totm nov 2023 SpaceX Discussion Thread

[XB-70A]

Pretty fun as the sky was clear enough to see the first stage descending back during one of the braking burns.

I took some shots too, but the result is (obviously) a bit disappointing. At 16.5 km from the LC-40 and at night it's hard to imagine something better.

[Jaff]

So can someone in the know answer a question.

 

do they always fill up a rocket with fuel or do they only put as much fuel as they need in? 

 

If its the former do they expend the first stage and have excess in the upper stage or do they cut the first stage off early and have the upper stage expend all its fuel? 

[Nightfury]

17 hours ago, sh1pman said:

Landing during a thunderstorm would surely be awesome to watch though

And the explosion when the booster tips over... hot and bright

[cubinator]

1 hour ago, Jaff said:

So can someone in the know answer a question.

 

do they always fill up a rocket with fuel or do they only put as much fuel as they need in? 

 

If its the former do they expend the first stage and have excess in the upper stage or do they cut the first stage off early and have the upper stage expend all its fuel? 

I believe the second stage vents any excess fuel after orbit insertion.

[Jaff]

2 minutes ago, cubinator said:

I believe the second stage vents any excess fuel after orbit insertion.

So S1 is always full. 

 

That means with heavier PL the first stage doesn’t go as high/far as it would with a lighter PL? 

 

Just trying to compare it a bit with KSP as I build families of rockets and take fuel out of the bottom tanks to give it a suitable dV  when realistically I should be just keeping to full and flying it as far as it will go 

[PB666]

10 hours ago, Ultimate Steve said:

I believe they said largest, but I might have misheard. Not as heavy.

Largest to go to GTO, iirc. I have no idea if this was accurate, it was late and I was watching another TV program at the time. 

[sevenperforce]

3 hours ago, Jaff said:

So can someone in the know answer a question.

do they always fill up a rocket with fuel or do they only put as much fuel as they need in? 

If its the former do they expend the first stage and have excess in the upper stage or do they cut the first stage off early and have the upper stage expend all its fuel? 

All modern launch vehicles launch with full tanks. Propellant is cheap, and a launch vehicle's aerodynamic performance is based heavily on its initial TWR off the pad, so if you launched with less-than-full tanks you'd need to completely recalculate your entire ascent.

In most cases, the first stage (or stages) are burned to minimum residual shutdown (MRS), meaning that the engines are commanded to shut down just before they run out of propellant. Running liquid engines dry will blow them up. The terminal stage then proceeds to the target orbit. For GTO launches, where it is advantageous to raise apogee as high as possible, the terminal stage may also continue burning to MRS after reaching the target GTO, as any additional dV for the payload will save it fuel on the inclination change and circularization. For launches where a more precise orbit is desired, like sending a spacecraft to the ISS, the engine shuts down with significant propellant reserves. These are vented following spacecraft separation; since the tanks are pressurized, propellant venting can often be used in lieu of a deorbit burn, to hasten the stage's entry and disposal.

Of course, even though propellant is cheap, rockets are not, and so you do not want to waste too much performance. So, how do you avoid wasting performance? Expendable rockets adjust performance by either adding an upper stage on top or adding strap-on boosters beneath. The Atlas V and Delta IV families from ULA can strap on 1-5 solid-fueled boosters, adding additional dV to the first stage to increase the staging velocity of the upper stages. Soyuz, in contrast, can launch with the Fregat upper stage if Stage III is not enough; the Fregat can also have larger tanks added to it, or even a toroidal drop tank.

SpaceX does it differently. Since SpaceX recovers its first stage, it precisely adjusts upper-stage performance by reserving propellant on the first stage. The more propellant it can reserve on the first stage, the easier the re-entry and landing will be.

 

[CatastrophicFailure]

2 hours ago, sevenperforce said:

For launches where a more precise orbit is desired, like sending a spacecraft to the ISS, the engine shuts down with significant propellant reserves. These are vented following spacecraft separation; since the tanks are pressurized, propellant venting can often be used in lieu of a deorbit burn, to hasten the stage's entry and disposal.

Just this kind of venting was observed on the Zuma mission, over northern Africa, shortly before the upper stage re-entered:

2 hours ago, sevenperforce said:

For GTO launches, where it is advantageous to raise apogee as high as possible, the terminal stage may also continue burning to MRS after reaching the target GTO, as any additional dV for the payload will save it fuel on the inclination change and circularization.

So, question here: On this kind of MRS super-synchronous shutdown, they still need to dispose of the upper stage after separation. Any ideas how they actually give it this nudge? They could adjust the burn attitude so that while raising the apogee up to SSO, the perigee on the return also dips into the atmosphere, but that would place the payload at risk if there was some problem and it was unable to fire its own thruster at apogee to begin circularizing.

The on-board RCS could probably give the needed kick at apogee to drop the perigee enough, but after the FH launch I kind of got the impression SpaceX wasn't accustomed to maintaining contact/control with the upper stage that long after launch (3 hours or more, after passing the Van Allen belts, too).

[sevenperforce]

5 minutes ago, CatastrophicFailure said:

So, question here: On this kind of MRS super-synchronous shutdown, they still need to dispose of the upper stage after separation. Any ideas how they actually give it this nudge? They could adjust the burn attitude so that while raising the apogee up to SSO, the perigee on the return also dips into the atmosphere, but that would place the payload at risk if there was some problem and it was unable to fire its own thruster at apogee to begin circularizing.

The on-board RCS could probably give the needed kick at apogee to drop the perigee enough, but after the FH launch I kind of got the impression SpaceX wasn't accustomed to maintaining contact/control with the upper stage that long after launch (3 hours or more, after passing the Van Allen belts, too).

The parking orbit from where GTO insertion is performed is high enough to prevent significant drag during coast, but it has enough drag to rapidly deorbit a second stage if it makes a couple of passes.

The Falcon 9 can also vent its tank pressure shortly after separation but do so on a normal vector, changing the argument of periapsis and lowering periapsis simultaneously. This will ensure re-entry on the very next orbit.

[sevenperforce]

33 minutes ago, CatastrophicFailure said:

...that would place the payload at risk if there was some problem and it was unable to fire its own thruster at apogee to begin circularizing.

Just an aside...if the payload is unable to fire its own thruster at apogee, the payload is DOA anyway. A GEO satellite is worth exactly $0 if it cannot get to GEO.

It is possible to do a direct ascent burn to GTO, where there is no coast period and the upper stage never reaches an orbit with a periapsis out of the atmosphere. The Ariane 5, for example, usually does this. Ariane 5 can do this because the Guana launch site is almost exactly at the equator, but launches from the Cape or from higher-inclination sites cannot; they need to launch down toward the equator and then coast until they cross 0 latitude.

The GTO injection burn must take place at 0 latitude because the destination orbit needs to be equatorial. If you tried to do a GTO injection burn anywhere other than at 0 latitude, you'd end up with a perigee at a nonzero latitude, which means an apogee at a nonzero latitude, which means the spacecraft has to spend additional propellant below apogee to circularize at a huge dV cost.

Even with a GTO injection burn over the equator, the GTO orbit is still going to be inclined. The longer the coast period, the less inclined it will be; that's why SpaceX launches as close to the beginning of the launch window as possible. A delay of an hour doesn't make too big of a difference, though. 

A GTO injection burn could dog-leg to cancel inclination over the equator, but because changing inclination is so much cheaper at apogee, it is far more efficient to simply raise apogee a little higher (to supersynch) with a longer burn, and let the payload do the inclination change.

Edited March 6, 2018 by sevenperforce

[Jaff]

Really interesting and kind of answers my question. But space x landing boosters aside, does the fact that the first stage is full of fuel and burns to pretty much empty mean that a lighter payload has the first stage burn faster/longer? 

[sevenperforce]

9 minutes ago, Jaff said:

Really interesting and kind of answers my question. But space x landing boosters aside, does the fact that the first stage is full of fuel and burns to pretty much empty mean that a lighter payload has the first stage burn faster/longer? 

Well, that depends on what you mean by "faster/longer". For a given vehicle configuration, the first stage burn time will always be about the same; they have a certain amount of fuel and they want to burn at full thrust to minimize gravity losses, so burn time is simply propellant load divided by engine propellant mass flow rate. Of course there is a throttledown for Max-Q and there is sometimes a throttledown just before MECO, but those don't change much from mission to mission.

However, for a lighter payload, the launch stack will be higher, faster, and further downrange at MECO, because it has been lifting less mass. The mass of the payload is only a few percent of the total vehicle mass, of course, so the difference isn't going to be huge, but it's measurable.

SpaceX has a LOT more propellant on its upper stage, relative to its first stage, than most launch vehicles. The job of the first stage is really just to get the second stage + payload up and out of the atmosphere; the second stage usually provides over 90% of the energy for orbital insertion. For heavier payloads, the second stage needs more of a "kick" because there is more to lift (or farther to go). The decision about whether to expend the first stage, land the first stage maximally downrange on an ASDS, land the first stage partially downrange, or RTLS depends on how propellant is reserved vs how much is given as an extra kick to the second stage.

