totm nov 2023 SpaceX Discussion Thread

[Exoscientist]

 A quarter-scale version of the BFR upper stage can serve as an SSTO able to carry a Dragon 2 to orbit:

A Small Raptor Spaceship.

https://exoscientist.blogspot.com/2017/10/a-small-raptor-spaceship.html

 

  Bob Clark

 

[MatterBeam]

1 hour ago, Exoscientist said:

 

 Actually what he said in the video presentation is that the BFR upper stage can be SSTO but the full two-stage BFR can carry more than an order of magnitude more payload. Since the payload for the reusable two-stage BFR is 150 tons, the payload for the BFR upper stage as a reusable SSTO might be, say, 10 to 15 tons.

 Part of the calculation in my blog post was to suggest using winged landing for the SSTO you might lose less of the payload in the reusable case. I estimate less than 10% loss with winged landing, as opposed to 70% to 80% loss with the vertical, propulsive landing approach. 

 If this is true you might want to make also the lower stage do a winged landing. For instance if the 10% loss using winged landing also holds for the two-stage BFR, then instead of losing 40% payload from 250 tons to 150 tons for reusability, it would be only 10% loss from 250 tons to 225 tons. So it is important to do such trades between the different landing modes to see which would result in the smallest loss in payload for reusability.

 

  Bob Clark

 

The 'order of magnitude' comment is only valid if we consider the 75 ton BFR as expendable. The 85 ton 'likely' dry mass of the BFR means it has only 9.6km/s of deltaV with the vacuum engines. After losses at sea-level, it does indeed have no cargo capacity. So, the 75 ton 'design' dry would allow for a 10 ton payload. However... it must be expendable because even a bare minimum landing deltaV of 400m/s would require 9.86 tons of propellant with the 330s Isp landing engines. The actual deltaV for landing being planned is 3-4 times larger. 

1 hour ago, Exoscientist said:

 A quarter-scale version of the BFR upper stage can serve as an SSTO able to carry a Dragon 2 to orbit:

A Small Raptor Spaceship.

https://exoscientist.blogspot.com/2017/10/a-small-raptor-spaceship.html

  Bob Clark

What do you think of the 'SFR' concept I proposed here and submitted to discussion of the SpaceX subreddit here?

[tater]

 

[tater]

 

[Nightside]

The next few months are packed with launches for SpaceX, can they take the risk of a Falcon Heavy test failure damaging their facilities when they have so many paying customers? 

How is the Texas facility coming along?

[tater]

Boca Chica is not very far along in terms of launch facilities, the crews are at KSC/Cape Canaveral. 

SLC 40 will be around should 39A receive damage, but it would be pretty bad for commercial crew.

[DAL59]

A small falcon rocket would be a good idea, but could it get a small crew to Mars via orbital refueling?

[NSEP]

On 25/10/2017 at 8:42 PM, tater said:

 

Im too darn curious now its hurting my brain. 

[Exoscientist]

15 hours ago, MatterBeam said:

The 'order of magnitude' comment is only valid if we consider the 75 ton BFR as expendable. The 85 ton 'likely' dry mass of the BFR means it has only 9.6km/s of deltaV with the vacuum engines. After losses at sea-level, it does indeed have no cargo capacity. So, the 75 ton 'design' dry would allow for a 10 ton payload. However... it must be expendable because even a bare minimum landing deltaV of 400m/s would require 9.86 tons of propellant with the 330s Isp landing engines. The actual deltaV for landing being planned is 3-4 times larger. 

What do you think of the 'SFR' concept I proposed here and submitted to discussion of the SpaceX subreddit here?

 

 You have to keep in mind there are two versions of the upper stage, the spaceship and the tanker. It is unlikely Elon is referring to the payload of the spaceship version when he is making that payload comparison to the full two-stage BFR, because clearly he is taking the tanker version as the upper stage in the two-stage payload estimate. For instance see this image from the presentation:

 

 From the context of his discussions in the video he is also talking about the reusable payload because that is the only operational mode they are considering for the BFR.

