SpaceX Falcon 9 - Inmarsat 5 F4 - May 15, 2017 - DISCUSSION

[edkyle99]

Was the Iridium payload adapter on top of a regular payload adapter? If so, then it should be counted as payload. If it was in /place/ of the regular payload adapter, then you can count it as Stage 2. We're talking about the maximum payload mass a Falcon 9 has proven it could launch, and if the former is true, then it'd be factually incorrect to assert it has proven less.

No, the Iridium payload adapter was the payload adapter, or at least part of the adapter.  Why count this dead weight?  If the adapter were lighter, Iridium might have been able to put another satellite on board!

 - Ed Kyle

[gospacex]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

No. A rocket engine's thrust is a result of it throwing many thousands of pounds of propellant out of the nozzle, opposite the desired direction of travel independent of whatever is behind it. Tom Mueller had a great practical example in his recent interview. A rocket engine works on the same principle as sitting in the back of the wagon, and throwing a brick out the back of it. You'll get a small impulse from throwing a brick opposite the direction you want to travel. If you could throw thousands of pounds of bricks out the back continuously, you'd move very, very quickly.

Your explanation is correct, but it does not explain why presence of S1 does not change the picture.

Naively, gas impinging on S1 results in gas cloud between S1 and S2 having higher pressure and pushing on S2 a little more than if S1 would not be there.

This does not happen because exhaust is supersonic and any changes in pressure from below the nozzle can not propagate through the gas and affect the nozzle and S2.

[envy887]

Was the Iridium payload adapter on top of a regular payload adapter? If so, then it should be counted as payload. If it was in /place/ of the regular payload adapter, then you can count it as Stage 2. We're talking about the maximum payload mass a Falcon 9 has proven it could launch, and if the former is true, then it'd be factually incorrect to assert it has proven less.

No, the Iridium payload adapter was the payload adapter, or at least part of the adapter.  Why count this dead weight?  If the adapter were lighter, Iridium might have been able to put another satellite on board!

 - Ed Kyle

Iridium used a pair of dispensers mounted atop the standard SpaceX PAF:
https://spaceflightnow.com/wp-content/uploads/2016/07/IRIDIUM_Test_Prep_183_KHarris.jpg
https://assets.cdn.spaceflightnow.com/wp-content/uploads/2017/02/03171820/C04EdfTUsAEiHPC-2.jpg

It's the same PAF as the one Imarsat 5 F4 used:
https://assets.cdn.spaceflightnow.com/wp-content/uploads/2017/05/10184932/C_dCn50XgAIDsvL.jpg

The dispensers don't appear to be part of the PAF at all. IMO the dispensers are non-separating payload, while the PAF is a standard part of the second stage - but it all depends how one defines payload.

[edkyle99]

- but it all depends how one defines payload.

Yes! 

 - Ed Kyle

[abaddon]

No, the Iridium payload adapter was the payload adapter, or at least part of the adapter.  Why count this dead weight?  If the adapter were lighter, Iridium might have been able to put another satellite on board!

The Iridium dispenser is also the payload adapter?  I had assumed so, do you have confirmation?  Ah, thanks @envy887, it is not:

Iridium used a pair of dispensers mounted atop the standard SpaceX PAF

In any case, you're absolutely right, if the dispenser was lighter - like a payload adapter - it could have put more payload into orbit, which is exactly my point.

From https://spaceflightnow.com/2016/12/30/iridium-satellites-closed-up-for-launch-on-falcon-9-rocket/:

With a full load of propellant, each satellite weighs nearly 1,900 pounds (860 kilograms), and when combined with the specially-designed multi-spacecraft mounting dispenser, the Iridium Next package will be weigh in at more than 20,000 pounds, the heaviest payload launched by SpaceX to date.

we know the dispenser is at least 460kg.

This means we know a single satellite going to the same orbit as the Iridium constellation could have massed a little over nine metric tonnes at a minimum with downrange ASDS recovery of the first stage.

To be clear; I would not count the dispenser as payload for this flight.  However, it does demonstrate payload capability for a satellite that does not require the dispenser.

Since this is all off-topic for Inmarsat 5 F4, I'll bow out at this point.

[macpacheco]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

In order for such an effect to help, it has to throw rocket exhaust back at the engine/stage bottom. The main purpose of the flame trench is precisely to protect the rocket from its own acoustic energy.
Even if the 1st stage helps at all in the initial acceleration, its not even a tiny fraction of a second, for all practical purposes the effect is insignificant !

[rsdavis9]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

No. A rocket engine's thrust is a result of it throwing many thousands of pounds of propellant out of the nozzle, opposite the desired direction of travel independent of whatever is behind it. Tom Mueller had a great practical example in his recent interview. A rocket engine works on the same principle as sitting in the back of the wagon, and throwing a brick out the back of it. You'll get a small impulse from throwing a brick opposite the direction you want to travel. If you could throw thousands of pounds of bricks out the back continuously, you'd move very, very quickly.

