Author Topic: FH payload to Mars, when fully re-usable  (Read 6947 times)

Offline LouScheffer

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FH payload to Mars, when fully re-usable
« on: 02/11/2018 03:20 PM »
The FH just put the roadster, estimated at 1250 kg, on an orbit with Mars apogee.   But the first launch left a lot on the table.   How much can a recoverable FH lift to Mars?

First, it appears the first flight used a very tame throttle profile, with the center core not even at full thrust for liftoff, and only 30 seconds of single-core operation after separation.  A more aggressive throttle up at liftoff, then further down during 3 booster flight,  could save more fuel for after booster sep, giving some of the benefit of cross-feed.  A crude guess indicates they could get perhaps 500 m/s more staging velocity with this.

Second, SpaceX had the ASDS close to shore.   Putting it further out means no boostback burn.  Another crude guess indicates they could get 500 m/s from this.

Of course, they cannot use all 1000 m/s for increased staging, since they need to save some for slowing down the booster so not to fry it on entry.   This trades off about 4:1, since the booster mass is about 1/4 of the whole stack near staging.   So perhaps they can increase the staging velocity by 800 m/s, then slow the booster by the same amount, then land fully downrange.

Now plugging in some estimates for the second stage (ISP=348, empty mass 4.5t, fuel = 111.5t) gives the delta-V for a 1.25t payload.   If the second stage can provide 800 m/s less, it can loft about 3t instead of 1.25t.

Finally, there is some improvements that are hard to estimate.  Block 5 will increase the thrust, which will decrease the gravity losses.  Also there is no 6 hour coast on a normal Mars injection.  The total firing time of the second stage on the first FH mission was shorter than usual by about 23 seconds.   Some of this might be no need to throttle for this payload, but a lot might be boiloff.  This could result in a large increase in capability - even 10 more seconds of firing could contribute 500 m/s or so.

So all things combined, it would not surprise me if FH could loft 3.8t (the mass of Mars Science Laboratory) to Mars and still recover all boosters.  Of course there are lots and lots of estimates, handwaving, and assumptions behind this conclusion.

Offline guckyfan

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Re: FH payload to Mars, when fully re-usable
« Reply #1 on: 02/11/2018 03:41 PM »
Elon Musk once mentioned they might be able to fly RedDragon fully reusable but was not sure about the central core. So 10t may be the limit or whatever mass a RedDragon with limited payload may have.

Offline meithan

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Re: FH payload to Mars, when fully re-usable
« Reply #2 on: 02/11/2018 05:22 PM »
One estimate can be obtained from NASA's Launch Vehicle Performance Website. I'm attaching the performance curves for the Falcon Heavy in both expendable and recoverable modes, as well as Atlas V 551 and Delta IV Heavy for comparison.

The Roadster was launched with C3 = 12 km/s. At that energy (a typical value for decent Mars launch windows), according to these figures the max Falcon Heavy payload in recoverable mode is around 4000 kg, while in expendable mode it's about 9500 kg. I'm not sure how reliable or updated these figures are, though.

An interesting thing to note in the figure is how at launch energies below C3 = 30 km/s (for comparison, Jupiter transfer requires about 80 km/s) the Falcon Heavy (expendable) is king, but for higher launch energies the Delta IV Heavy seems to have better performance. I'd guess it's because the DIVH has a more efficient hydrolox-based upper stage.
« Last Edit: 02/20/2018 10:49 PM by meithan »

Offline hkultala

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Re: FH payload to Mars, when fully re-usable
« Reply #3 on: 02/11/2018 08:52 PM »
The FH just put the roadster, estimated at 1250 kg, on an orbit with Mars apogee.   But the first launch left a lot on the table.   How much can a recoverable FH lift to Mars?

First, it appears the first flight used a very tame throttle profile, with the center core not even at full thrust for liftoff,

Where is this claim based on?

Quote
and only 30 seconds of single-core operation after separation. 

A more aggressive throttle up at liftoff, then further down during 3 booster flight,  could save more fuel for after booster sep, giving some of the benefit of cross-feed.  A crude guess indicates they could get perhaps 500 m/s more staging velocity with this.

Throttling down hurts isp.

And the gravity losses would be worse.

And if the center core would be flying at higher velocity during the staging, it would also need to spend more delta-v for braking burn (called boostback burn).

And your calculations are simply badly wrongly calculated. You calculating the delta-v gotten from "almost empty" tanks twice. If the booster separation happens earlier, you are getting the delta-v of some time of tanks semi-full, not more tanks-empty time.

And, because of more fuel needed for braking burn, in reality actually the tanks would have MORE propellant at the last 30 second of the burn, meaning LESS delta-v for it.

Quote
Second, SpaceX had the ASDS close to shore.   

No, it did not.

Quote
Putting it further out means no boostback burn.  Another crude guess indicates they could get 500 m/s from this.

The ship was NOT very close to the shore. There were rumours about it, but those rumours were incorrect. The ship was much further away than the staging point.

It did NOT boost BACK. the "boostback burn" was just practically braking most of the horizontal velocity, which had to be done anyway. If it was not done by the "boostback burn" it would have needed a much longer "entry burn".

