Author Topic: BFS - how bad can it be? (and still get to Mars with little delay)  (Read 13094 times)

Offline speedevil

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This post is hypothetical, and wondering about how 'badly' BFR/S can go, and still get to Mars with little difference in the outcome, with problems that would doom projects with smaller margins and more fragile architecture.
I have no particular information or suspicion that this will be the case, other than the unfortunate tendency of complex engineering projects to not work to schedule and spec. If BFS can do Mars with failures that would doom other systems, that is a massive plus in the likelyhood of getting to Mars on time.(edit)

It would of course be nice if BFS worked fully from the go, with 85 ton or so dry mass, 1100 tons of fuel, able to SSTO with some nominal payload, and do 150 tons of cargo to and back from orbit with the aid of BFR, and even Mars return aerobraking.

How bad can it be and still be really interesting?
As a pure hypothetical, not based on any inside knowledge, I was wondering about the heatshield.

With a nominal payload of 150 tons, which has 25 tons of landing propellant (700m/s or so delta-v), this means it can orbit 260 tons, with an initial vehicle weight of 1335 tons.

With an ISP of 380, this is an equivalent delta-v to orbit of 6100m/s.
So, with 150 tons, we can get to 6100m/s and still have 25 tons of left over propellant for landing nominally.

If we drop the payload down to 50 tons, we now start with 1235 tons, and to get 6100m/s delta-v takes our final mass down to 240 tons, as the dry mass is slightly worse.

But, we have 130 tons of fuel left over.

This is enough to remove about 3450m/s of orbital velocity, taking it down to 4.25, which is going to make things at least 6 times less hot -  an even smaller number than this considering that this 3450m/s can be used to keep the BFR up in the tenuous atmosphere, aerobraking gently, if that is better.

For 150 (actually 175) ton payloads, you can launch it emptying the tanks, and refuel three times to get back down.

Getting to Mars or the moon would need more like 25 refills, and getting back from Mars or the moon is not happening with a heat-shield that can't do even earth reentry. (light aerobraking, followed by on-near-hyperbolic orbit refuelling, maybe)

This seems to imply that entire failure of the heat-shield program to produce something as expected, wouldn't significantly impact performance in many areas, and at the very least allow massive capacity to be used until the heat-shield works.

Price to orbit for 'small' payloads of 50 tons remains the same as baseline, missions involving multiple retankings may may not even go up very much if capital dominates and reflight works well.

Capacity to orbit remains unchanged, though flight rate per pair of BFS drops as you've got to re-tank more.
But if rapid reuse works, this pretty much doesn't matter until you're actually constrained by number of launches.

Similar results as the above happen if BFR drastically underperforms, or there is significant weight growth on both systems (but the heat-sheild works).

It only fails to deliver in the face of adversity if it's not rapidly reusable.


Or, well, explodes a lot.
« Last Edit: 02/23/2018 12:32 pm by speedevil »

Offline whitelancer64

Re: BFS - how bad can it be?
« Reply #1 on: 02/15/2018 02:41 pm »
What's this about a failure of the heat shield program???
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline speedevil

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Re: BFS - how bad can it be?
« Reply #2 on: 02/15/2018 03:10 pm »
What's this about a failure of the heat shield program???

This is not based on anything other than a thought experiment.
It is at the least reasonably plausible that the initial heat-shield may not work to specs, and I was wondering how much this could be mitigated without wholly changing the nature of the system.

Offline neoforce

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Re: BFS - how bad can it be?
« Reply #3 on: 02/15/2018 03:12 pm »
I don't do kerbal or understand the specific rocket equation calculations, so not sure if these questions are obvious.  Just trying to understand some pieces of this.

With a nominal payload of 150 tons, which has 25 tons of landing propellant (700m/s or so delta-v), this means it can orbit 260 tons, with an initial vehicle weight of 1335 tons.

Is that 25 tons for Mars landing?  (the smaller tanks inside the bigger tanks)  So its lifting 175 tons which is 150 tons of cargo, and 25 tons of propellant reserved for Mars.

Quote
For 150 (actually 175) ton payloads, you can launch it empty, and refuel three times to get back down.

What does get back down mean in this case?  Do you mean from earth orbit?  (as opposed to the earlier mention of 25 tons of landing propellant)

Offline whitelancer64

Re: BFS - how bad can it be?
« Reply #4 on: 02/15/2018 03:14 pm »
Please make it clear in your post that this is just something you are positing as an assumption for a thought-experiment.