Your explanation is correct, but it does not explain why presence of S1 does not change the picture.

Naively, gas impinging on S1 results in gas cloud between S1 and S2 having higher pressure and pushing on S2 a little more than if S1 would not be there.

This does not happen because exhaust is supersonic and any changes in pressure from below the nozzle can not propagate through the gas and affect the nozzle and S2.

In the example it is like the bricks you throw out bounce off something and hit you. Thereby giving you a little extra impulse at the expense of getting hit with bricks.

[edkyle99]

This was NOT a flight to GTO!  This used a super-synchronous transfer orbit.

Don't the parameters at insertion (315 km altitude, 10,025 m/s velocity) indicate a subsynchronous transfer orbit?

Isn't a supersynchronous transfer a type of GTO?

 - Ed Kyle

[rsdavis9]

This was NOT a flight to GTO!  This used a super-synchronous transfer orbit.

Don't the parameters at insertion (315 km altitude, 10,025 m/s velocity) indicate a subsynchronous transfer orbit?

 - Ed Kyle

I calculated it naively with the elliptical orbit and got almost exactly a apogee of geo sync. i.e. 35786 km above earth surface.

v=(u*(2/r-1/a))^.5
u=GM
r=perigee from center of earth
a=semimajor axis

[LouScheffer]

This was NOT a flight to GTO!  This used a super-synchronous transfer orbit.

Don't the parameters at insertion (315 km altitude, 10,025 m/s velocity) indicate a subsynchronous transfer orbit?

I calculated it naively with the elliptical orbit and got almost exactly a apogee of geo sync. i.e. 35786 km above earth surface.

v=(u*(2/r-1/a))^.5
u=GM
r=perigee from center of earth
a=semimajor axis

Velocity seems relative to launch site, since it's 0 at launch and not 408 m/s.  Adding this in, you get  an inertial frame speed of 10435, for a apogee of about 62000 km, or super-synchronous.

[kevin-rf]

Am I reading it wrong, or has Space-Track.org not published a TLE yet?

Seems the newest entry is still 42697, ISS Debris...

[mn]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

No. A rocket engine's thrust is a result of it throwing many thousands of pounds of propellant out of the nozzle, opposite the desired direction of travel independent of whatever is behind it. Tom Mueller had a great practical example in his recent interview. A rocket engine works on the same principle as sitting in the back of the wagon, and throwing a brick out the back of it. You'll get a small impulse from throwing a brick opposite the direction you want to travel. If you could throw thousands of pounds of bricks out the back continuously, you'd move very, very quickly.

(mass of brick) * (acceleration experienced by brick being thrown by you) = (force of brick leaving your hand, pushing you and wagon forward)

F = Ma

I'm not a rocket scientist so I hope i'm allowed stupid questions.

Suppose there's a wall behind the wagon and I manage to throw enough rocks to fill the space between the wagon and the wall, and I continue throwing rocks, Now i'm not just throwing rocks in a vacum, I'm essentially pushing against this 'wall of rocks' in front of me, wouldn't that give me a little bit more thrust?

Edit: never mind, I see this was already discussed before I posted, somehow missed the last page of the thread before posting.

[BabaORileyUSA]

Am I reading it wrong, or has Space-Track.org not published a TLE yet?

Seems the newest entry is still 42697, ISS Debris...

18 SPCS is in the process of publishing Elset One to Space-track.  42698 and 42699 will be the numbers.

[Crispy]

I'm not a rocket scientist so I hope i'm allowed stupid questions.

Suppose there's a wall behind the wagon and I manage to throw enough rocks to fill the space between the wagon and the wall, and I continue throwing rocks, Now i'm not just throwing rocks in a vacum, I'm essentially pushing against this 'wall of rocks' in front of me, wouldn't that give me a little bit more thrust?

Any forces that act on the exhaust of a rocket engine can only propagate through that exhaust as fast as the speed of sound. Because the exhaust is moving far far faster than that, nothing downstream of the nozzle can possibly affect it.

[ppnl]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

No. A rocket engine's thrust is a result of it throwing many thousands of pounds of propellant out of the nozzle, opposite the desired direction of travel independent of whatever is behind it. Tom Mueller had a great practical example in his recent interview. A rocket engine works on the same principle as sitting in the back of the wagon, and throwing a brick out the back of it. You'll get a small impulse from throwing a brick opposite the direction you want to travel. If you could throw thousands of pounds of bricks out the back continuously, you'd move very, very quickly.

(mass of brick) * (acceleration experienced by brick being thrown by you) = (force of brick leaving your hand, pushing you and wagon forward)

F = Ma

I'm not a rocket scientist so I hope i'm allowed stupid questions.

Suppose there's a wall behind the wagon and I manage to throw enough rocks to fill the space between the wagon and the wall, and I continue throwing rocks, Now i'm not just throwing rocks in a vacum, I'm essentially pushing against this 'wall of rocks' in front of me, wouldn't that give me a little bit more thrust?

Edit: never mind, I see this was already discussed before I posted, somehow missed the last page of the thread before posting.

Only if some of the rocks you threw bounced back and hit  you. And if enough of them did that to make any measurable difference it would probably be a bad thing.

[Pete]

If the plume pushed against the 1st stage would the intial aceleration be larger than straight into vaccum?

No. A rocket engine's thrust is a result of it throwing many thousands of pounds of propellant out of the nozzle, opposite the desired direction of travel independent of whatever is behind it. Tom Mueller had a great practical example in his recent interview. A rocket engine works on the same principle as sitting in the back of the wagon, and throwing a brick out the back of it. You'll get a small impulse from throwing a brick opposite the direction you want to travel. If you could throw thousands of pounds of bricks out the back continuously, you'd move very, very quickly.

Your explanation is correct, but it does not explain why presence of S1 does not change the picture.

Naively, gas impinging on S1 results in gas cloud between S1 and S2 having higher pressure and pushing on S2 a little more than if S1 would not be there.

This does not happen because exhaust is supersonic and any changes in pressure from below the nozzle can not propagate through the gas and affect the nozzle and S2.

In the example it is like the bricks you throw out bounce off something and hit you. Thereby giving you a little extra impulse at the expense of getting hit with bricks.

If you want to be REALLY pedantic...
The presence of S1 in the exhaust stream likely *decreases* the thrust of S2 by an infinitestimal amount.
.
No, really.
Why:
Having the exhaust plume impinge on a stationary object directly behind the S2 nozzle will cause quite a large amount of gaseous matter to "accumulate" in the area, leading to a very-slightly-less-than-vacuum environment.
This will directly result in a very slightly lower engine ISP, which mean less thrust.

The decrease in thrust is likely to be very very very small, but still larger than the "air pressure" from the bouncing gas molecules hitting S1, bouncing off, and hitting S2.

[ClayJar]

I'm not a rocket scientist so I hope i'm allowed stupid questions.

Suppose there's a wall behind the wagon and I manage to throw enough rocks to fill the space between the wagon and the wall, and I continue throwing rocks, Now i'm not just throwing rocks in a vacum, I'm essentially pushing against this 'wall of rocks' in front of me, wouldn't that give me a little bit more thrust?

If you filled up the space with rocks and you kept shoving more in, what you would have then would be analogous to pneumatic or hydraulic pushers, some types of which SpaceX uses for certain separation events.  In that case, the force results from the pressure of the fluid in the confined space being exerted over a given area.  Since the fluid does have mass and velocity, there would be some negligible thrust caused by the fluid flowing into the space, but basically all the force is from a fluid pushing against whatever it is surrounding it.

The thing about pressure in fluids exerting forces on their surroundings is that the forces are the result of the particles (atoms, molecules, whatever) in the fluid bouncing off things.  The higher the pressure in a pneumatic cylinder, the more particles will be bouncing off the piston at the end, so the more force will result.  The thing to remember, however, is that the particles have some finite speed, and if you're moving away fast enough, you can outrun them.  You only need to be moving away fast enough that as one bounces off another off another and so on like a chain of falling dominos, they can't catch up and hit you -- in other words, the force they're trying to exert on you can't move upstream faster than the speed of sound:

This does not happen because exhaust is supersonic and any changes in pressure from below the nozzle can not propagate through the gas and affect the nozzle and S2.

In order to have any effect upstream on S2, you would have to build up enough exhaust in the vacuum of space with nothing to contain it to make a bubble with enough matter and pressure to have a high enough local speed of sound to let the forces from the colliding particles catch up to the second stage that is very rapidly departing.  (Any particles that happen to bounce off S1 back toward S2 will run into the exhaust plume and never have a chance to ricochet off S2.)

[rsdavis9]

Why:
Having the exhaust plume impinge on a stationary object directly behind the S2 nozzle will cause quite a large amount of gaseous matter to "accumulate" in the area, leading to a very-slightly-less-than-vacuum environment.
This will directly result in a very slightly lower engine ISP, which mean less thrust.

The decrease in thrust is likely to be very very very small, but still larger than the "air pressure" from the bouncing gas molecules hitting S1, bouncing off, and hitting S2.

and that is because anything not completely vacuum reduces supersonic exhaust velocity which reduces thrust?

[Silmfeanor]

From the update thread -

42698   INMARSAT 5-F4   2017-025A      1401.67min   24.50deg   69839km   381km      
42699   FALCON 9 R/B   2017-025B           1410.43min   24.47deg   70181km   384km

Definitly Super-synchronous.

[guckyfan]

From the update thread -

42698   INMARSAT 5-F4   2017-025A      1401.67min   24.50deg   69839km   381km      
42699   FALCON 9 R/B   2017-025B           1410.43min   24.47deg   70181km   384km

Definitly Super-synchronous.

Certainly. Only thing is that upper stage will stay up there for a long time.