Author Topic: Fast transits and aerocapture  (Read 5812 times)

Offline MikeAtkinson

  • Full Member
  • ****
  • Posts: 1702
  • Bracknell, England
  • Liked: 472
  • Likes Given: 59
Re: Fast transits and aerocapture
« Reply #20 on: 10/29/2016 09:16 AM »
If you refuel more often, you can STILL send 450 tons on a fast trajectory.

Where would you see those other refuelling positions?


L1/L2 is the obvious option. But there are at least two others.

I find it very doubtful they can do efficient aerobraking with such a heavy payload and very fast entry. Maybe with aerobraking into orbit first.

So do I, 300 tonnes is probably pushing it initially.

Note the presentation is carefully worded to say "Cargo to Mars", not cargo landed on Mars. It is talking about the  Spaceship transfer capacity in terms of delta-v, not the landing capacity.

Offline guckyfan

  • Senior Member
  • *****
  • Posts: 6472
  • Germany
  • Liked: 1639
  • Likes Given: 1524
Re: Fast transits and aerocapture
« Reply #21 on: 10/29/2016 12:09 PM »
There is another issue I thougt about. They will want a fast transfer for crew. I think they will also want to test Mars entry of a ship with a similar speed and similar mass before they send people. That would limit the mass of the first cargo ship. They could send two in 2022, one fast and one on a slow trajectory with 300t.

But that may stretch their resources so early in the development. Maybe 1 cargo with 150t fast in 2022. 1 crew fast in 2024 plus one cargo 300t in 2024. Distribution of cargo in a way that it would not endanger the crew if they lose the big cargo ship.

Offline envy887

  • Senior Member
  • *****
  • Posts: 3195
  • Liked: 1515
  • Likes Given: 1027
Re: Fast transits and aerocapture
« Reply #22 on: 10/29/2016 07:44 PM »
Note the presentation is carefully worded to say "Cargo to Mars", not cargo landed on Mars. It is talking about the  Spaceship transfer capacity in terms of delta-v, not the landing capacity.

Why then would the presentation explicitly show the amount of delta-v reserved for landing 450 tonnes of payload?

Offline Vultur

  • Full Member
  • ****
  • Posts: 1086
  • Liked: 154
  • Likes Given: 11
Re: Fast transits and aerocapture
« Reply #23 on: 10/30/2016 12:16 AM »
Their is a great irony involved in mars return trajectories, a trajectory that allows the vehicle to make one round trip every synod will involve a VERY long duration is space or nearly 1.3 years for the return leg.  This is clearly unacceptable for human flight being longer then any human has ever spent in space in one stretch and that was in LEO which is far milder.

'Clearly unacceptable' to whom? To current NASA, sure, but probably not to private efforts ~10 years from now.

It is only marginally longer than the longest Mir stay (over 14 months). Sure that was in LEO, but zero-gravity is the same there, and the radiation isn't that bad. (Higher than NASA accepted astronaut doses, sure, but those are very conservative.)

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 27335
  • Minnesota
  • Liked: 7283
  • Likes Given: 4990
Re: Fast transits and aerocapture
« Reply #24 on: 10/30/2016 01:07 AM »
If you refuel more often, you can STILL send 450 tons on a fast trajectory.

Where would you see those other refuelling positions?


L1/L2 is the obvious option. But there are at least two others.

I find it very doubtful they can do efficient aerobraking with such a heavy payload and very fast entry. Maybe with aerobraking into orbit first.

So do I, 300 tonnes is probably pushing it initially.

Note the presentation is carefully worded to say "Cargo to Mars", not cargo landed on Mars. It is talking about the  Spaceship transfer capacity in terms of delta-v, not the landing capacity.
Disagree.

Again, you can use some propellant for slowing down and only aerocapture into Mars orbit (perhaps elliptical), then refuel again.

From the presentation, it seemed clear to me the 450tons was for payload to the ground. It would take much fuller tanks to enable that (hence more refueling in multiple places), but it's most certainly possible. Supersonic retropropulsion is uniquely scalable. If you have enough propellant, you can land a lot.

As far as SpaceX "not really thinking through" some of these other options, I strongly disagree. They've thought through a very wide trade space. What was presented was just the core of their approach, not anywhere close to the limit of their analysis or the architecture's flexibility.

A single person devoted to crawling and analyzing the trade space could generate an enormous multitude of options. With a team of people working on this, each option can be analyzed and considered in some significant depth.