 I like the idea you are creating a small scale version of the stage with your SFR. However, I think you taking too large an estimate of the BFR tanker dry mass to estimate the dry mass of your SFR.

 

  Bob Clark

Edited October 27, 2017 by Exoscientist

[MatterBeam]

59 minutes ago, Exoscientist said:

 From the context of his discussions in the video he is also talking about the reusable payload because that is the only operational mode they are considering for the BFR.

 I like the idea you are creating a small scale version of the stage with your SFR. However, I think you taking to large an estimate of the BFR tanker dry mass to estimate the dry mass of your SFR.

I see. Do you have a reference for the '50 ton BFR tanker' figure I have seen in many places?

[sh1pman]

10 minutes ago, MatterBeam said:

I see. Do you have a reference for the '50 ton BFR tanker' figure I have seen in many places?

If it is indeed just 50 ton, it could completely replace Falcon 9 for LEO launches with up to 20ish ton payloads while being fully reusable. And being an SSTO. This doesn't sound right...

[tater]

The 75-85 ton figure for BFS is the crew vehicle.

Under 50t is estimated based on the stated dry mass of last year's ITS versions (the tanker was substantially lighter than the passenger version, and the ratio is assumed for the 9m version).

[Exoscientist]

5 hours ago, MatterBeam said:

I see. Do you have a reference for the '50 ton BFR tanker' figure I have seen in many places?

 It comes from consideration of the dry mass of the original ITS spaceship, as in this image:

 

 Elon in the video says the dry mass of the half-size BFR spaceship is 85 metric tons. If the dry mass were simply half that of the ITS version it would have been 75 tons. So the actual dry mass is larger than just by proportional scaling by a factor of 85/75 = 1.133.

 In that image above the ITS tanker is given as having a dry mass of 90 tons. So if the BFR tanker were proportionally half-size, that would put its mass at 45 tons. But the additional scaling factor of 1.133 would put it at 51 tons, which I rounded off to 50 tons.

  Bob Clark 

Edited October 27, 2017 by Exoscientist

[DAL59]

All your base

 

are belong to Musk

 

[Exoscientist]

4 hours ago, sh1pman said:

If it is indeed just 50 ton, it could completely replace Falcon 9 for LEO launches with up to 20ish ton payloads while being fully reusable. And being an SSTO. This doesn't sound right...

 According to Elon the reusable BFR tanker would have a payload of, perhaps, 10 to 15 tons. That's an estimate because all he says is as a reusable the BFR tanker payload is more than an order of magnitude less than that of the full two-stage reusable BFR's payload which is 150 tons.

  Bob Clark

[DAL59]

This thread has reached 100,000 views!  That's 1/3 the population of Iceland.  

[Ultimate Steve]

13 hours ago, DAL59 said:

This thread has reached 100,000 views!  That's 1/3 the population of Iceland.  

That's 30 of my town!

[sh1pman]

2 hours ago, Ultimate Steve said:

That's 30 of my town!

or 0.8% of my city.

[Vanamonde]

And it's got 0% to do with the thread topic! 

Please get back on-track. 

[TheDestroyer111]

On 10/26/2017 at 11:24 AM, MatterBeam said:

What do you think of the 'SFR' concept I proposed here and submitted to discussion of the SpaceX subreddit here?

I wonder how you want to ISRU RP-1 for Merlin or NTO/MMH for SuperDraco while on Mars. There is no oil for kerosene, so you would have to synthesise some bigger hydrocarbons out of methane, and for NTO/MMH there is not much nitrogen in the Martian atmosphere.

And if you want to go for LEO, fully reusable F9/FH should IMO be enough to dominate the medium-lift market while BFR handles bigger launches. After all, there must be a reason why SpaceX does it their way, right?