[Elthy]

For everyone except SpaceX the burntime remains the same, with a lighter payload you will just have a higher acceleration = more speed at separation. For SpaceX a lighter payload actually means a shorter burn on the first stage, since the second stage has more DeltaV. This way the first stage can use more fuel for the boostback/reentry/landing.

[sevenperforce]

12 minutes ago, Elthy said:

For everyone except SpaceX the burntime remains the same, with a lighter payload you will just have a higher acceleration = more speed at separation.

There are a few exceptions. The core burn time for an Atlas V 501 is the same as a 521, for example. But a Delta IV Heavy has a longer core-stage burn time, because the core is throttled down while the side boosters remain at full throttle.

The Delta IV and Atlas V have a baseline configuration, and they add first-stage boosters to increase performance. The Soyuz family has a baseline configuration and adds an upper stage with optional extra upper stage tanks to increase performance. The Proton family also adds various optional upper stages of various sizes to increase performance. The Ariane 5 basically has just one configuration and wastes a lot of performance on smaller payloads. The Falcon 9 uses a very overpowered baseline configuration and uses recovery as a way to decrease performance, since recovery saves costs.

Edited March 6, 2018 by sevenperforce

[sevenperforce]

Comparing the ULA approach (add first-stage boosters to increase performance) to the Soyuz/Proton approach (add upper stages to increase performance) has an interesting cost/benefit breakdown. Adding upper stages gives you a lot of modularity and allows you to fine-tune your performance quite a bit, and your launch and ground support structure doesn't have to change at all. Plus, small upper stages are usually cheaper than a big booster.

On the other hand, the vehicle's baseline configuration already needs to have terminal guidance capabilities on the upper stage, so if you've already sunk a lot of money into developing a really capable upper stage (think Centaur) then adding another stage on top of that doesn't make much sense. Plus, solid boosters are really, really cheap...sometimes cheaper than a small upper stage. 

[1101]

25 minutes ago, sevenperforce said:

However, for a lighter payload, the launch stack will be higher, faster, and further downrange at MECO, because it has been lifting less mass. The mass of the payload is only a few percent of the total vehicle mass, of course, so the difference isn't going to be huge, but it's measurable.

As an example, a comparison of Apollo 11 to Skylab 1 gives some interesting results - 3 second shorter S-IC burn time for Skylab, but peak accelerations prior to CECO/MECO were ~4.4g for skylab, ~3.9g for Apollo 11 with it's squishy LM and humans.  Skylab MECO velocity was 2,565.3m/s, Apollo 11 2,402.7m/s.

Figures from the Flight Evaluation reports for both missions:

Skylab

Apollo 11

So overall Delta-v would be really similar for S-IC performance across both launches (only 150m/s difference), burn times nearly identical.  I presume small increments in mission procedures and payload type (crewed vs uncrewed) are other causes for the differences. 

So, as burn time is a function of tank size and fuel burn rate, more or less the same for most (non-recoverable) boosters. 

SpaceX, I get the impression at least, flies steeper (especially for RTLS) to give the second stage more time for a longer burn, and reduce velocity to be removed for recovery, but correct me if I'm wrong.

[sevenperforce]

14 minutes ago, 1101 said:

As an example, a comparison of Apollo 11 to Skylab 1 gives some interesting results - 3 second shorter S-IC burn time for Skylab, but peak accelerations prior to CECO/MECO were ~4.4g for skylab, ~3.9g for Apollo 11 with it's squishy LM and humans.  Skylab MECO velocity was 2,565.3m/s, Apollo 11 2,402.7m/s.

Figures from the Flight Evaluation reports for both missions:

Skylab

Apollo 11

So overall Delta-v would be really similar for S-IC performance across both launches (only 150m/s difference), burn times nearly identical.  I presume small increments in mission procedures and payload type (crewed vs uncrewed) are other causes for the differences. 

There are actually more significant changes. Apollo 11-13 used a constant mixture ratio in the Saturn V first stage, but Apollo 14-17 and Skylab used a variable mixture ratio. By burning more oxy-rich at launch and more fuel-rich later on, the F-1 engines had a higher TWR at liftoff and a higher isp at the end of the burn. I'm actually surprised that the difference was only three seconds.

Also note that Skylab was much lighter weight than the Apollo lunar stack.

Quote

So, as burn time is a function of tank size and fuel burn rate, more or less the same for most (non-recoverable) boosters. 

SpaceX, I get the impression at least, flies steeper (especially for RTLS) to give the second stage more time for a longer burn, and reduce velocity to be removed for recovery, but correct me if I'm wrong.

