, but the increasing density of the atmosphere slows it down to sub-sonic speed.

7. By this time, the vehicle is almost directly over the LZ, free-falling tail down at around 1000 km/h. Its attitude and trajectory are being trimmed and corrected using the grid fins. The landing burn kicks in at around 4000m altitude, rapidly decelerating the vehicle to a soft landing about 8 minutes after lift off.

Corrections, additions, amendments and comments welcome.

Yeah, that looks about right, although I don't know enough of the maths to verify the numbers.

Now what you've described is what seems to happen for launch-site landings.

If the payload is heavier and there isn't enough fuel for a boostback burn, I get the impression that they simply omit step 4 and place one of their barges downrange where ballistics indicate the stage would come down.

[Peter B]

Incidentally, I googled "falcon 9 first stage trajectory" and got some interesting pictures. For landings back at the launch site, it appears the first stage continues to climb even after the boostback burn, with the graphic suggesting that for that burn the stage is aimed above the horizon.

[smartcooky]

Incidentally, I googled "falcon 9 first stage trajectory" and got some interesting pictures. For landings back at the launch site, it appears the first stage continues to climb even after the boostback burn, with the graphic suggesting that for that burn the stage is aimed above the horizon.

Oh, Stage 1 goes UP after separation. The only reasons I can think of for doing that would be

(a) to use gravity in slowing the vehicle down, and
(b) to give them more room for maneuvering the vehicle.

I noticed on this video....

...that just after staging, the RCS kicks stage 1 sideways. That must be to put it in the correct orientation to fire the main engines in order to retrograde the stage and send it back to the Cape while gravity helps to slow the stage down.

Also, the stage is not sent on a trajectory that will take it directly back to the LZ. The ballistic trajectory is deliberately short so that in case of engine failure it will crash into the sea.  Once the landing burn starts, the grid fins maneuvre the stage over to the landing pad.   


ETA: Found this....(click to toggle size)

[ka9q]

My reading of the technical broadcast is that first stage shutdown occurs at about 2:23 when the earth-fixed velocity is 1676 m/s and the altitude is 63.2 km. Much of that velocity is upward, not downrange, as shown by the decrease in velocity as the altitude continues to climb rapidly. You can also see from the video that at staging the launch vehicle is still pitched up well above horizontal to gain altitude. (Part of the vertical component of thrust is also needed to overcome gravity.)

Stage 2 ignition occurs at 2:38 at a earth-fixed velocity of 1619 m/s and altitude of 79.8 km. This refers to the second stage but the first stage is still close by in the video, the two having coasted together to this point.

If the two stages rose 79.8 - 63.2 = 16.6 km in only 15 seconds, that's an average vertical velocity component of 1107 m/s. If the average total earth-fixed velocity during this time is, say, 1648 m/s, then the downrange component is sqrt( 1648² - 1107²) = 1221 m/s. That's the component that needs to be cancelled by the boostback burn. The vertical component will be turned into potential energy as the stage reaches apogee. This gets turned back into kinetic energy as the stage falls, with some of it removed by the entry burn but the majority removed by atmospheric drag.

I suspect, but do not know, that the stage is flown back into the atmosphere at a non-zero angle of attack both to increase drag (and dissipate energy from falling) and to produce horizontal lift to push the stage in the desired direction. I'd have to study the videos more closely for clues to the relative wind, e.g. from rocket plumes and debris.
 

[Peter B]

And now SpaceX has launched and recovered a recycled first stage.

http://www.abc.net.au/news/2017-03-31/spacexs-recycled-falcon-9-rocket-blasts-off-in-world-first/8404218

I see it was from the Kennedy Space Center but I'm not sure if it was from Pad 39A.

And while I recognise some of the subject matter experts here aren't cheerleaders for SpaceX, their efforts continue to give me chills of excitement.

[gwiz]

I see it was from the Kennedy Space Center but I'm not sure if it was from Pad 39A.

It was.

[smartcooky]

My reading of the technical broadcast is that first stage shutdown occurs at about 2:23 when the earth-fixed velocity is 1676 m/s and the altitude is 63.2 km. Much of that velocity is upward, not downrange, as shown by the decrease in velocity as the altitude continues to climb rapidly. You can also see from the video that at staging the launch vehicle is still pitched up well above horizontal to gain altitude. (Part of the vertical component of thrust is also needed to overcome gravity.)

Stage 2 ignition occurs at 2:38 at a earth-fixed velocity of 1619 m/s and altitude of 79.8 km. This refers to the second stage but the first stage is still close by in the video, the two having coasted together to this point.

If the two stages rose 79.8 - 63.2 = 16.6 km in only 15 seconds, that's an average vertical velocity component of 1107 m/s. If the average total earth-fixed velocity during this time is, say, 1648 m/s, then the downrange component is sqrt( 1648² - 1107²) = 1221 m/s. That's the component that needs to be cancelled by the boostback burn. The vertical component will be turned into potential energy as the stage reaches apogee. This gets turned back into kinetic energy as the stage falls, with some of it removed by the entry burn but the majority removed by atmospheric drag.

I suspect, but do not know, that the stage is flown back into the atmosphere at a non-zero angle of attack both to increase drag (and dissipate energy from falling) and to produce horizontal lift to push the stage in the desired direction. I'd have to study the videos more closely for clues to the relative wind, e.g. from rocket plumes and debris.
 

I missed this reply earlier, so a belated thanks for getting back to me on this.