BTW the BFR design is still not final, and they could just substract one more meter from the rocket's diameter and use their 1000 kN subscale development engines

Edited October 28, 2017 by TheDestroyer111

[Rune]

On 27/10/2017 at 2:52 AM, Exoscientist said:

 It comes from consideration of the dry mass of the original ITS spaceship, as in this image:

 

 Elon in the video says the dry mass of the half-size BFR spaceship is 85 metric tons. If the dry mass were simply half that of the ITS version it would have been 75 tons. So the actual dry mass is larger than just by proportional scaling by a factor of 85/75 = 1.133.

 In that image above the ITS tanker is given as having a dry mass of 90 tons. So if the BFR tanker were proportionally half-size, that would put its mass at 45 tons. But the additional scaling factor of 1.133 would put it at 51 tons, which I rounded off to 50 tons.

  Bob Clark 

A this point, that short of analysis is very close to pulling out the numbers of your metaphorical bottom, IMO. The only  number I short of trust is the stated payload to LEO, and that is because even though it must be little more than a wish at this point, it's probably also a design goal the rest of the ship can be designed around.

 

Rune. But if itmakes you happy to do such things, don't let me stop you.

[DAL59]

17 hours ago, Vanamonde said:

And it's got 0% to do with the thread topic! 

Please get back on-track.

Sure.  So, where are the solar panels on the new version?  He didn't mention them in the conference.   

[Exoscientist]

3 hours ago, Rune said:

A this point, that short of analysis is very close to pulling out the numbers of your metaphorical bottom, IMO. The only  number I short of trust is the stated payload to LEO, and that is because even though it must be little more than a wish at this point, it's probably also a design goal the rest of the ship can be designed around.

Rune. But if itmakes you happy to do such things, don't let me stop you.

 

 I agree we can't be sure of the actual dry mass of the BFR tanker, but it is certainly less than the 85 tons of the BFR spaceship since it won't contain the passenger quarters and supplies for 100 colonists on a six month journey to Mars. 

  Bob Clark

[tater]

3 hours ago, Rune said:

A this point, that short of analysis is very close to pulling out the numbers of your metaphorical bottom, IMO. The only  number I short of trust is the stated payload to LEO, and that is because even though it must be little more than a wish at this point, it's probably also a design goal the rest of the ship can be designed around.

Rune. But if itmakes you happy to do such things, don't let me stop you.

The dry mass of the tanker is certainly lower than the dry mass of the "spaceship" version fitted out for crew. That this fact would stay the same with the downsize from 12m to 9m is uncontroversial I would think.

Musk also said that the BFS cargo/tanker variant could take a small cargo to LEO as an SSTO. You could just as well work backwards to that, and at least rough out what the dry mass could not exceed for this to be the case.

 

[MatterBeam]

3 hours ago, TheDestroyer111 said:

I wonder how you want to ISRU RP-1 for Merlin or NTO/MMH for SuperDraco while on Mars. There is no oil for kerosene, so you would have to synthesise some bigger hydrocarbons out of methane, and for NTO/MMH there is not much nitrogen in the Martian atmosphere.

And if you want to go for LEO, fully reusable F9/FH should IMO be enough to dominate the medium-lift market while BFR handles bigger launches. After all, there must be a reason why SpaceX does it their way, right?

BTW the BFR design is still not final, and they could just substract one more meter from the rocket's diameter and use their 1000 kN subscale development engines

The point of the SFR design is to keep SpaceX in business in the short term, and in the long term if the Mars mission doesn't pan out - this would make the BFR much too big for any market. The SFR would focus on Earth.

With that in mind, I was suggesting different low-thrust propulsion systems that SpaceX could install on the SFS stage for the landing burn. Merlins or SuperDracos exist already and their separate propellant feeds can be installed in the already-separated header tanks. Although, if it turns out that developing new lower-pressure Raptors is the better option, then we can use those instead. We don't have enough data on the Raptors to cut either way.

The SFR launches on an F9 booster, so it is basically a reusable upper stage based on the BFS design.