Author Topic: BFR GTO capability (Read 28092 times)

Alvian@IDN · « **on:** 09/21/2018 04:58 am »

Considering BFR will be replacing all of the Falcons, GTO market will be still one of the important market for BFR, even though GEO market now decrease & LEO smallsats increase.

Did BFS has a capabilities to do a second burn, deploy a satellite in GTO (like Falcon 9 S2), but it will do a retrograde burn (maybe in periapsis to lower the apoapsis to below the Karman line) ?

Maybe considering to ask Elon Musk in his upcoming Reddit AMA.

Nomadd · « **Reply #1 on:** 09/21/2018 05:05 am »

Are you asking if a spaceship designed to land on Mars can do a burn a few hours after it's launched?

JonathanD · « **Reply #2 on:** 09/21/2018 05:09 am »

If it can't do a second burn to get to GTO, Mars EDL will be a very brief experiment in lithobraking.

su27k · « **Reply #3 on:** 09/21/2018 05:17 am »

Yeah, of course it can do a 2nd burn. The real question is does it need orbital refueling before it does GTO burn. The 2017 version doesn't, but with the lower Isp engine and potentially higher dry mass, not sure if the 2018 version can still do single launch GTO mission.

JonathanD · « **Reply #4 on:** 09/21/2018 05:21 am »

Quote from: su27k on 09/21/2018 05:17 am

Yeah, of course it can do a 2nd burn. The real question is does it need orbital refueling before it does GTO burn. The 2017 version doesn't, but with the lower Isp engine and potentially higher dry mass, not sure if the 2018 version can still do single launch GTO mission.

On-orbit refueling for a GTO mission would be crazy. I have to think they'd throw on some vacuum raptors if they had to.

su27k · « **Reply #5 on:** 09/21/2018 05:50 am »

Quote from: JonathanD on 09/21/2018 05:21 am

On-orbit refueling for a GTO mission would be crazy. I have to think they'd throw on some vacuum raptors if they had to.

It's not that crazy if you have a full and rapid reusable launch vehicle. The delta-v to GTO is substantial, it's only 700m/s or so lower than TLI, it's a big ask to require your reusable 2nd stage to do single launch to GTO when the first stage does RTLS, I think the total delta-v required for the 2nd stage would be similar to SSTO.

Semmel · « **Reply #6 on:** 09/21/2018 06:35 am »

My question exactly in the engineering thread. GTO performance is the only relevant performance number early on. We have the thrust per engine, we have the ISP of the sea level engines and we have the payload to LEO. We have an approximation of the fuel volume, one can derive the mass from that. We don't have the dry mass (85 mT is not relevant for Sat delivery) but maybe the above info is enough to get an estimate?

hkultala · « **Reply #7 on:** 09/21/2018 07:59 am »

Quote from: su27k on 09/21/2018 05:17 am

Yeah, of course it can do a 2nd burn. The real question is does it need orbital refueling before it does GTO burn. The 2017 version doesn't, but with the lower Isp engine and potentially higher dry mass, not sure if the 2018 version can still do single launch GTO mission.

If the craft weights about 90 tonnes and can lift 110 tonnes to LEO, it means i could lift a 8-tonne satellite plus 102 tonnes of fuel to orbit.

Unfortunately it does not have extra tanks for this fuel and it it wants to have this fuel available in main tanks, it has to leave this fuel unused in the main tanks, burning less than "optimal for maximum payload" for the main burn to LEO, meaning the mass to LEO drops a bit.

This means that in practice, without extra tanks, with the 8-tonne satellite, it could lift slightly smaller amount of fuel to LEO, maybe like 90 tonnes.

LEO to GTO is about 2.5km/s.

90+90+8 = 188 tonnes initial mass, 98 tonnes final mass, 357 second specific impulse gives about 2.28 km/s delta-v after LEO.

So, GTO not achievable with these numbers with 8-tonne satellite.

But, if the satellite launcher version of BFS is only 80 tonnes instead of 90 tonnes, and it can lift slightly more to LEO (100 tonnes of fuel + 8-tonne satellite), then we get about 2.65 km/s delta-v after LEO, enough for GTO.

So, close call. I think they are nailing the details in such way that it will work for the biggest common GTO satellites, as GTO satellites are important source of income.

speedevil · « **Reply #8 on:** 09/21/2018 09:38 am »

Quote from: hkultala on 09/21/2018 07:59 am

So, close call. I think they are nailing the details in such way that it will work for the biggest common GTO satellites, as GTO satellites are important source of income.

At least for this question, it's easy to answer - if you believe the presentation.(*)
That states it can do 3300m/s with a VIP of say 5% of the stage dry mass with 250m/s of landing fuel.

You have 820m/s left over after GTO, meaning you can lift a VIP-mass sat to GTO+300m/s or so - doing part of its insertion burn - and then return to earth after doing a 300m/s burn to lower perigee again, with some margin.

Or, lift a ~20 ton satellite to GTO.
The point in the last post about the tanks not being big enough is wrong(*) as it must be able to do 3300m/s after LEO.

*) The outlined moon mission needs a dry weight of 60 tons or so, if it has 1100 tons initial mass. Which is ... ambitious.
So, either it's awesomer that we thought, or something has to give. Either a tanker, or the vehicle has to grow/be respecced.
This obviously means basing numbers on it is problematic.

If you assume '100t+ to LEO' includes ~150 tons, then all the problems with the presentation go away.

If, on the other hand, you assume they are implicitly assuming a refueling for the moon mission, 100+ to LEO may be the 'worst case best case' that they are aiming at.
100 tons would be more-or-less the point at which they can do all current F9 GTO payloads without retanking.

Semmel · « **Reply #9 on:** 09/21/2018 11:06 am »

Quote from: speedevil on 09/21/2018 09:38 am

At least for this question, it's easy to answer - if you believe the presentation.(*)
That states it can do 3300m/s with a VIP of say 5% of the stage dry mass with 250m/s of landing fuel.

I must have missed it, was that number in the presentation?

speedevil · « **Reply #10 on:** 09/21/2018 11:43 am »

Quote from: Semmel on 09/21/2018 11:06 am

Quote from: speedevil on 09/21/2018 09:38 am
At least for this question, it's easy to answer - if you believe the presentation.(*)
That states it can do 3300m/s with a VIP of say 5% of the stage dry mass with 250m/s of landing fuel.

I must have missed it, was that number in the presentation?

Implicitly.
Lunar injection to any orbit which gets you close to the moon from LEO with a hair of margin is 3300m/s.
250m/s of landing fuel is what you get if you integrate gravity losses over the landing burn which begins at 100m/s and lasts 16s.

Alvian@IDN · « **Reply #11 on:** 09/21/2018 02:09 pm »

OR, simply put the small kick/third stage installed with satellite. Deployed earlier in LEO

After spacecraft deployed, that third stage do a retrograde burn, lower the periapsis so it will burn up, so it won't ended being a GTO space junk.

But this maybe will increase the spacecraft price (if the kick stage manufactured by satellite maker), or considered as "wasteful" by SpaceX, who're at that time didn't even know about "expendable" thing thanks to BFR.

speedevil · « **Reply #12 on:** 09/21/2018 02:15 pm »

Quote from: Alvian@IDN on 09/21/2018 02:09 pm

OR, simply put the small kick/third stage installed with satellite. Deployed earlier in LEO
After spacecraft deployed, that third stage do a retrograde burn, so it won't ended being a GTO space junk.

Indeed.
Something looking very like a F9S2 would be quite adequate to get from a LEO vehicle with 100 tons payload to GTO with a 20 ton payload, and then back to its original orbit empty, so it could be picked up and reused.
A very much more modest kick stage would be just fine if you just want it to get the payload to GTO and burn up.

Alvian@IDN · « **Reply #13 on:** 09/21/2018 02:19 pm »

Quote from: speedevil on 09/21/2018 02:15 pm

Quote from: Alvian@IDN on 09/21/2018 02:09 pm
OR, simply put the small kick/third stage installed with satellite. Deployed earlier in LEO
After spacecraft deployed, that third stage do a retrograde burn, so it won't ended being a GTO space junk.

Indeed.
Something looking very like a F9S2 would be quite adequate to get from a LEO vehicle with 100 tons payload to GTO with a 20 ton payload, and then back to its original orbit empty, so it could be picked up and reused.
A very much more modest kick stage would be just fine if you just want it to get the payload to GTO and burn up.

SpaceX (of course in BFR's early days) could use the remaining of Merlin 1D vac in storage for that kick stage, like NASA do with RS-25

I'm still waiting for Elon's AMA though, we will see what he thinks about this.

matthewkantar · « **Reply #14 on:** 09/21/2018 02:29 pm »

Refueling for a GEO mission is acceptable if you take ten eight ton sats with you, drop them where they need to be, pick up five dead GEO sats and then return to LEO, tank up again and return to the launch site.

edited to add a missing "if"

wannamoonbase · « **Reply #15 on:** 09/21/2018 02:42 pm »

Taking an 80-90 ton vehicle to GTO seems to be the problem with the 8 ton payload.

SpaceX maybe much better off developing a small reusable space tug for doing the final push.

Cheapchips · « **Reply #16 on:** 09/21/2018 02:57 pm »

Longer term, shouldn't they be retanking and providing customers with larger satellite options? Or would a bigger tug still be the way to go?

Elmar Moelzer · « **Reply #17 on:** 09/21/2018 04:31 pm »

Quote from: hkultala on 09/21/2018 07:59 am

If the craft weights about 90 tonnes

Where is that assumption coming from? The cargo version should have a significantly lower dry mass than the 85 ton passenger version. I would assume a cargo version to be at most 65 tonnes.

RedLineTrain · « **Reply #18 on:** 09/21/2018 06:59 pm »

SpaceX is already launching 7+ ton birds to GTO-2200 rather than GTO-1800. I would imagine the theme could be expanded upon for BFR.

Semmel · « **Reply #19 on:** 09/21/2018 07:24 pm »

Quote from: speedevil on 09/21/2018 11:43 am

Quote from: Semmel on 09/21/2018 11:06 am
Quote from: speedevil on 09/21/2018 09:38 am
At least for this question, it's easy to answer - if you believe the presentation.(*)
That states it can do 3300m/s with a VIP of say 5% of the stage dry mass with 250m/s of landing fuel.

I must have missed it, was that number in the presentation?
Implicitly.
Lunar injection to any orbit which gets you close to the moon from LEO with a hair of margin is 3300m/s.
250m/s of landing fuel is what you get if you integrate gravity losses over the landing burn which begins at 100m/s and lasts 16s.

Well, Musk never explicitly said that it is done without refuelling. Unless we get that information, I would not count on this performance.