SpaceX Falcon 9 : Iridium NEXT Flight 2 (June 25, 2017) : Discussion

[macpacheco]

Unless Block 5 changes things, the Iridium launches still require the ASDS. 10 mt to 800 km to a near polar inclination means they need the ASDS. That said, they want to use the ASDS less, not more. The biggest reason being the added time required for recovery not to mention the downrange weather risks. It will be interesting to see the trades between launching a 3-core RTLS FH and a downrange recovered ASDS F9.

Please take a look at this:
https://twitter.com/elonmusk/status/726560848177561600?lang=en

M1D just have even more thrust available that is still being unused. Block IV/V are just two intermediate steps towards what SpaceX considers safe/reliable and just as importantly margins that don't wear the M1D such that they can still be reflown ~10 times between major refurbs and perhaps reusable 100x total.

The private reports that the last 2 launches were mixed Block III/IV rockets, the information as Block IV upper stage/Block III booster, but that doesn't quite match the observation, of a shorter booster flight time, which suggests the extra thrust is actually on the booster (higher thrust = higher fuel flow = shorter total burn time). Either way a fully Block IV rocket will get a little more payload to the same orbit.
And Block V is still another step forwards.

SpaceX states F9 ultimately can put 8.3 tons to GTO-1800 m/s. Worst case that's with zero margins, but they state it can be done.
Why think that's bogus or whatever ?
Can we please stop with this I'll believe when I see it behavior. M1D have been tested to higher thrust. At the same time, its a good idea to slowly open up the thrust levels so that no surprises are seen in flight, with the Block IV/V just arbitrary throttle limits set as intermediate goals.

[gongora]

Unless Block 5 changes things, the Iridium launches still require the ASDS. 10 mt to 800 km to a near polar inclination means they need the ASDS. That said, they want to use the ASDS less, not more. The biggest reason being the added time required for recovery not to mention the downrange weather risks. It will be interesting to see the trades between launching a 3-core RTLS FH and a downrange recovered ASDS F9.

Please take a look at this:
https://twitter.com/elonmusk/status/726560848177561600?lang=en

M1D just have even more thrust available that is still being unused. Block IV/V are just two intermediate steps towards what SpaceX considers safe/reliable and just as importantly margins that don't wear the M1D such that they can still be reflown ~10 times between major refurbs and perhaps reusable 100x total.

The private reports that the last 2 launches were mixed Block III/IV rockets, the information as Block IV upper stage/Block III booster, but that doesn't quite match the observation, of a shorter booster flight time, which suggests the extra thrust is actually on the booster (higher thrust = higher fuel flow = shorter total burn time). Either way a fully Block IV rocket will get a little more payload to the same orbit.
And Block V is still another step forwards.

SpaceX states F9 ultimately can put 8.3 tons to GTO-1800 m/s. Worst case that's with zero margins, but they state it can be done.
Why think that's bogus or whatever ?
Can we please stop with this I'll believe when I see it behavior. M1D have been tested to higher thrust. At the same time, its a good idea to slowly open up the thrust levels so that no surprises are seen in flight, with the Block IV/V just arbitrary throttle limits set as intermediate goals.

You linked a tweet that says the M1D thrust will be upgraded.  We all know that already.  What is the point?  The person you were responding too was wondering if the Block 5 upgrades will allow RTLS of the Iridium flights.  That is a perfectly reasonable thing to wonder.  Have you done any analysis to see if that is possible?  (It's probably possible, but not by any huge margin.  The NASA launch performance calculator shows it needing about 15% more performance than F9 FT.)  Just waving your hands and throwing out GTO numbers is not a convincing argument that specific payloads to specific orbits from Vandenberg won't need ASDS landings anymore.

[mn]

... (higher thrust = higher fuel flow = shorter total burn time) ...

Does it have to be that way? isn't it possible to get higher thrust by improving isp, so you can get more thrust out of the same fuel flow?

I understand that we may already know what SpaceX is doing with regards to the M1D and we may already know they are increasing fuel flow, just wondering if that is always the case?

Edit: correct quote.

