Space X forging ahead with BFR

louis · 2018-04-26 03:06:08

This is a very informative video - sadly it's in robot voice but don't let that put you off:

https://www.youtube.com/watch?v=6yC8rOikn2c

Looks like Space X are forging ahead on all BFR fronts and the 2022 BFR cargo mission is a strong possibility.

Interesting that Shotwell is also talking about a 3 month transit time for human BFRs. I've now seen so many references to 3-4 month transit times that I think we should be assessing the Space X mission on that basis, rather than the longer transit times mentioned in the past.

GW Johnson · 2018-04-26 08:36:29

For the outbound trip to Mars, my numbers posted over at "exrocketman" show a 5.07 km/s delta-vee needed to depart LEO onto a min-energy Hohmann transfer ellipse to Mars. My numbers show that 101 tons of propellant must be reserved for landing on Mars with 150 tons of payload, starting with max 1100 tons of propellant. I show 3.36 km/s extra delta-vee capability available to speed up the outbound trip, even allowing for boil-off losses and a small midcourse correction budget. There would be more available to speed up the trip if outbound payload were reduced.

For the return trip at 50 tons return payload, I'm showing 6.78 km/s delta-vee required to launch from the surface of Mars directly to a min-energy Hohmann transfer ellipse back to Earth, again starting with max 1100 tons of propellant. I show only 1.37 km/s extra delta-vee capability available to speed up the return trip. That's after reserving 57 tons for the Earth landing, and allowing for boiloff losses and a small midcourse budget. Again, reducing return payload would increase this speed-up capability slightly.

Hohmann transfer is nominally 8.5 months one way. Looking at faster trajectories is beyond what I know how to do with pencil and paper. All I can determine are the excess delta-vee capabilities, and those only within the limits of my reverse-engineering assumptions.

Again, if 0.38 gee at Mars for months to years results in fully Earth-fit people, then their health is degraded by exposure to 0-gee for x-months home. Maybe 3 months is possible, maybe it's closer to 6 or even 8 months. But when they arrive home, it's a free return trajectory from the interplanetary orbit, at an entry speed near 17 km/s. This does not scale linearly, but it's typically 4 gees at 8 km/s entry from LEO, 11 gees at 11 km/s entry from the moon. If you scale linearly, that's a 25 gee entry, which is not survivable even by fully-fit people. I've been guessing 12-15 gees, which is survivable by fully fit people.

People exposed to 0-gee for times of months have never ridden home at more than 4 gees! Period.

It is the flight home that is the crew killer, due to high gees at the free-return entry. They need to be fully Earth fit to survive that. Period.

There is no propellant budget to execute a delta-vee into LEO for an easy 4 gee return. That exceeds 5.1 km/s delta-vee, and it just ain't there. And it ain't gonna be there without flying BFR/BFS's as throwaway tankers, without big rendezvous delta-vee budgets, and without betting lives on rendezvous-and-docking while deep in interplanetary space. Not a smart thing to attempt at all.

As I have tried to indicate, there is no guarantee that 0.38 gee provides adequate Earth-level fitness for the months home at 0-gee to degrade somewhat. Less than full Earth-level fitness at free-return entry is a lethal risk to the ship's occupants. So, send two BFS's home at the same time, dock them tail-to-tail, and spin them end-over-end at 4 rpm. That at least gets you around 3/4 gee in the occupied spaces.

Louis, I agree with you that Spacex is forging ahead with this thing, and will be flying these missions with this hardware, or something very close to it. Where I disagree most strongly is with the way you (and seemingly Spacex) want to disregard the microgravity disease risk in your thinking.

GW

Last edited by GW Johnson (2018-04-26 08:41:27)

Oldfart1939 · 2018-04-26 13:39:44

It appears to GW and myself that both NASA and SpaceX want to simply sweep the issue of microgravity physical degradation under the rug. Bob Zubrin certainly didn't in his Mars Direct architecture, and he's been thumping that tub for going on 30 years. Absent any form of gravity and associated physical conditioning (and no, wearing weighted space suits will NOT work), it seems that the number of imponderables included in the Musk plan for Mars is doomed to ultimately fail on the return trip. So...in my humble assessment...it should realistically be a one way journey until we get a better handle on things.

louis · 2018-04-26 15:59:30

I don't think anyone's minimising the issue of mG negative physical effects. But so far no one has died very early from such effects even though they have been in zero G for over one year and in zero G for several years cumulatively. I have never claimed weighed suits will "work" in zero G but they are an obvious way of replicating the gross effects of earth gravity.

For the sake of clarity - how long do you think the BFR transit times will be Earth to Mars and Mars to Earth.

Oldfart1939 wrote:

It appears to GW and myself that both NASA and SpaceX want to simply sweep the issue of microgravity physical degradation under the rug. Bob Zubrin certainly didn't in his Mars Direct architecture, and he's been thumping that tub for going on 30 years. Absent any form of gravity and associated physical conditioning (and no, wearing weighted space suits will NOT work), it seems that the number of imponderables included in the Musk plan for Mars is doomed to ultimately fail on the return trip. So...in my humble assessment...it should realistically be a one way journey until we get a better handle on things.

Oldfart1939 · 2018-04-26 17:24:26

Louis-

To reiterate--the loss of bone density is unaffected by even massive amounts of exercise aboard the ISS, and bone samples indicate a situation similar to that of extreme osteoporosis in returning astronauts. As GW has pointed out, a too fast return from Mars with the concomitant g forces associated with direct atmospheric entry give me pause as to whether I would want to be a guinea pig for these experiments--even as much as I would love to go to Mars. I envision something happening that would not be at all pretty, such as collapse of the entire chest architecture from broken ribs weakened by decalcification.

I did some paper calculations and came up with a 180 day transit time to Mars on a Hohmann transfer ballistic ellipse, and that's just about the extreme limit of transit time if Zubrin's numbers are to be trusted. But SOMEHOW, we need to provide artificial gravity, if only at a moderate fraction of a single g. It would certainly make life aboard the spacecraft a lot more pleasant with functional toilets and stove top cooking possible.

louis · 2018-04-26 19:55:00

Well I am not sure you are right there. My understanding is that exercising in zero G does retard bone loss. Do you have a citation to the opposite?

Clearly moving around on Mars in a weighted suit will provide muscles and bones with the right amount of stress on Mars.

Well you don't sound too sure about your 180 days transit time! I think we need to investigate what is the basis for all these statements about a 3-4 month transit time that I am hearing. The quicker transit time sounds plausible to me given we will have refuelling in Earth orbit and a weaker gravity environment to escape from on the return journey.

In principle, do you accept that :4 month transit - 2 years in 1 G simulation on Mars - and 4 months return: is very unlikely to present any more negative health outcomes than have been experienced in relation to long term passage in the ISS?

Oldfart1939 wrote:

Louis-
To reiterate--the loss of bone density is unaffected by even massive amounts of exercise aboard the ISS, and bone samples indicate a situation similar to that of extreme osteoporosis in returning astronauts. As GW has pointed out, a too fast return from Mars with the concomitant g forces associated with direct atmospheric entry give me pause as to whether I would want to be a guinea pig for these experiments--even as much as I would love to go to Mars. I envision something happening that would not be at all pretty, such as collapse of the entire chest architecture from broken ribs weakened by decalcification.
I did some paper calculations and came up with a 180 day transit time to Mars on a Hohmann transfer ballistic ellipse, and that's just about the extreme limit of transit time if Zubrin's numbers are to be trusted. But SOMEHOW, we need to provide artificial gravity, if only at a moderate fraction of a single g. It would certainly make life aboard the spacecraft a lot more pleasant with functional toilets and stove top cooking possible.

Oldfart1939 · 2018-04-26 20:43:20

Louis-

The only thing walking around Mars in a weighted suit will accomplish, biologically, it will make the wearer tired. Weighted suits are NOT a substitute for gravity.
Re: departure velocity for 180 day Hohmann transfer; the deltaV required for this trajectory is 5.08 km/sec. Ref: R. Zubrin in The case for Mars, 2011, p. 96, Table 4.2 "Free return trajectories between Earth and Mars."

Last edited by Oldfart1939 (2018-04-26 20:47:02)

louis · 2018-04-27 03:41:29

Come on, a weighted suit will definitely retard muscle and bone loss. That seems to be the conclusion of this discussion at least:

https://space.stackexchange.com/questio … le_rich_qa

Oldfart1939 wrote:

Louis-
The only thing walking around Mars in a weighted suit will accomplish, biologically, it will make the wearer tired. Weighted suits are NOT a substitute for gravity.
Re: departure velocity for 180 day Hohmann transfer; the deltaV required for this trajectory is 5.08 km/sec. Ref: R. Zubrin in The case for Mars, 2011, p. 96, Table 4.2 "Free return trajectories between Earth and Mars."