Not that there are no holes anywhere in their analysis. For instance, solar flare radiation is fairly isotropic and anyway spirals around field lines that come in at an angle from the Sun, so pointing the tanks either to the Sun or really anywhere won't guard from most flare radiation. Luckily, you can just reposition supplies and shelter in the center of the ship to get more than sufficient shielding.

But anyway, architecturally, we know from L2 that they've considered a LOT of options.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: Fast transits and aerocapture
« Reply #25 on: 10/30/2016 03:00 AM »
Their is a great irony involved in mars return trajectories, a trajectory that allows the vehicle to make one round trip every synod will involve a VERY long duration is space or nearly 1.3 years for the return leg.  This is clearly unacceptable for human flight being longer then any human has ever spent in space in one stretch and that was in LEO which is far milder.

'Clearly unacceptable' to whom? To current NASA, sure, but probably not to private efforts ~10 years from now.

It is only marginally longer than the longest Mir stay (over 14 months). Sure that was in LEO, but zero-gravity is the same there, and the radiation isn't that bad. (Higher than NASA accepted astronaut doses, sure, but those are very conservative.)

Musk has all but admitted it himself with his emphasis on a fast transit time to mars to outrun the health issues inherent in deep space, to speak nothing of the human factors of confinement for that length of time.  The radiation dosages are not just higher then NASA acceptable dosages they are significantly higher, your looking at an extra year in which dosages are not reduced by shielding on mars and more time for solar flares to occur which even if shielded add to cumulative dosage.  Lastly the fact that a vehicle and it's mission might be private in no way negates NASA or the FFA's regulatory power or responsibility to see that the trip is up to safety standards, if it doesn't meet their standards their astronauts won't use it, and they will never let civilians fly on something that they won't put their own people on.

Offline vapour_nudge

  • Full Member
  • ****
  • Posts: 439
  • Australia
  • Liked: 183
  • Likes Given: 293
Re: Fast transits and aerocapture
« Reply #26 on: 10/30/2016 03:33 AM »
There is another issue I thougt about. They will want a fast transfer for crew. I think they will also want to test Mars entry of a ship with a similar speed and similar mass before they send people. That would limit the mass of the first cargo ship. They could send two in 2022, one fast and one on a slow trajectory with 300t.

But that may stretch their resources so early in the development. Maybe 1 cargo with 150t fast in 2022. 1 crew fast in 2024 plus one cargo 300t in 2024. Distribution of cargo in a way that it would not endanger the crew if they lose the big cargo ship.
Hmm. A lot of extreme optimism here. So, within 5 years SpaceX have to develop and man rate a new rocket, test it, create all the infrastructure to build it in the first place, send all that cargo and then crew, which all needs to land without a hitch and the crew too and then that has to be set up and readied for further transits. And all this while they are yet investigating another unforeseen failure of a medium lift rocket.

They're doing great things but if it happens at all, it is a long way off and fast transits are just one small thing. If there were the same optimism for the economy we'd be in a good place. I'd like to say I hope I'm wrong but I just don't see any of it happening within ten years if at all

Fast transits wouldn't be a problem if we were aiming at the Moon or some goal more obtainable.
Having ranted all that, aero capture needs more focus to get the probes into a working orbit more quickly. I'd like to see a lot more probes and science perhaps using the full use of Musks's rocket payload capability before wasting any money on silly adventures like sending crew to Mars
« Last Edit: 10/30/2016 03:41 AM by vapour_nudge »

Offline gospacex

  • Senior Member
  • *****
  • Posts: 3028
  • Liked: 535
  • Likes Given: 604
Re: Fast transits and aerocapture
« Reply #27 on: 10/30/2016 05:32 AM »
Their is a great irony involved in mars return trajectories, a trajectory that allows the vehicle to make one round trip every synod will involve a VERY long duration is space or nearly 1.3 years for the return leg.  This is clearly unacceptable for human flight being longer then any human has ever spent in space in one stretch and that was in LEO which is far milder.

'Clearly unacceptable' to whom? To current NASA, sure, but probably not to private efforts ~10 years from now.

It is only marginally longer than the longest Mir stay (over 14 months). Sure that was in LEO, but zero-gravity is the same there, and the radiation isn't that bad. (Higher than NASA accepted astronaut doses, sure, but those are very conservative.)