[envy887]

...
The private reports that the last 2 launches were mixed Block III/IV rockets, the information as Block IV upper stage/Block III booster, but that doesn't quite match the observation, of a shorter booster flight time, which suggests the extra thrust is actually on the booster (higher thrust = higher fuel flow = shorter total burn time). Either way a fully Block IV rocket will get a little more payload to the same orbit.
And Block V is still another step forwards.
...

The Inmarsat booster actually had a longer burn time at lower acceleration than Echostar-23. Combined with the heavier payload to higher orbit, and the late LOX load, this indicates more fuel mass onboard but a similar thrust level. However, both are likely Block 3 boosters (as was the Iridium-1 booster). Inmarsat was definitely run at a higher thrust level than SES-10, which is a Block 1 booster.

The NASA LSP information is likely for Block 1, since it was last updated around when CRS-8 flew. I think it's likely that a Block 5 booster will provide the 15% increase necessary to RTLS, but it's hard to get a definitive answer based on the info we have so far.

[envy887]

... (higher thrust = higher fuel flow = shorter total burn time) ...

Does it have to be that way? isn't it possible to get higher thrust by improving isp, so you can get more thrust out of the same fuel flow?

I understand that we may already know what SpaceX is doing with regards to the M1D and we may already know they are increasing fuel flow, just wondering if that is always the case?

Edit: correct quote.

When you throttle a rocket engine up, you're really just spinning the turbopumps faster, which pumps mass into the chamber faster and raises the chamber pressure. The exhaust velocity does increase with chamber pressure, especially in the atmosphere, but the mass flow rate increases faster then exhaust velocity. The two are linked and can't be changed separately just by throttling.

There's another variable here though, and that's fuel mass available. Because SpaceX is subcooling, the fuel mass in the tanks is a function of both volume and temperature, and temperature is a function of time. Late LOX load leaves more LOX mass in the tanks, so the equation above (higher thrust = higher fuel flow = shorter total burn time) only holds for identical loading timelines.

[Lar]

Will we ever see a east cost land landing

Do you mean west coast? We have had several east coast land landings.

[baldusi]

... (higher thrust = higher fuel flow = shorter total burn time) ...

Does it have to be that way? isn't it possible to get higher thrust by improving isp, so you can get more thrust out of the same fuel flow?

I understand that we may already know what SpaceX is doing with regards to the M1D and we may already know they are increasing fuel flow, just wondering if that is always the case?

Edit: correct quote.

When you throttle a rocket engine up, you're really just spinning the turbopumps faster, which pumps mass into the chamber faster and raises the chamber pressure. The exhaust velocity does increase with chamber pressure, especially in the atmosphere, but the mass flow rate increases faster then exhaust velocity. The two are linked and can't be changed separately just by throttling.

There's another variable here though, and that's fuel mass available. Because SpaceX is subcooling, the fuel mass in the tanks is a function of both volume and temperature, and temperature is a function of time. Late LOX load leaves more LOX mass in the tanks, so the equation above (higher thrust = higher fuel flow = shorter total burn time) only holds for identical loading timelines.

Don't forget that Merlin 1 are gas generator engines. To increase Pc, you need to increase massflow through the gas generator, which is dump overboard and basically offers no thrust. So you gain thrust and Pc, which also increases isp, but then you lose also a bigger proportion of your mass through the GG. At certain point, increasing the Pc decreases overall isp due to the increased fraction of massflow deviated to the gas generators.

[deruch]

Don't forget that Merlin 1 are gas generator engines. To increase Pc, you need to increase massflow through the gas generator, which is dump overboard and basically offers no thrust. So you gain thrust and Pc, which also increases isp, but then you lose also a bigger proportion of your mass through the GG. At certain point, increasing the Pc decreases overall isp due to the increased fraction of massflow deviated to the gas generators.

Interesting.  Suggests the possibility of improved rocket performance without a change in Isp by increasing the efficiency of the GG and thereby having to divert less prop mass to a stream that provides no effective thrust.  Would be the equivalent of loading more propellants.  Given talk of alterations to the turbines to deal with cracking issues, maybe it's not farfetched to consider such a change as well?

[baldusi]

Don't forget that Merlin 1 are gas generator engines. To increase Pc, you need to increase massflow through the gas generator, which is dump overboard and basically offers no thrust. So you gain thrust and Pc, which also increases isp, but then you lose also a bigger proportion of your mass through the GG. At certain point, increasing the Pc decreases overall isp due to the increased fraction of massflow deviated to the gas generators.

Interesting.  Suggests the possibility of improved rocket performance without a change in Isp by increasing the efficiency of the GG and thereby having to divert less prop mass to a stream that provides no effective thrust.  Would be the equivalent of loading more propellants.  Given talk of alterations to the turbines to deal with cracking issues, maybe it's not farfetched to consider such a change as well?

The only way to improve the performance without sacrificing extra mass, is if they somehow improved the turbine efficiency. Either by improving the mechanical design or by increasing the gas generator output temperature, both extremely difficult task that would probably mean a Merlin 1E.

[Jakusb]

Matt Desch‏ @IridiumBoss  25m25 minutes ago
Replying to @CJDaniels77
Start shipping satellites this weekend; rocket stages show up next week...

link to tweet

And the related core seems underway too:

first_stage_spotted_westbound @Reddit

[Raul]

Not yet mentioned, that Iridium-2 planned ASDS position is only 300km downrange, compare to Iridium-1 with 372km downrange.
That's for 20% closer to launch pad with same payload -  probably thanks to different F9 performance.

[kevin-rf]

-  probably thanks to different F9 performance.

Or a more lofted trajectory... Or some sort of partial boost back test.... Or

[macpacheco]

-  probably thanks to different F9 performance.

Or a more lofted trajectory... Or some sort of partial boost back test.... Or

There are unconfirmed/unofficial information the last 2 F9 flights used Block IV upper stage with Block III booster. Assuming higher thrust levels, less gravity losses are experienced which produces more useful Delta V. This would leave more performance on the booster for recovery burns.
Eventually a Block V booster should be able to RTLS or at least land on ASDS fairly close to Vandy for those Iridium launches.

But if its going to be an ASDS landing, for west coast launches having ASDS very close to VAFB isn't useful as they are unloading recovered boosters in the LA area. 200km downrange recovery is likely easier, unless RTLS is operational.

Right now its far more useful to let boosters have more performance for a longer re-entry burn so its subject to less heating than to prefer a tight RTLS profile.

If I were a reflight customer, I would prefer a nice ASDS landing than a hot RTLS one. Just saying.

[manoweb]

How can a barge landing ever be "less hot" than returning to the launch site? I mean theoretically it can but if you have the fuel to slow down you can very well just return home.

Also: does not the base at Vandenberg have a port, isn't there anything that could be used instead of Los Angeles? This assumes they do not need to return the rocket back at the factory for refurbishment purposes (and I expect in th long run they would like to avoid this)

[macpacheco]

How can a barge landing ever be "less hot" than returning to the launch site? I mean theoretically it can but if you have the fuel to slow down you can very well just return home.

Also: does not the base at Vandenberg have a port, isn't there anything that could be used instead of Los Angeles? This assumes they do not need to return the rocket back at the factory for refurbishment purposes (and I expect in th long run they would like to avoid this)

Even if there's a port at Vandy, so far there's no word of a Vandy refurb facility, if the booster is either going to be refurbed in Hawthorne, McGregor or Florida (like the first Iridium launch was), what's the point in disembarking closer to VAFB ?

Because the RTLS boostback apparently is 100% a horizontal burn towards the landing site, vertical apogee is the same, what makes typical RTLS nicer landings is there's more fuel left, so the re-entry burn starts a few seconds sooner, reducing peaking and total re-entry heating.
If boostback burns are slightly upward, you get a higher apogee, so more fuel must be spent for the same thermal profile. If boostback is slightly downward, you get less apogee, but a shorter parabolic arc back towards the LZ which might fall short.

But if the boostback fuel is instead used for an even longer re-entry burn, it can soften the re-entry even more.

But on typical high performance GTO launches, the booster apogee is even higher, aka, faster re-entry interface speed, and even without boostback, there isn't much fuel for re-entry, SpaceX is force to make due.

The expectation is with Block V F9+FH, the F9/FH line will be set such that a longer re-entry burn is doable on every F9 launch. The fundamental question is if those 6100kg to super sync launches will ever be sold even for Block V F9, or if the customer will only be offered a FH contract. Maybe reflown FHs will be as cheap as expendable F9 launches.
Remember I'm talking about moving forward, not about what has been sold already, and lets recall that the Inmarsat launch was originally a FH launch that got shifted to a F9 expendable to avoid loosing the contract, it was likely far from the usual.

With future CRS Block V launches, we could see even much softer re-entries.

Ok all of that are educated guesses from an arm chair rocket engineer. But all conclusions are very logical. I've said this much even on L2 (the same jist) and got only upvotes, nobody argued I was wrong.

[manoweb]

OK so you mean: instead of expending fuel to return to launch site, just use it to perform a longer re-entry burn and just land on the barge, applying less heat and possibly acceleration stresses to preserve the rocket.

Well landing at a shorter distance to the launch site, meaning the barge positioned just after the MECO point, and then using fuel to zero out horizontal velocity but not reverse direction, could deliver the "mellowest" re-entry, again if there is enough fuel and the aim is to preserve the rocket for as many re-uses as possible.

Another thing: yes there isn't a Vandenberg refurbishment facility; a possible theory is that launch rate from this location is low enough that they might get to "full, rapid reuse" (close to the "24h" one) and just need the regular integration facility etc

[macpacheco]

OK so you mean: instead of expending fuel to return to launch site, just use it to perform a longer re-entry burn and just land on the barge, applying less heat and possibly acceleration stresses to preserve the rocket.

Well landing at a shorter distance to the launch site, meaning the barge positioned just after the MECO point, and then using fuel to zero out horizontal velocity but not reverse direction, could deliver the "mellowest" re-entry, again if there is enough fuel and the aim is to preserve the rocket for as many re-uses as possible.

Another thing: yes there isn't a Vandenberg refurbishment facility; a possible theory is that launch rate from this location is low enough that they might get to "full, rapid reuse" (close to the "24h" one) and just need the regular integration facility etc

24h refurb is mostly about how much resources are required for the refurb.
Say SpaceX has the ability to launch every day. And has enough payloads to fill such awesome cadence. Its far more likely a few dozen boosters would be recycled rather than as few as possible. Otherwise the more extensive refurb expected every 10 launches would be a substantial bottleneck.
Recycling the very same booster again and again only makes sense with zero refurb like aspired by the ITS video of the same booster launching the spaceship then launching the tanker in short cadence, also assuming the booster can fly 100x between refurbs and the high cadence is temporary such as only during synods.
If larger tonnage to a target or a substantially higher number of satellites launch throughput is desired a larger payload stack is a better bet than a launch every day aspiration.

[manoweb]

I might take years to fly a single "Vandenberg" booster 10 times. But if such booster does not need extensive refurbishment they would not need a designated facility at Vandenberg. When a more extensive refurb is needed they might get it to the factory.
What I am trying to say, is that they may end up not needing an additional, refurbishment dedicated facility to perform RTLS or even tow the barge back at Vandenberg

[IanThePineapple]

I might take years to fly a single "Vandenberg" booster 10 times. But if such booster does not need extensive refurbishment they would not need a designated facility at Vandenberg. When a more extensive refurb is needed they might get it to the factory.
What I am trying to say, is that they may end up not needing an additional, refurbishment dedicated facility to perform RTLS or even tow the barge back at Vandenberg

Very true, I think they could even get by by just using one side of the HIF as a booster refurb facility, and have 2 Vandenberg boosters, since they'll only have like 3-5 Vandy launches per year.

[FishInferno]

I might take years to fly a single "Vandenberg" booster 10 times. But if such booster does not need extensive refurbishment they would not need a designated facility at Vandenberg. When a more extensive refurb is needed they might get it to the factory.
What I am trying to say, is that they may end up not needing an additional, refurbishment dedicated facility to perform RTLS or even tow the barge back at Vandenberg

Very true, I think they could even get by by just using one side of the HIF as a booster refurb facility, and have 2 Vandenberg boosters, since they'll only have like 3-5 Vandy launches per year.

Not once the internet constellation gets going.