Oldfart1939 · 2018-04-27 07:59:45

Louis-
You need to do some research into the physiological effects of microgravity. I have done so, about 11 years ago when I was working on a research proposal regarding bone decalcification and osteoporosis.

Terraformer · 2018-04-27 08:39:29

What's the current thinking on the cause of bone loss? If it's caused by lack of stress on the bones, that should in theory be simple to fix - make sure the exercise is high impact. We could test this quite easily with bed rest patients. Have them exercise in a horizontal direction using an elastic/spring device and harness to put their bones under pressure and impacts, and see how they compare with the control group.

Oldfart1939 · 2018-04-27 08:53:03

Bone loss is mediated through hormonal regulators, and I'll give a brief and somewhat incomplete description of what happens. Bone tissue, like all bodily tissues, aren't simply fixed for all time. These cells in bone undergo continuous turnover and replacement of older cells by new ones. The stem cells involved for bone construction are OSTEOBLASTS, and those designed for removing and elimination of older and less healthy cells are OSTEOCLASTS. These 2 classes of cells are continually working to maintain a healthy skeletal architecture. In the condition called osteoporosis, there are too many osteoclasts produced which slowly converts once solid bones into a more sponge-like consistency and thus weaker than necessary for stressful activities. There has been much research done in studying the regulatory mechanism for this process, but it is undoubtedly hormonally related, since it's most prevalent in post-menopausal females.

louis · 2018-04-27 09:12:23

The evidence from ISS and other space flights is that stress exercise retards bone and muscle loss. That why they do the exercises.

Terraformer wrote:

What's the current thinking on the cause of bone loss? If it's caused by lack of stress on the bones, that should in theory be simple to fix - make sure the exercise is high impact. We could test this quite easily with bed rest patients. Have them exercise in a horizontal direction using an elastic/spring device and harness to put their bones under pressure and impacts, and see how they compare with the control group.

louis · 2018-04-27 09:16:36

But, I am sure you agree, hormones react to the internal/external environment. That is why astronauts do stress exercise on board the ISS isn't it? And I cannot see how replicating the stresses and strains of a 1G environment on Mars, through use of weights to complement the 0.38G, could do anything other than aid recovery after zero G.

Oldfart1939 wrote:

Bone loss is mediated through hormonal regulators, and I'll give a brief and somewhat incomplete description of what happens. Bone tissue, like all bodily tissues, aren't simply fixed for all time. These cells in bone undergo continuous turnover and replacement of older cells by new ones. The stem cells involved for bone construction are OSTEOBLASTS, and those designed for removing and elimination of older and less healthy cells are OSTEOCLASTS. These 2 classes of cells are continually working to maintain a healthy skeletal architecture. In the condition called osteoporosis, there are too many osteoclasts produced which slowly converts once solid bones into a more sponge-like consistency and thus weaker than necessary for stressful activities. There has been much research done in studying the regulatory mechanism for this process, but it is undoubtedly hormonally related, since it's most prevalent in post-menopausal females.

Oldfart1939 · 2018-04-27 11:14:13

Louis, et. al.-
I was interrupted earlier and didn't finish my exposition about hormonal regulation of bone architecture. The onset of osteoporosis in menopausal females is entirely unrelated to exercise. And as such, there are additional changes in metabolism, blood chemistries, and other effects. The concept that these changes can be regulated or overcome through exercise is ludicrous.

So far, no one has isolated a "gravity receptor," biologically. What we are seeing, however, is a cascade of effects related to null gravity; bone decalcification is but one of these. The deterioration of vision, seemingly irreversible, and loss of cardiac efficiency are just a few. I'm skeptical about the changes to the immune system, but that too, is subject to hormonal regulation.

What I'm getting at here, is we don't know enough about the triggering of all these deleterious effects to counter them. Using extreme exercise protocols was a WAG type approach to health maintenance at the ISS. But in spite of the exercise protocols used, we still see decalcification of bone, vision problems, and weakened and somewhat atrophied muscles.

My research proposal in 2007 addressed this problem by using common osteoporosis treatment/regulation using CALCITONIN, the hormone which mediates calcium uptake through stimulation of osteoblast production. There is also a hormonal regulator which suppresses osteoclast production: amylin. My proposal suggested an in-space rat experiment using these hormones for 90 days in various combinations, along with an untreated control group, with a litter mate control set of subjects retained on Earth. At the time, NASA exhibited ZERO interest in the project. I declined to pursue this any further due to the onset of my wife's terminal illness.

Last edited by Oldfart1939 (2018-04-27 11:16:17)

GW Johnson · 2018-04-27 12:45:29

More than anything else, it is the weakened heart that poses the risk with high gee entry. Too much stress, and it just quits. Dead crew, 2-3 minutes from touchdown. 4 gees at 8 km/s from LEO, 11 gees at 11 km/s coming back from the moon. A guess: 12-15 gees during direct entry at Earth from the interplanetary trajectory, at something in the vicinity of 17 km/s. (If you scale linearly it's 25 gees, but I don't believe that.)

To the best of my knowledge (imperfect as it is) the exercise protocols developed on ISS do little good for that weak heart issue. That would take some kind of aerobics, something not so very feasible on ISS. It takes the combination of Earth-level resistance forces at modestly-high whole-body motion speeds to get the aerobic effect. Waving your arms and legs around won't do it. You have to actually run, as best I understand it. Not possible at 0-gee, very hard at low gee.

GW

louis · 2018-04-27 13:29:29

As I've said already, I very much doubt any human mission to Mars will take place prior to a Mars-analogue mission, preferably involving a period on the Moon with four months in zero G either end. Then we'll see how well people can cope with such a mission.

Oldfart1939 wrote:

Louis, et. al.-
I was interrupted earlier and didn't finish my exposition about hormonal regulation of bone architecture. The onset of osteoporosis in menopausal females is entirely unrelated to exercise. And as such, there are additional changes in metabolism, blood chemistries, and other effects. The concept that these changes can be regulated or overcome through exercise is ludicrous.
So far, no one has isolated a "gravity receptor," biologically. What we are seeing, however, is a cascade of effects related to null gravity; bone decalcification is but one of these. The deterioration of vision, seemingly irreversible, and loss of cardiac efficiency are just a few. I'm skeptical about the changes to the immune system, but that too, is subject to hormonal regulation.
What I'm getting at here, is we don't know enough about the triggering of all these deleterious effects to counter them. Using extreme exercise protocols was a WAG type approach to health maintenance at the ISS. But in spite of the exercise protocols used, we still see decalcification of bone, vision problems, and weakened and somewhat atrophied muscles.
My research proposal in 2007 addressed this problem by using common osteoporosis treatment/regulation using CALCITONIN, the hormone which mediates calcium uptake through stimulation of osteoblast production. There is also a hormonal regulator which suppresses osteoclast production: amylin. My proposal suggested an in-space rat experiment using these hormones for 90 days in various combinations, along with an untreated control group, with a litter mate control set of subjects retained on Earth. At the time, NASA exhibited ZERO interest in the project. I declined to pursue this any further due to the onset of my wife's terminal illness.

louis · 2018-04-27 13:48:18

GW, I am not sure where this 12-15 G thing is coming from...is your claim that there is no way to slow down?

I am not finding anyone else taking that view...see this discussion for instance:

https://www.reddit.com/r/SpaceXLounge/c … gured_out/

Everyone else seem to think Space X will have ways to bring their craft down to 3-5g.

NASA seem to think there is a protective function for the heart from exercise:

https://www.nasa.gov/mission_pages/stat … ardio.html

The issue is how effective it is, and which forms of exercise are best at protecting cardiac function.

GW Johnson wrote:

More than anything else, it is the weakened heart that poses the risk with high gee entry. Too much stress, and it just quits. Dead crew, 2-3 minutes from touchdown. 4 gees at 8 km/s from LEO, 11 gees at 11 km/s coming back from the moon. A guess: 12-15 gees during direct entry at Earth from the interplanetary trajectory, at something in the vicinity of 17 km/s. (If you scale linearly it's 25 gees, but I don't believe that.)
To the best of my knowledge (imperfect as it is) the exercise protocols developed on ISS do little good for that weak heart issue. That would take some kind of aerobics, something not so very feasible on ISS. It takes the combination of Earth-level resistance forces at modestly-high whole-body motion speeds to get the aerobic effect. Waving your arms and legs around won't do it. You have to actually run, as best I understand it. Not possible at 0-gee, very hard at low gee.
GW

GW Johnson · 2018-04-28 10:08:29

Louis:

The reddit thing is readers of a blog site bandying guesses about, not folks who have actually run real numbers. The NASA cardio thing is a plan, not a result. That answer is not known yet. But we already do know that stress applied to a weak heart kills.