There is very little savings available by doing a smaller "boostback burn".

Your "1000m/s" if total bogus. It's maybe about 150m/s in reality.
« Last Edit: 02/11/2018 08:55 PM by hkultala »

Online Robotbeat

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Re: FH payload to Mars, when fully re-usable
« Reply #4 on: 02/12/2018 12:40 AM »
One estimate can be obtained from NASA's Launch Vehicle Performance Website. ...
This is a very good website to use, but note that the performance figures are older and much lower than what are on SpaceX's website. For instance, to TMI (i.e. about c3= 7km^2/s^2, optimistically), Falcon Heavy gets >16t, but the KSC website just gives 10t. So at high energy, I'd bet the actual figures SpaceX thinks they can do are at least 60% higher than on the KSC website.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline ulm_atms

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Re: FH payload to Mars, when fully re-usable
« Reply #5 on: 02/12/2018 01:14 AM »
Where is this claim based on?

This is what the claim is based on:

https://twitter.com/johnkrausphotos/status/960985711825620993

The whole way up you can tell that the center core was not throttled up as high as the side boosters.

Offline meithan

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Re: FH payload to Mars, when fully re-usable
« Reply #6 on: 02/12/2018 01:28 AM »
One estimate can be obtained from NASA's Launch Vehicle Performance Website. ...
This is a very good website to use, but note that the performance figures are older and much lower than what are on SpaceX's website. For instance, to TMI (i.e. about c3= 7km^2/s^2, optimistically), Falcon Heavy gets >16t, but the KSC website just gives 10t. So at high energy, I'd bet the actual figures SpaceX thinks they can do are at least 60% higher than on the KSC website.

Ah, I suspected the figures might not be updated. You're right, they do seem lower than the current official figures.

Any idea where one can get an updated performance curve for FH like those shown there?

Online Robotbeat

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Re: FH payload to Mars, when fully re-usable
« Reply #7 on: 02/12/2018 02:31 AM »
One estimate can be obtained from NASA's Launch Vehicle Performance Website. ...
This is a very good website to use, but note that the performance figures are older and much lower than what are on SpaceX's website. For instance, to TMI (i.e. about c3= 7km^2/s^2, optimistically), Falcon Heavy gets >16t, but the KSC website just gives 10t. So at high energy, I'd bet the actual figures SpaceX thinks they can do are at least 60% higher than on the KSC website.

Ah, I suspected the figures might not be updated. You're right, they do seem lower than the current official figures.

Any idea where one can get an updated performance curve for FH like those shown there?
The KSC website is the best we got, combined with SpaceX's figures. People have also made their own models, but I prefer the KSC+SpaceX route.
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Offline deruch

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Re: FH payload to Mars, when fully re-usable
« Reply #8 on: 02/12/2018 11:13 AM »
Quote
Second, SpaceX had the ASDS close to shore.   

No, it did not.

Quote
Putting it further out means no boostback burn.  Another crude guess indicates they could get 500 m/s from this.

The ship was NOT very close to the shore. There were rumours about it, but those rumours were incorrect. The ship was much further away than the staging point.

It did NOT boost BACK. the "boostback burn" was just practically braking most of the horizontal velocity, which had to be done anyway. If it was not done by the "boostback burn" it would have needed a much longer "entry burn".

There is very little savings available by doing a smaller "boostback burn".

Your "1000m/s" if total bogus. It's maybe about 150m/s in reality.

The ASDS position was known from SpaceX's FCC applications/grants for use of radio spectrum.  It was positioned ~340km downrange, which is certainly "close to shore" compared to flying the Falcon Heavy on a ballistic trajectory the way they do for GTO launches on F9.  That was the point.  This distance was also considerably closer inshore than SpaceX uses for those GTO launches.  He isn't claiming that FH center core actually reversed direction from the boostback burn. 
Shouldn't reality posts be in "Advanced concepts"?  --Nomadd

Online Demidrol

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Re: FH payload to Mars, when fully re-usable
« Reply #9 on: 02/12/2018 11:27 AM »
The ASDS position was known from SpaceX's FCC applications/grants for use of radio spectrum.
Here's the real position of ASDS based on MarineTrafic data

Offline LouScheffer

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Re: FH payload to Mars, when fully re-usable
« Reply #10 on: 02/12/2018 01:26 PM »

Your "1000m/s" if total bogus. It's maybe about 150m/s in reality.

Here's another way of showing much higher MECO without the complication of performance loss from the long coast.

Consider BulgaraSat at 3.7 tonnes.   F9 recoverable put this into a 250x65000x24o GTO.  This takes 2800 m/s from LEO.

Now FH recoverable can put this same payload to Mars, according to the NASA chart above (and maybe more).  That takes LEO+3600 (see, for example, Hohmann Transfer, section "Application to Interplanetary Travel".

Now the second stage is the same in both cases.  So the only place the extra delta-v can come from is increased velocity at MECO.  BulgariaSat staged at 2362 m/s.   So a FH recoverable flight needs to stage at a minimum 3162 m/s (actually slightly more since it can throw heavier things to Mars).  The FH flight staged at about 2640 m/s, so at the very least it's capable of 522 m/s more at MECO than it demonstrated on the first flight.