Right now it sounds like you've heard major news that there's something seriously wrong with the heat shield, when this isn't the case.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline whitelancer64

Re: BFS - how bad can it be?
« Reply #5 on: 02/15/2018 03:17 pm »
I don't do kerbal or understand the specific rocket equation calculations, so not sure if these questions are obvious.  Just trying to understand some pieces of this.

With a nominal payload of 150 tons, which has 25 tons of landing propellant (700m/s or so delta-v), this means it can orbit 260 tons, with an initial vehicle weight of 1335 tons.

Is that 25 tons for Mars landing?  (the smaller tanks inside the bigger tanks)  So its lifting 175 tons which is 150 tons of cargo, and 25 tons of propellant reserved for Mars.

Quote
For 150 (actually 175) ton payloads, you can launch it empty, and refuel three times to get back down.

What does get back down mean in this case?  Do you mean from earth orbit?  (as opposed to the earlier mention of 25 tons of landing propellant)

I think he's assuming that heat shields don't work, and that aerobraking while retro-rocket firing will be necessary to land the BFS.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline speedevil

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Re: BFS - how bad can it be?
« Reply #6 on: 02/15/2018 03:20 pm »
Is that 25 tons for Mars landing?  (the smaller tanks inside the bigger tanks)  So its lifting 175 tons which is 150 tons of cargo, and 25 tons of propellant reserved for Mars.
The nominal BFS uses around 25 tons of propellant on landing, this means that it can raise 175 tons to orbit, normally 25 is reserved for landing.

Quote
Quote
For 150 (actually 175) ton payloads, you can launch it emptying the tanks, and refuel three times to get back down.

What does get back down mean in this case?  Do you mean from earth orbit?  (as opposed to the earlier mention of 25 tons of landing propellant)

The hypothesis here is that the heat-shield program has issues, which can be mitigated by a lot of retrobraking before reentry.
(this is not based on anything other than the fact that the heat-shield is one of the obvious risks)

This doesn't actually affect at all capability of launching things to orbit, which remains at 175 tons, but you need 155 tons of fuel to be loaded on, before you can safely land in this case.

Or, you can get this extra landing fuel by reducing the payload from the nominal 150, to 50.

Above typo in red also, which may have confused.
« Last Edit: 02/15/2018 03:23 pm by speedevil »

Offline neoforce

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Re: BFS - how bad can it be?
« Reply #7 on: 02/15/2018 03:27 pm »
Is that 25 tons for Mars landing?  (the smaller tanks inside the bigger tanks)  So its lifting 175 tons which is 150 tons of cargo, and 25 tons of propellant reserved for Mars.
The nominal BFS uses around 25 tons of propellant on landing, this means that it can raise 175 tons to orbit, normally 25 is reserved for landing.


So that 25 tons that is reserved for landing applies to either Mars or Earth?  Even with the differences of EDL in the two systems, it is expected 25 tons covers the landing in either direction. (The nominal BFS, not the thought experiment if the heat shield doesn't work as well as desired.)

Offline speedevil

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Re: BFS - how bad can it be?
« Reply #8 on: 02/15/2018 03:30 pm »
Is that 25 tons for Mars landing?  (the smaller tanks inside the bigger tanks)  So its lifting 175 tons which is 150 tons of cargo, and 25 tons of propellant reserved for Mars.
The nominal BFS uses around 25 tons of propellant on landing, this means that it can raise 175 tons to orbit, normally 25 is reserved for landing.


So that 25 tons that is reserved for landing applies to either Mars or Earth?  Even with the differences of EDL in the two systems, it is expected 25 tons covers the landing in either direction. (The nominal BFS, not the thought experiment if the heat shield doesn't work as well as desired.)

Around this number.
Most of the entry is aerodynamic in either case (nominally).
The IAC presentation showed about 700m/s propulsion on descent to Mars. Earth is broadly similar, as though there is more gravity, there is also a prepared surface at a known position, so descent can be harder and the atmosphere is a lot denser at the surface.

Offline TomH

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Re: BFS - how bad can it be?
« Reply #9 on: 02/17/2018 09:09 pm »
The hypothesis here is that the heat-shield program has issues, which can be mitigated by a lot of retrobraking before reentry.

In scientific studies, even a hypothesis is normally based on some existing fact, something that seems to have been observed. Studies with a hypothesis which posits something already known to be false are not given serious scientific consideration. Your presupposition that heat shields don't work is incorrect. In any case, the only way to actually test your hypothesis through valid scientific investigation is to build the thing (or start with smaller scale models) and try it. Scientific experimentation is done by conducting actual trials. Your thought experiment is a not well formed example of what is known as deductive logic or deductive reasoning. Deductive reasoning requires sound facts to begin with, and what you posit regarding heat shields is already known to be incorrect, therefore any conclusions that you draw are faulty as well.
« Last Edit: 02/17/2018 09:11 pm by TomH »

Offline RotoSequence

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Re: BFS - how bad can it be?
« Reply #10 on: 02/17/2018 09:16 pm »
The hypothesis here is that the heat-shield program has issues, which can be mitigated by a lot of retrobraking before reentry.

In scientific studies, even a hypothesis is normally based on some existing fact, something that seems to have been observed. Studies with a hypothesis which posits something already known to be false are not given serious scientific consideration. Your presupposition that heat shields don't work is incorrect. In any case, the only way to actually test your hypothesis through valid scientific investigation is to build the thing (or start with smaller scale models) and try it. Scientific experimentation is done by conducting actual trials. Your thought experiment is a not well formed example of what is known as deductive logic or deductive reasoning. Deductive reasoning requires sound facts to begin with, and what you posit regarding heat shields is already known to be incorrect, therefore any conclusions that you draw are faulty as well.

The original post is meant as a thought experiment, and concludes that a lack of an effective heat shield would not stop BFS from performing its mission. No failure of the heatshield program is known or expected.
« Last Edit: 02/17/2018 09:17 pm by RotoSequence »

Offline speedevil

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Re: BFS - how bad can it be?
« Reply #11 on: 02/17/2018 09:38 pm »
The hypothesis here is that the heat-shield program has issues, which can be mitigated by a lot of retrobraking before reentry.

Studies with a hypothesis which posits something already known to be false are not given serious scientific consideration. Your presupposition that heat shields don't work is incorrect.

So you know that BFS heatshield will work first time, without delays, to final performance?

I was interested to find that even if BFS gains 50 tons, or the heatshield doesn't work fully, it can pretty much do the nominal mission outlined, albeit at a cost of several times more launches - and importantly - no more expendable launches.
It all seems plausible it'll work out in the end.
Having the margin in the design for it to significantly underperform in any of several aspects and still 'work' (100t on mars) means hitting deadlines may become more likely.

Offline TomH

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Re: BFS - how bad can it be?
« Reply #12 on: 02/17/2018 09:58 pm »
So you know that BFS heatshield will work first time, without delays, to final performance?

That is a quite different statement than what you first posited.

Offline speedevil

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Re: BFS - how bad can it be?
« Reply #13 on: 02/17/2018 09:59 pm »
So you know that BFS heatshield will work first time, without delays, to final performance?

That is a quite different statement than what you first posited.

Quote
This seems to imply that entire failure of the heat-shield program to produce something as expected, wouldn't significantly impact performance in many areas, and at the very least allow massive capacity to be used until the heat-shield works.

Quote
It would of course be nice if BFS worked fully from the go,
Also implying that this scenario would be temporary.
« Last Edit: 02/17/2018 10:05 pm by speedevil »

Offline TomH

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Re: BFS - how bad can it be?
« Reply #14 on: 02/17/2018 10:02 pm »
The hypothesis here is that the heat-shield program has issues, which can be mitigated by a lot of retrobraking before reentry.

In scientific studies, even a hypothesis is normally based on some existing fact, something that seems to have been observed. Studies with a hypothesis which posits something already known to be false are not given serious scientific consideration. Your presupposition that heat shields don't work is incorrect. In any case, the only way to actually test your hypothesis through valid scientific investigation is to build the thing (or start with smaller scale models) and try it. Scientific experimentation is done by conducting actual trials. Your thought experiment is a not well formed example of what is known as deductive logic or deductive reasoning. Deductive reasoning requires sound facts to begin with, and what you posit regarding heat shields is already known to be incorrect, therefore any conclusions that you draw are faulty as well.

The original post is meant as a thought experiment, and concludes that a lack of an effective heat shield would not stop BFS from performing its mission. No failure of the heatshield program is known or expected.

This part does imply failure:

This seems to imply that entire failure of the heat-shield program to produce something as expected, wouldn't significantly impact performance in many areas, and at the very least allow massive capacity to be used until the heat-shield works.

Offline RotoSequence

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Re: BFS - how bad can it be?
« Reply #15 on: 02/17/2018 10:36 pm »
This part does imply failure:

This seems to imply that entire failure of the heat-shield program to produce something as expected, wouldn't significantly impact performance in many areas, and at the very least allow massive capacity to be used until the heat-shield works.

^ The above is bolded for emphasis ^

If the quoted text had read "failure of the heat-shield program to produce something as expected, won't/will not significantly impact performance in many areas, and at the very least allow massive capacity to be used until the heat-shield works," I would agree with you  ;D
« Last Edit: 02/17/2018 10:37 pm by RotoSequence »

Offline Hominans Kosmos

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We should still figure out how bad can it be in dry vehicle mass and specific impulse to still be reasonably able to meet near term goals. Tonnage to mars by 202X. I assume some of this discussion could be found scatteres about in other threads. I intend to find some of that for quoting here tomorrow.

Offline speedevil

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We should still figure out how bad can it be in dry vehicle mass and specific impulse to still be reasonably able to meet near term goals. Tonnage to mars by 202X. I assume some of this discussion could be found scatteres about in other threads. I intend to find some of that for quoting here tomorrow.

Specific impulse is a non-issue.
For launching bulk cargo into LEO at least.

Taking the numbers from the first post, 6100m/s delta-v for the first stage, with an initial weight of 1335 tons, the final weight for 150 tons in orbit is 260 tons, counting landing fuel.
This is with 380 ISP engines.

If you get instead the same ISP as current M1Ds, that takes it down to 225 tons, or 130 tons payload.

It's comfortably over 100 tons, even if we assume similar underperformance on the first stage.

For obvious reasons, dry weight growth of 50 tons does the same.

Considering both, this means that you need an extra tanker flight to refuel, only very marginally affecting payload to Mars, and you need to do some on-orbit transfer of cargo or fluids to make up the difference.

A slightly more complex architecture can swap for ISP.
For example, a final filled tanker just able to return to earth after transferring half its payload over would entirely mitigate any ISP or weight issues. Just mean you may need 10, not 6 launches per mission.

ISP would matter if you want fast trips back of a lightweight BFS and are unable to do retanking in orbit due to ISRU or other constraints. But this is not till 6 years at least after the first Mars landing.


Offline oldAtlas_Eguy

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The original F9 [v1.0] missed it's design goal of 12.5mt to LEO and only was able to do ~10mt to LEO.

If BFR/BFS has a similar performance shortfall in it's first design iteration it would have a payload to Mars of ~120mt. It would also require 1 additional tanker flight to fuel up the BFS: 6 vs the design goal of 5.

Even if it's performance got as bad as only 94mt vs 150mt with requiring 8 tanker flights it would still meet it's goal of being able to adequately support Mars missions delivering 90+mt to Mars surface and being able to return to Earth after refueling on the Mars surface. The return payload would be very small probably less than 20mt. But still large enough for a manned return trip of a dozen persons.

The BFR/BFS design has a design margin on it's performance in it's ability to satisfy mission goals of ~40%. This is a very big margin. So SpaceX should be able to develop a BFR/BFS that will satisfy the very minimum of Mars mission goals. Once experience in operating BFR/BFS is obtained the information gathered will indicate where performance can be improved without decreasing reliability or increasing costs.

Offline vaporcobra

The better thought experiment with respect to the heatshield issue here would be not a total failure, but of needing a far heavier shield that requires significant refurbishment after high-energy reentries. This would make the consequences far less extreme, as SpaceX knows full well how to build a heatshield that can survive LEO reentry, and it will be a relatively easy prospect to test that assumption w/ S2 recovery efforts if Cargo Dragon/Crew Dragon can't provide clean data.

Other consequences would be:
-All spaceships that land on Mars would be unable to return directly to the surface of the Earth until TPS refurbishment can be successfully done on Mars
-Spaceships could still be used as orbital tugs or ad-hoc propellant depots (opportunity cost is minimal if they can't return to Earth's surface)

The fact that SpaceX itself is already operating under the assumption that the first spaceships will indefinitely remain on Mars and be used as preliminary habitats lends a huge amount of credence to this line of thought, whether that assumption is the result of lack of dedicated habitat development, uncertainty about TPS reusability for back-to-back interplanetary reentries, or something else.

The inability to immediately reuse spaceships sent to Mars is perhaps the biggest hurdle for current SpaceX planning, that's either going to significantly stretch out timelines or require major recurring investments/huge profit margins to be sustained for years.

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