Musk has all but admitted it himself with his emphasis on a fast transit time to mars to outrun the health issues inherent in deep space, to speak nothing of the human factors of confinement for that length of time.  The radiation dosages are not just higher then NASA acceptable dosages they are significantly higher, your looking at an extra year in which dosages are not reduced by shielding on mars and more time for solar flares to occur which even if shielded add to cumulative dosage.  Lastly the fact that a vehicle and it's mission might be private in no way negates NASA or the FFA's regulatory power or responsibility to see that the trip is up to safety standards, if it doesn't meet their standards their astronauts won't use it, and they will never let civilians fly on something that they won't put their own people on.

I sure hope US government has no power to decide how people can or cannot risk their own lives. (Risking other people's lives is another matter).

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: Fast transits and aerocapture
« Reply #28 on: 10/30/2016 06:10 AM »

I sure hope US government has no power to decide how people can or cannot risk their own lives. (Risking other people's lives is another matter).

Selling a transport service IS risking other peoples lives.  If Musk himself wants to ride in the rocket that would be risking his own life, once he starts selling a service his has to meet safety standards no matter what kind of waver people sign.

Offline MikeAtkinson

  • Full Member
  • ****
  • Posts: 1702
  • Bracknell, England
  • Liked: 472
  • Likes Given: 59
Re: Fast transits and aerocapture
« Reply #29 on: 10/30/2016 06:25 AM »
Note the presentation is carefully worded to say "Cargo to Mars", not cargo landed on Mars. It is talking about the  Spaceship transfer capacity in terms of delta-v, not the landing capacity.

Why then would the presentation explicitly show the amount of delta-v reserved for landing 450 tonnes of payload?

I think that makes my point. This is in terms of delta-v, there may be other constraints which limit the amount of cargo landed. There are at least two constraints which potentially limit cargo to lower than 450 tonnes.

First, cargo density, 450 tonnes at 0.2 tonne/m3 is 2250 m3 of cargo, we don't know the usable cargo volume of the spaceship but it is possibly less than that.

Second, heatshield and general EDL performance, the 450 tonnes payload might only be possible for 4th or 5th generation heatshield technology (ITS will use 3rd generation PICA-X).

Offline MikeAtkinson

  • Full Member
  • ****
  • Posts: 1702
  • Bracknell, England
  • Liked: 472
  • Likes Given: 59
Re: Fast transits and aerocapture
« Reply #30 on: 10/30/2016 06:46 AM »

So do I, 300 tonnes is probably pushing it initially.

Note the presentation is carefully worded to say "Cargo to Mars", not cargo landed on Mars. It is talking about the  Spaceship transfer capacity in terms of delta-v, not the landing capacity.
Disagree.

Again, you can use some propellant for slowing down and only aerocapture into Mars orbit (perhaps elliptical), then refuel again.

From the presentation, it seemed clear to me the 450tons was for payload to the ground. It would take much fuller tanks to enable that (hence more refueling in multiple places), but it's most certainly possible. Supersonic retropropulsion is uniquely scalable. If you have enough propellant, you can land a lot.

I think we are slightly talking across each other. I think 450 tonnes cargo is an eventual stretch aim, not a short term goal.

Eventually, there will be cargo transfer in orbit - 150 tonnes is a lot of cargo to transfer (it takes the crew of ISS a  day or two to unload 2 tonnes from Dragon).

Eventually, there will be refueling in other places than LEO and Mars surface - Mars orbit, L1/L2, etc.

Eventually, there will be improvements in EDL.

But none of these are needed short term and are a distraction from the large number of things that are needed over the next 20 years.

Offline Vultur

  • Full Member
  • ****
  • Posts: 1086
  • Liked: 154
  • Likes Given: 11
Re: Fast transits and aerocapture
« Reply #31 on: 10/30/2016 07:30 AM »
Musk has all but admitted it himself with his emphasis on a fast transit time to mars to outrun the health issues inherent in deep space, to speak nothing of the human factors of confinement for that length of time.

By the nature of a colonization vehicle, return passengers will be much fewer than outbound, and the vehicle is big.

Also, people over-state the confinement issues IMO; people are greatly variable in this regard, and Mars colonists will be heavily self-selected for not being bothered by confinement. Plenty of people I know already spend the great majority of their waking hours in a chair staring at a screen...

(And consider the Antarctic winter quarters in the 1900s and 10s, over a dozen people in a small hut for months...)


Quote
Lastly the fact that a vehicle and it's mission might be private in no way negates NASA or the FFA's regulatory power or responsibility to see that the trip is up to safety standards,

NASA has no regulatory power, and currently the FAA has very little authority over the commercial space industry due to a Congressional moratorium. This was extended to 2023... I see no reason to expect it necessarily won't be extended again (and again). 2023 is late enough to see if Musk's plans look likely to pan out... if they do, I think there will be enough public enthusiasm to make "government interference" look unpopular and therefore unlikely.