With BFS return from Mars, there is no braking burn before atmospheric entry at Earth, because there is no available propellant budget in this BFR/BFS design for that purpose. There is only aerobraking, and you are running into the Earth from behind, at a velocity with respect to the sun that is considerably higher than Earth's orbital velocity about the sun. Inherent orbital mechanics, that is, and no amount of wishful thinking will ever change that.

That is a large velocity difference with respect to the Earth, exceeding Earth's escape speed by a considerable margin. And, gravity accelerates you a bit more as you close to the edge of the atmosphere. Plus you may be flying a faster-than-Hohmann trajectory. That is where the roughly 17 km/s entry velocity is coming from. In the old US customary units days, for Mars mission plans during the Apollo days, this was reported as a nominal 50,000 ft/sec entry. You convert the units for yourself (3.2808333... feet per meter, 1000 m per km). It's the same speed. Nothing has changed, not in all these decades.

Ballistic entry peak gees depend mostly upon two thing: how shallow your entry angle is, and how fast your speed is when entry begins. Steep angles and higher entry speeds raise peak gees (and peak heating). That's just inherent physics. No amount of wishful thinking can change it.

With entry speeds exceeding escape (Earth escape 11 km/s), there is the danger of bouncing off the atmosphere at shallow entry angles, like a rock skipped on water. You must counter that with lift downwards during entry by flying at the appropriate off-angle, until your speed is well below escape. Works for any shape body, not just capsules or winged spaceplanes. Only the details differ.

Downlift to prevent the bounce-off is exactly what Musk's entry sequence depicts for the Mars landing. There's been no depiction published of the Earth landing, but it has to work EXACTLY the same way. Physics is just physics. Downlift does steepen the trajectory, and increase peak gees by the way. Later in the trajectory, you go to uplift, which shallows the trajectory and lessens gees. At Mars, that is how you get enough altitude at the end of this sequence for the landing to work in that thin air. Min is 4-5 km at about Mach 3, the tail-first switch occurs closer to 10 km altitude after climbing, which lowers speed to nearer Mach 1.

Here on Earth, climbing is not needed, since the air is very much thicker, and you come out of hypersonics at Mach 3 very much higher up than at Mars. Falling more broadside is enough to limit terminal velocity to transonic or even high-subsonic, until you are low enough to turn tail-first and fire up the rockets to land. Again, it's just physics: has to be that way.

I have not run the calculations for ballistic entry at Earth at 17 km/s. I have run them for 11 km/s to simulate an Apollo return, and I got the same 11 gees peak that those crews actually saw. My model is crude, so I didn't exactly match the Apollo entry angle, but I did match the extreme sensitivity to entry angle.

So, I don't know what the true gees will be for direct entry returning from Mars. But the speed is higher, and what Apollo saw at 11 km/s was 11 gees. Mars return will be more, possibly a lot more. Hopefully not so much more.

GW

Last edited by GW Johnson (2018-04-28 10:28:21)

SpaceNut · 2018-04-28 11:40:33

https://en.wikipedia.org/wiki/Escape_velocity

The escape velocity from Earth is about 11.186 km/s (6.951 mi/s; 40,270 km/h; 25,020 mph) at the surface.

https://nssdc.gsfc.nasa.gov/planetary/f … nfact.html

The Moons Escape velocity is about 2.38 km/s and when you add in the earths gravitational pull you end up the much higher velocity coming back home.

Propulsion Estimates for High Energy Lunar Missions Using Future Propellants

page 3 has the table of what GW is talking about...

louis · 2018-04-28 16:58:04

GW,

So do you agree with the following quote:

"The BFS doesn’t need an entry burn, it’ll have insulated tiles like the Space Shuttle and use aerodynamic braking to subsonic speeds." ?

https://www.quora.com/Elon-Musk-just-co … l-in-space

Is your point that relying on that type of braking still produces the v. high G forces?

I am not competent to query your basis for saying a braking burn will not be available but Musk is clearly fully aware of the issue of high speed return to Earth:

QUOTE:

“The ship part is, by far, the hardest because that’s going to come in from super-orbital velocities, like interplanetary Mars transfer velocities, moon transfer velocities. These are way harder than coming from Earth orbit.”

The spaceship’s high-speed returns will stress the craft’s heat shield and structure beyond the temperatures and pressures experienced by a capsule re-entering the atmosphere from Earth orbit, or by a descending rocket stage.

“Testing that ship out is the real tricky part,” Musk said Feb. 6. “The booster, I think — I don’t want to get too complacent — but I think we understand reusable boosters. Reusable spaceships that can land propulsively, that’s harder. We’re starting with the hard part first.”

END QUOTE

Then there's this:

https://spaceflightnow.com/2018/03/13/m … next-year/

Sounds to me like Musk knows exactly what the problem is and is addressing it. Here's another quote from the above article referencing the heat shield:

QUOTE:

Musk said last month that SpaceX will likely conduct the first phase of spaceship testing at its South Texas launch site near Brownsville. Another option could be “ship-to-ship” flights at sea, he said.

“Most likely, it’s going to happen at our Brownsville location because we’ve got a lot of land with nobody around, so if it blows up, it’s cool,” he said. “By hop test, I mean it’ll go up several miles and come down. The ship is capable of single stage to orbit if you fully loaded the tanks, so we’ll do flights of increasing complexity. We really want to test the heat shield material, (and do) something like fly out, turn around, accelerate back real hard, and come in hot to test the heat shield.”

END QUOTE

Clearly the heat shield is going to be v. important in the re-entry sequence. I simply don't accept that Musk is unaware of the need to minimise the stress on the returning crew. You don't really believe he would be that negligent either do you?

GW Johnson wrote:

Louis:
The reddit thing is readers of a blog site bandying guesses about, not folks who have actually run real numbers. The NASA cardio thing is a plan, not a result. That answer is not known yet. But we already do know that stress applied to a weak heart kills.
With BFS return from Mars, there is no braking burn before atmospheric entry at Earth, because there is no available propellant budget in this BFR/BFS design for that purpose. There is only aerobraking, and you are running into the Earth from behind, at a velocity with respect to the sun that is considerably higher than Earth's orbital velocity about the sun. Inherent orbital mechanics, that is, and no amount of wishful thinking will ever change that.
That is a large velocity difference with respect to the Earth, exceeding Earth's escape speed by a considerable margin. And, gravity accelerates you a bit more as you close to the edge of the atmosphere. Plus you may be flying a faster-than-Hohmann trajectory. That is where the roughly 17 km/s entry velocity is coming from. In the old US customary units days, for Mars mission plans during the Apollo days, this was reported as a nominal 50,000 ft/sec entry. You convert the units for yourself (3.2808333... feet per meter, 1000 m per km). It's the same speed. Nothing has changed, not in all these decades.
Ballistic entry peak gees depend mostly upon two thing: how shallow your entry angle is, and how fast your speed is when entry begins. Steep angles and higher entry speeds raise peak gees (and peak heating). That's just inherent physics. No amount of wishful thinking can change it.
With entry speeds exceeding escape (Earth escape 11 km/s), there is the danger of bouncing off the atmosphere at shallow entry angles, like a rock skipped on water. You must counter that with lift downwards during entry by flying at the appropriate off-angle, until your speed is well below escape. Works for any shape body, not just capsules or winged spaceplanes. Only the details differ.
Downlift to prevent the bounce-off is exactly what Musk's entry sequence depicts for the Mars landing. There's been no depiction published of the Earth landing, but it has to work EXACTLY the same way. Physics is just physics. Downlift does steepen the trajectory, and increase peak gees by the way. Later in the trajectory, you go to uplift, which shallows the trajectory and lessens gees. At Mars, that is how you get enough altitude at the end of this sequence for the landing to work in that thin air. Min is 4-5 km at about Mach 3, the tail-first switch occurs closer to 10 km altitude after climbing, which lowers speed to nearer Mach 1.
Here on Earth, climbing is not needed, since the air is very much thicker, and you come out of hypersonics at Mach 3 very much higher up than at Mars. Falling more broadside is enough to limit terminal velocity to transonic or even high-subsonic, until you are low enough to turn tail-first and fire up the rockets to land. Again, it's just physics: has to be that way.
I have not run the calculations for ballistic entry at Earth at 17 km/s. I have run them for 11 km/s to simulate an Apollo return, and I got the same 11 gees peak that those crews actually saw. My model is crude, so I didn't exactly match the Apollo entry angle, but I did match the extreme sensitivity to entry angle.
So, I don't know what the true gees will be for direct entry returning from Mars. But the speed is higher, and what Apollo saw at 11 km/s was 11 gees. Mars return will be more, possibly a lot more. Hopefully not so much more.
GW

louis · 2018-04-28 17:29:43

I don't think anyone's denying that...I think it's simply a question of "Are Space X a complete bunch of idiots who have totally ignored this issue?" My betting is that they are not. It would be a remarkable coincidence if the design of the BFS left it with absolutely no propellant for a braking burn on its return to Earth.

SpaceNut wrote:

https://en.wikipedia.org/wiki/Escape_velocity
The escape velocity from Earth is about 11.186 km/s (6.951 mi/s; 40,270 km/h; 25,020 mph) at the surface.
https://nssdc.gsfc.nasa.gov/planetary/f … nfact.html
The Moons Escape velocity is about 2.38 km/s and when you add in the earths gravitational pull you end up the much higher velocity coming back home.
Propulsion Estimates for High Energy Lunar Missions Using Future Propellants
page 3 has the table of what GW is talking about...

elderflower · 2018-04-29 03:26:53

Braking burns , like everything else so far devised by humans, can go wrong. Consider Schiaparelli....
Then the fall back is entry at whatever speed and angle the attitude thrusters can manage.

GW Johnson · 2018-04-29 09:00:53

Louis is putting words in my mouth that I didn't say. I reverse-engineered the performance of the 2017 presentation's 9 m diameter BFS/BFR two-stage vehicle, and posted those results over at my "exrocketman" site. The numbers are there for all to see.

Those results for the return voyage at the max stated 50 ton return payload start from a fully-fueled BFS leaving Mars with all 1100 tons of propellant on board. My best estimates say they need to reserve something like 60 tons of that propellant to cover boiloff losses, a small midcouse manever, and the final retropropulsive landing on Earth.

Almost all of the rest is required to leave Mars on a min-energy Hohmann transfer. I show a reserve delta-vee capability beyond that burn, without touching the 60 ton reserve, of only 1.6 km/s. Compare that to around 7+ to leave Mars on a min-energy transfer trajectory. You can come home faster than 8.5 months, but not so very much faster. Not so much at all.

There is no other propellant on board. Period. End of issue.

Now, if they save that 1.6 km/s capability and use it for a braking burn right before Earth entry, then they fly the Hohmann min-energy trajectory home from Mars (the full 8.5 months!). Under favorable orbital conditions, entry speed would be 13 km/s or a little more. If you brake right before entry, that slows you to 11 or 12 km/s at entry, roughly.

Apollo came back from the moon at 11 km/s, and experienced 11 gees doing so. It was a capsule, BFS is a lifting body craft, not the same. But it really is in the same class of gees. Something like 11+ gees. Higher, if you choose a higher energy trajectory and hit Earth's air closer to 17 km/s.

There IS NO WAY AROUND THAT! If you shallow-out to lower peak gees, you bounce off the atmosphere like a skipped stone, at speeds above escape. Dead crew lost in space!

You are looking at around (as a minimum) peak 11-12 gees imposed for a minute or so on a crew after suffering at the very least the medical damage of 8.5 months exposure to 0-gee. That presumes Mars 0.38 gee is fully therapeutic, and we don't know that, yet. Any way else you might slice it, you are looking at high-gee stress on significantly-weakened hearts.

Or, I hope they think of sending BFS's home 2 at a time, and providing spin gravity closer to 1 gee than 0.38 gee, by docking tail-to-tail, and spinning the pair up at 4 to 4.6 rpm.

If you cannot change the flight dynamics (and you cannot !!!), then change the health (or lack thereof).

GW

Last edited by GW Johnson (2018-04-29 09:21:15)

elderflower · 2018-04-29 09:12:23

What masses are included in the assumed 50Te return payload? How much of it could be jettisoned on arrival, prior to initiating the braking manoeuver?

GW Johnson · 2018-04-29 09:24:41

Hi Elderflower:

I dunno, no one has said. How about the guess of 25-50 people with the 1-2 tons each of air, water, and food to keep them alive for 9 months? I would think crews would be rotating home after their tour on Mars. It'll be a long time yet before there is a city with permanent residents.

GW

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