IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)

[Negan]

The above hypothesis I raised was a Red Dragon, launching on a falcon heavy a couple of years before, so the rover would be in the ton range, at most, to prepare the site for BFR landing in 2022.

But there is no Red Dragon. SpaceX decided it was inadequate or not needed for the purpose you're suggesting. Plus what you've suggested does nothing to help protect the surface from the rocket engine plume high pressure impingement that NASA has determined is a big risk for landers with landed mass greater than 20 metric tons, and the reason they've been looking at technologies to create landing/launch pads.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150015509.pdf

Maybe this isn't really an issue at all, but if it is, I'm highly doubtful one Red Dragon launched with a FH could deliver everything that's needed, but one BFS in Mars Orbit surely could. It also opens up the possibility of creating a lander with is much more mass and volume. A ten ton dozer on the surface would surely more valuable than a one ton rover trying to play the part. It might be a trade of developing and intermediate lander or risk those first BFS's tipping over and destroying themselves and all the valuable equipment on board.

As far as launching anything in 2020 to Mars, I don't see that happening at all. The 2022 date is aspirational and dependent on the technology needed is ready to go. I don't see any reason to believe they wouldn't push it to the right to lower risk.

[speedevil]

The above hypothesis I raised was a Red Dragon, launching on a falcon heavy a couple of years before, so the rover would be in the ton range, at most, to prepare the site for BFR landing in 2022.

But there is no Red Dragon. SpaceX decided it was inadequate or not needed for the purpose you're suggesting

They did not quite do that, as I understand it.
Red Dragon was cancelled as a program post it being discovered that crew dragon was not going to be able to propulsively land, so any savings in development cost went away, combined with the firming up of plans for ITS/BFR and how to pay for it.

At least some of the risk for landing can be mitigated by careful surveying of the landing pad, and minimal prep.
Enough is another question entirely.

BFS to LMO is harder than getting it to the surface possibly, even with aerobraking.

Takeoff requires enormously more thrust than landing however, the last few meters can be at minimal thrust on one engine, for perhaps 100 tons.
Takeoff is going to need 350++.

A 'takeoff mat' that has to last two seconds, once may also be a useful device.

This does depend on how well commercial crew goes, of course.

If it goes well, by this time there may be a used dragon 2 or several that could be cannibalised, as well as falcon heavy launching regularly.

[Negan]

BFS to LMO is harder than getting it to the surface possibly, even with aerobraking.

Why? Aerobraking has been successful with Mars orbiters. What about the other orbiters that are there? Those insertion techniques are not possible?

[envy887]

BFS to LMO is harder than getting it to the surface possibly, even with aerobraking.

Why? Aerobraking has been successful with Mars orbiters. What about the other orbiters that are there? Those insertion techniques are not possible?

Delta-v wise, it's harder to get to LMO than to the surface after a fast transfer. If you do a slow transfer, it's easier to get to LMO assuming you use aerobraking after propulsive capture to a high orbit.

[meekGee]

Thousands?

I would expect stuff like implements to be farmed out to CAT or another commercial vendor even if Tesla makes the chassis.

I can't see that at all. CAT makes stuff from heavy steel. They know nothing about designing lightweight gear more suitable to being flown a couple of hundred million miles through space.

And BTW, I used to own a tractor and about 20 implements/attachments. Most of these type items are not made by CAT. CAT makes the chassis and engines of immense machines. Most tractors use either towed implements, made by a dozen or so other companies, or attachments driven by the PTO (power take off). These things are made by people who are used to designing implements whose utility depends upon high mass and fabricated via simple means from basic steel. They have little experience in fabricating low mass/high strength tools that depend upon advanced geometrical structures and advanced materials to achieve a task. I guarantee you, things that have to be transported hundreds of millions of miles through space and landed on another planet will be designed and engineered by someone other than any of the companies who build traditional tractor and earthmoving gear.

IIRC, CAT is one of the companies with the highest R&D/revenue budget fraction.

If a company has R&D in its DNA and upper management still has a spark in it, don't count them out.

[john smith 19]

If it goes well, by this time there may be a used dragon 2 or several that could be cannibalised, as well as falcon heavy launching regularly.

The attractive part of this plan is that they have the basis for a lander. The downside is that they'd need to build a rover and the power system to keep it charged. The upside is that if it arrives in the previous launch window it has 24 months to scan the area and prep the ground. Assuming a 50m square that's needs the rover to cover about 4m/day. That seems feasible, if the rover is big enough (and they have no serious power outages. I'm assuming a PV power system). It's not doing research, it's basically running a grid pattern, with something like a snow plow on the front to push most stuff out of the way (if possible).

The question is how well developed is the propulsive landing system and how serious would the mods be to retro-fit it to Crew Dragon?

[speedevil]

The attractive part of this plan is that they have the basis for a lander. The downside is that they'd need to build a rover and the power system to keep it charged.

A rover with an arm seems a really, really handy thing to have if you are trying to setup a solar array for BFS on the surface, or to drag a propellant hose from one BFS to another, or indeed to pretty much any surface ops out of the range of a crane.

The power system would differ, but a surface rover might help quite a lot to reduce the risk of further rovers that would be doing actual productive work around ISRU.

[Lar]

IIRC, CAT is one of the companies with the highest R&D/revenue budget fraction.

If a company has R&D in its DNA and upper management still has a spark in it, don't count them out.

Also CAT is actually on record as already doing research and prototyping in this area. Something TomH completely missed.

[zodiacchris]

Could we be overthinking the problem a bit? What is the distance between the extended leg pads, and what is the tipping angle of the BFS? It would appear that one or two of the legs would have to sink in pretty deep to cause a topple.

The first two ships will be uncrewed, so just land them in a non-cratered and boulder free area. More likely than not they will stay upright. Get a remote controlled/autonomous grader out and scrape a suitable pad. Fit the grader with a geotech probe that assesses load bearing capability of the substrate and if it checks out, land the next one on the prepared surface.

We use road graders to build 10s of thousand kilometers of unsealed tracks here in Australia and then run 50 tonne 40m long road trains over them, generally works very well.

I’d save the painting of circles and logos until later (when the crew gets really bored), and use the first two ships as markers/ beacons for landing, in combination with radar.

But then as a geologist I tend to oversimplify things a bit sometimes 🤔

[TripD]

Once the first crews start landing, why not trench out an area under the nozzles enough to replace the regolith with treated marscrete. This could reduce flying debris during launch. Anything outside of the perimeter of the nozzles isn't as likely to strike the craft.  Or has it been established that chances of damage diminish quickly enough upon launch that any ground treatment is unnecessary?

[Lar]

The first two ships will be uncrewed, so just land them in a non-cratered and boulder free area. More likely than not they will stay upright. Get a remote controlled/autonomous grader out and scrape a suitable pad. Fit the grader with a geotech probe that assesses load bearing capability of the substrate and if it checks out, land the next one on the prepared surface.

Do that PLUS sinter the surface and you've got a pretty good pad I think.  Making a flame trench can come later

[speedevil]

We use road graders to build 10s of thousand kilometers of unsealed tracks here in Australia and then run 50 tonne 40m long road trains over them, generally works very well.

A jet of compressed gas moving at many times the speed of sound can cause the subsurface to get pressurised and move really quite large chunks in unexpected ways - certainly large enough to cause damage.
Determining the properties of the surface in detail with 5cm imagery lets you find some classes of surface that would cause problems for the lander - it's great for finding holes and boulders.

It is not so great for working out how tightly packed sand is, or if an apparently smooth surface is riddled with cracks.
And going from optical measurements to physical properties of the surface - something like 'