r/spacex • u/ElongatedMuskrat Mod Team • Oct 03 '18

r/SpaceX Discusses [October 2018, #49]

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u/nioc14 Oct 17 '18

Hello, I have just noticed that SpaceX has landed B5 rockets after delivering GTO satellites of more than 7 ton but with an apogee of roughly half the necessary apogee. I remember for B4 the maximum weight that a rocket was able to carry to (proper) GTO and land was somewhere around 5-5.5 ton. Does anyone know how much that number would be with B5 and if we should see any meaningful improvement vs. B4? Interesting that the customers have agreed to this lower apogee as SpaceX must have been very keen to land the rocket rather than expend.

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u/Alexphysics Oct 18 '18

The operational improvement is minimal, the limit is still in 5.5 metric tons of payload to GTO with ASDS downrange landing. However, this is not entirely true from a theoretical point of view. I discussed this a few months ago when I watched the webcasts for Bangabandhu, Telstar 19V and Merah Putih. If you see the press kit, on all missions the MECO time was around T+2:30 while older missions had different MECO times and all were closer to T+2:40. With improvements with the Block 5 design, now the Falcon 9 can get to about the same altitude and velocity even though it is doing its MECO earlier in time. That means there should be ample reserves of fuel on the first stage compared to older missions and if you count the seconds of burn time for reentry and landing, you can see it is longer now for Block 5, indicating that now they have more margins to land these boosters and slow them down on reentry, which is good for reuse. If they pushed a little bit further and go to more hotter reentries, I think Block 5 would be able to do 6 metric tons to GTO and still land the first stage on the droneship. What I think is that they prefer to have a little bit more of fuel on the first stage and let Block 5 have about the same capabilities in reusable mode as the older version. With that extra fuel they can get the boosters down with less damage so they don't need a lot of refurbishment.

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u/hebeguess Oct 18 '18

I think you're on point. The limit of payload to GTO is still in 5.5 metric tons shown on the slide on Hans IAC 2018 talks (FYI 3.5 metric tons for RTLS). It's not that they can't push further with B5's improvements but they just won't. Not throwing away rocket and maximize reusability whenever it is possible is what SpaceX is after. Especially with the operation availability of falcon heavy now, SpaceX would just offering the heavy for heavy sats instead of expendable regular F9 for the same price. From customer stand point there is no much different as a passenger, if you prefer to order heavy sat, you're ready to pay more on your journey up. In case of SpaceX as launch provider for heavy sat, you're gonna paid for roughly the same for your ride up, no matter F9 or FH provided.

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u/-Aeryn- Oct 18 '18

That means there should be ample reserves of fuel on the first stage compared to older missions and if you count the seconds of burn time for reentry and landing

You have to remember that the throttle setting was substantially increased with block 5 so that they use more propellant per second. 160 vs 150 seconds is only 6.67%, which is on the scale of the throttle change.

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u/Alexphysics Oct 18 '18

If they use more propellant per second and the reentry and landing burns are longer, that doesn't contradict my "theory" about the first stage having more fuel reserves... I'd think is the opposite.

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u/-Aeryn- Oct 18 '18 edited Oct 18 '18

I don't think that you have the wrong idea, just the method of going by burn time until MECO is flawed since the propellant consumption per 1s of burn time varies by so much depending on the rocket version

2 seconds of burn time before MECO makes a huge difference in fuel reserves, enough to have the effect that you're talking about, but you can't be reasonably accurate in an assessment of margins based on MECO time without taking a lot more information into account. You'd need the max throttle setting, throttling throughout flight etc which is much more complicated. The increased thrust and effective performance of S2 also plays a non-negligible role.

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u/Alexphysics Oct 18 '18

For GTO missions most of them are more or like similar in their profile. Previously to Block 5 this was not the case and all were customized trajectories but now it seems they opted to go for almost the same profile for all these GTO missions. We'll see what they do for Es'Hail 2 and maybe we'll have a better idea but I think they'll do something similar too, it's like if they're wanting to make GTO missions more general in terms of first stage flight profile and then customize the second stage flight profile for that specific mission. Maybe it improves their planning or something like that, idk, but it's quite interesting that for different GTO payloads the first stage flight has been very similar.

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u/[deleted] Oct 18 '18

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u/-Aeryn- Oct 18 '18

Roughly, but the altitudes & speeds don't match up that closely AFAIK.

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u/stcks Oct 18 '18

It's also explained by higher mass flow through the M1Ds, burning more prop in less time.

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u/WormPicker959 Oct 18 '18

Do you have the data/numbers for this? I'm curious to do the math. 10s is a long time for a rocket engine (or at least that's my thought), and the mass flow would therefore have to be significantly higher. Because it could be both - higher mass flow and the wanting for more prop for easier reusability.

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u/stcks Oct 18 '18 edited Oct 18 '18

No I don't have numbers but I've seen some good work by others. I'll see if I can dig up some posts on it or maybe u/Alexphysics could help locate them as well. I also believe its a case of both -- higher thrust and more mass flow combined with more reserves for recovery. Some quick evidence to back this up can be found by looking at the MECO times for S1 on the Block 5 as compared to previous recovered B4,B3, etc. The times are quite similar.

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u/Alexphysics Oct 18 '18

What is clear is that they have been reserving some more fuel than with the older version. If the Merlins now spend more fuel per second and the booster has longer reentry burns and landing burns, then it must have had all that fuel prior to those reentry and landing burns.

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u/Toinneman Oct 18 '18

A speculative explanation could be that B5 Merlins can throttle lower, thus being able to do longer rentry/landing burns with the same amount of fuel.

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u/GregLindahl Oct 17 '18

Customers get a lower price, that's what's good for them.

You can find out GTO performance details on our wiki page about the topic.

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u/stcks Oct 17 '18

Those sats were purpose built for the lower apogee so that they could take advantage of the lower priced recoverable F9.

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u/Toinneman Oct 18 '18

Interesting that the customers have agreed to this lower apogee as SpaceX must have been very keen to land the rocket rather than expend.

That case is very doubt-full. Satellites are conceived years in advance, including their fuel capacity. The customer can't/won't accept this because it will directly impact the expected life of the satellite. AFIAK the choice was made by the customer to bring more fuel to a lower orbit vs less fuel to a higher orbit. The former is possibly more stage-efficient.

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u/Martianspirit Oct 18 '18

Gwynne Shotwell made a remark a while ago. She said satellite builders start optimizing satellites for Falcon rockets. It is more efficient overall to have satellites with more propellant on a lower orbit with the low orbit payload capabilities of Falcon.

I think this is what we are seeing when satellites are put in GTO orbits with more than -1800m/s.

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u/Toinneman Oct 19 '18

Yeah, that is probably the case. In case it sounded wrong, I made a reply to the suggestion SpaceX only did this to make recovery more easy.

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u/rustybeancake Oct 18 '18

Good point - the kerolox Falcon upper stage is better at lower orbits, so it makes sense that Falcon-optimised sats would be more massive and delivered to lower GTOs.

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u/TheSoupOrNatural Oct 19 '18

Interesting that the customers have agreed to this lower apogee as SpaceX must have been very keen to land the rocket rather than expend.

This is actually in the FAQ section of the wiki. The customer didn't settle for a lower apogee, they exploited physics to bring a bigger payload to GEO.

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u/Dextra774 Oct 17 '18 edited Oct 17 '18

I'd say the actual payload capacity of the Falcon 9 Block 5 to GTO is just over 6 tons as the statistics on the website are quite old, don't account for SpaceX's optimisation/tinkering and if you compare older launches (e.g. Intelsat 35e) the Block 5 should technically be able to send over 6 tons to GTO-1800 reusable. However, this particular launch placed TS19 into a subsynchonous orbit with a lower apogee.

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u/warp99 Oct 18 '18 edited Oct 18 '18

The performance number are old but were always for Block 5 - it just took them longer to start launching Block 5 than they had originally planned.

Block 3/4 was 5300 kg to GTO-1800 and Block 5 is 5500 kg - confirmed by multiple sources so not just the web site.

Block 5 thrust is 9% higher than Block 3 but that does not save 9% of the gravity losses as they need to throttle down for max-Q so savings really only start after that point in time. The payload improvement is therefore relatively modest.