Yes. The MVac is underpowered for the size of the second stage, so it needs a lofted trajectory. This means a steeper trajectory, which means less propellant for boostback.

Edited March 6, 2018 by sevenperforce

[1101]

24 minutes ago, sevenperforce said:

There are actually more significant changes. Apollo 11-13 used a constant mixture ratio in the Saturn V first stage, but Apollo 14-17 and Skylab used a variable mixture ratio. By burning more oxy-rich at launch and more fuel-rich later on, the F-1 engines had a higher TWR at liftoff and a higher isp at the end of the burn. I'm actually surprised that the difference was only three seconds.

 

I did kind of skim read those documents - one thing of note was that Skylab apparently got higher Isp (and about 18m/s more dV) than expected.  I didn't know that the S-IC could change mix ratio, for some reason thought that was just the J-2 stages.  Back to SpaceX I guess...

[Jaff]

4 hours ago, sevenperforce said:

Well, that depends on what you mean by "faster/longer". For a given vehicle configuration, the first stage burn time will always be about the same; they have a certain amount of fuel and they want to burn at full thrust to minimize gravity losses, so burn time is simply propellant load divided by engine propellant mass flow rate. Of course there is a throttledown for Max-Q and there is sometimes a throttledown just before MECO, but those don't change much from mission to mission.

However, for a lighter payload, the launch stack will be higher, faster, and further downrange at MECO, because it has been lifting less mass. The mass of the payload is only a few percent of the total vehicle mass, of course, so the difference isn't going to be huge, but it's measurable.

SpaceX has a LOT more propellant on its upper stage, relative to its first stage, than most launch vehicles. The job of the first stage is really just to get the second stage + payload up and out of the atmosphere; the second stage usually provides over 90% of the energy for orbital insertion. For heavier payloads, the second stage needs more of a "kick" because there is more to lift (or farther to go). The decision about whether to expend the first stage, land the first stage maximally downrange on an ASDS, land the first stage partially downrange, or RTLS depends on how propellant is reserved vs how much is given as an extra kick to the second stage.

I mean exactly what you explained - thanks  

 

so back to my kerbal analogy to keep my rockets realistic I should have my rockets first stage always full and it stages at whatever speed it gets to (dependant in PL mass) and waste the fuel in the upper stage if I don’t need it? 

And add moar boosters if it doesn’t have enough dV

 

i missed the Hispasat launch too, any news on the fairings? I thought they were supposed to be the 2.0 fairings hence the delay in the first place ?!

[tater]

No, @Jaffthe launch was fairing 1.0

[PB666]

47 minutes ago, Jaff said:

I mean exactly what you explained - thanks  

 

so back to my kerbal analogy to keep my rockets realistic I should have my rockets first stage always full and it stages at whatever speed it gets to (dependant in PL mass) and waste the fuel in the upper stage if I don’t need it? 

And add moar boosters if it doesn’t have enough dV

 

i missed the Hispasat launch too, any news on the fairings? I thought they were supposed to be the 2.0 fairings hence the delay in the first place ?!

Well if you had a model for the second stage, you could actually use FH for every rocket and have enough fuel in the second stage to re land once it dumped its payload, I wouldn't count on it. Elon Musk still considers the second stage an expendable part of the rocket, estimates that it cost in the 10 million dollar range, if we argue that a core and two boosters cost 58 million dollars you cannot justify the added launch cost to bring back the second stage. What they will probably is make the second stage cheaper (less costly to build).

SFRBs are not the way to go, they make a rocket more complex, as F9 did pair your side boosters the same diameter as the main rocket, if you are not going to use fuel feeds from the booster to the core, then what you do as you approach 250 m/s then action/toggle some of the cores engines off (assuming you have a multi-engine core), after passing MaxQ tag them back or leave them off until booster separation. This keeps fuel in the core.

Asparagas may not be used but it is handy in one particlar situtation, if you have a PL that is extremely bulky and you want to delay MaxQ until the static pressure is hideously low, then just go strait up and drop boosters, the FH shows how cheap boosters can be (10-20 million or so per each), over there in the corner ULA is charging 400 millon dollars, so dropping 4 boosters at 56 million on a set.

[CatastrophicFailure]

  

 

 

[CatastrophicFailure]

Via Twitter, some awesome slomo/timelapse of the last launch, including a piece of debris that goes flying right near the burning engines! 

 

[linuxgurugamer]

That projectile was flying away  from the rocket, it was nowhere near the engines

[PB666]

8 minutes ago, linuxgurugamer said:

That projectile was flying away  from the rocket, it was nowhere near the engines

Its a bird, its a plane its . . . . . . . .no it was a bird.