If this increase, compared to the test flight, does not come from a better throttle profile, or further downrange landings, where does it come from?


Offline John Alan

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Re: FH payload to Mars, when fully re-usable
« Reply #11 on: 02/12/2018 04:45 PM »
Musk just tweeted a 3rd ASDS is to be constructed specifically for east coast FH OP's...  8)

He says with core expend and both boosters ballistic to ASDS landings... only loose 10%... 
And he quoted $95m on twitter...  :o

Links and topic...
https://forum.nasaspaceflight.com/index.php?topic=39766.msg1787564#msg1787564
« Last Edit: 02/12/2018 04:52 PM by John Alan »

Offline Kansan52

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Re: FH payload to Mars, when fully re-usable
« Reply #12 on: 02/12/2018 04:49 PM »
OK, first glance, a larger drone ship with two boosters landing on Gravitas?

Offline John Alan

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Re: FH payload to Mars, when fully re-usable
« Reply #13 on: 02/12/2018 04:54 PM »
OK, first glance, a larger drone ship with two boosters landing on Gravitas?

On twitter he said two ships for two booster cores, center expended...  ;)
At only $5 million more... my guess is FH with 3 core recovery just went out the window...  ???
« Last Edit: 02/12/2018 04:56 PM by John Alan »

Offline acsawdey

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Re: FH payload to Mars, when fully re-usable
« Reply #14 on: 02/12/2018 05:06 PM »
OK, first glance, a larger drone ship with two boosters landing on Gravitas?

On twitter he said two ships for two booster cores, center expended...  ;)
At only $5 million more... my guess is FH with 3 core recovery just went out the window...  ???

No, I don't think so:

Quote
Not enough ignition fluid to light the outer two engines after several three engine relights. Fix is pretty obvious.

https://twitter.com/elonmusk/status/963107229523038211

Landing boosters on two drone ships and expending the core gets you 90% of expending all 3.


Offline John Alan

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Re: FH payload to Mars, when fully re-usable
« Reply #15 on: 02/12/2018 05:16 PM »
OK, first glance, a larger drone ship with two boosters landing on Gravitas?

On twitter he said two ships for two booster cores, center expended...  ;)
At only $5 million more... my guess is FH with 3 core recovery just went out the window...  ???

No, I don't think so:

Quote
Not enough ignition fluid to light the outer two engines after several three engine relights. Fix is pretty obvious.

https://twitter.com/elonmusk/status/963107229523038211

Landing boosters on two drone ships and expending the core gets you 90% of expending all 3.

My thinking when I said that was GEO birds and direct to geo flights...
Hmmm... build a big heavy satellite with a geo-1800 rocket kickstage...
Or just go all electric and pay +5 mil for direct to geo service...  ;)

But yes... they will fix the goof with running out of starting juice and do 2-RTLS and 1-ASDS when it makes sense.
« Last Edit: 02/12/2018 05:18 PM by John Alan »

Offline LouScheffer

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Re: FH payload to Mars, when fully re-usable
« Reply #16 on: 02/12/2018 05:45 PM »
Where is this claim based on?

This is what the claim is based on:

https://twitter.com/johnkrausphotos/status/960985711825620993

The whole way up you can tell that the center core was not throttled up as high as the side boosters.
Furthermore, the center core was even throttled down at liftoff.  This is exactly the opposite of what you would do for performance, where the center should be full throttle at liftoff to minimize gravity losses.   Then throttle down later in the trajectory when saving fuel becomes more important.  This John Kraus photo (also on the cover of Av Week) shows the center was throttled down from the beginning.

Offline meithan

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Re: FH payload to Mars, when fully re-usable
« Reply #17 on: 02/13/2018 01:33 AM »
What a coincidence, Musk tweeted just today about the NASA KSC Performance figures being outdated, and said that the "SpaceX GNC team is submitting updated numbers". Awesome!

Many interesting comments regarding performance in the Twitter thread.

Offline envy887

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Re: FH payload to Mars, when fully re-usable
« Reply #18 on: 02/13/2018 03:50 PM »
OK, first glance, a larger drone ship with two boosters landing on Gravitas?

On twitter he said two ships for two booster cores, center expended...  ;)
At only $5 million more... my guess is FH with 3 core recovery just went out the window...  ???

No, nobody really needs 90% of expendable FH performance right now. All existing commercial payloads can launch with full recovery. Even large DoD direct to GSO missions are likely within reach of FH with full recovery.

Offline sevenperforce

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Re: FH payload to Mars, when fully re-usable
« Reply #19 on: 02/20/2018 06:09 PM »
Falcon Heavy can send 3.17 tonnes on a Hohmann transfer to Mars with full recovery of all three boosters (side boosters RTLS, core on ASDS). Note that Falcon 9, on the other hand, can send up to 4.04 tonnes to Mars when flying expendable.

Expend the core and land each side booster on an ASDS, and you can send over 15 tonnes on TMI, for only $3M more than an expendable Falcon 9.

Source.

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