Anyway, this is a colonization vehicle; coming back to Earth is an edge case  ;)

EDIT: incomplete sentence
« Last Edit: 10/30/2016 07:31 AM by Vultur »

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 27335
  • Minnesota
  • Liked: 7283
  • Likes Given: 4990
Re: Fast transits and aerocapture
« Reply #32 on: 10/30/2016 06:43 PM »
Space is not safe. Period.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: Fast transits and aerocapture
« Reply #33 on: 10/30/2016 09:52 PM »
Musk has all but admitted it himself with his emphasis on a fast transit time to mars to outrun the health issues inherent in deep space, to speak nothing of the human factors of confinement for that length of time.

By the nature of a colonization vehicle, return passengers will be much fewer than outbound, and the vehicle is big.

Also, people over-state the confinement issues IMO; people are greatly variable in this regard, and Mars colonists will be heavily self-selected for not being bothered by confinement. Plenty of people I know already spend the great majority of their waking hours in a chair staring at a screen...

(And consider the Antarctic winter quarters in the 1900s and 10s, over a dozen people in a small hut for months...)


Quote
Lastly the fact that a vehicle and it's mission might be private in no way negates NASA or the FFA's regulatory power or responsibility to see that the trip is up to safety standards,

NASA has no regulatory power, and currently the FAA has very little authority over the commercial space industry due to a Congressional moratorium. This was extended to 2023... I see no reason to expect it necessarily won't be extended again (and again). 2023 is late enough to see if Musk's plans look likely to pan out... if they do, I think there will be enough public enthusiasm to make "government interference" look unpopular and therefore unlikely.

Anyway, this is a colonization vehicle; coming back to Earth is an edge case  ;)

EDIT: incomplete sentence

The congressional moratorium is not going to be extended indefinitely and would never be a basis for unregulated wild-west colonization of mars, that is pure fantasy as is the notion that public opinion will be behind the colonization to such a degree that it will be free from any safety or regulatory standards.

Offline JamesH65

  • Full Member
  • ****
  • Posts: 698
  • Liked: 419
  • Likes Given: 8
Re: Fast transits and aerocapture
« Reply #34 on: 10/31/2016 01:55 PM »
Musk has all but admitted it himself with his emphasis on a fast transit time to mars to outrun the health issues inherent in deep space, to speak nothing of the human factors of confinement for that length of time.

By the nature of a colonization vehicle, return passengers will be much fewer than outbound, and the vehicle is big.

Also, people over-state the confinement issues IMO; people are greatly variable in this regard, and Mars colonists will be heavily self-selected for not being bothered by confinement. Plenty of people I know already spend the great majority of their waking hours in a chair staring at a screen...

(And consider the Antarctic winter quarters in the 1900s and 10s, over a dozen people in a small hut for months...)


Quote
Lastly the fact that a vehicle and it's mission might be private in no way negates NASA or the FFA's regulatory power or responsibility to see that the trip is up to safety standards,

NASA has no regulatory power, and currently the FAA has very little authority over the commercial space industry due to a Congressional moratorium. This was extended to 2023... I see no reason to expect it necessarily won't be extended again (and again). 2023 is late enough to see if Musk's plans look likely to pan out... if they do, I think there will be enough public enthusiasm to make "government interference" look unpopular and therefore unlikely.

Anyway, this is a colonization vehicle; coming back to Earth is an edge case  ;)

EDIT: incomplete sentence

The congressional moratorium is not going to be extended indefinitely and would never be a basis for unregulated wild-west colonization of mars, that is pure fantasy as is the notion that public opinion will be behind the colonization to such a degree that it will be free from any safety or regulatory standards.

Whoever said there would be no safety or regulatory standards? There will be safety, there will be standards. Just lower/different ones that would be acceptable to the general public here on Earth, by necessity. Because as someone above said, Space is dangerous.

I used to race cars. That is, by definition, dangerous. You sign forms stating you accept the risk. Not only that, I maintained them myself(!). However, there are still safety standards, but by their very nature, those standards are different to those that govern road cars. My race car has no ABS, no airbags, limited crash protection (compared with a modern road car). It does however, have safety features not usually seen on a road car. Roll cage, full harness seatbelts, in built extinguishers etc.

The point? Different scenarios need different rules. Don't judge an interplanetary transport system with the same mindset as you judge an airliner.

Tags: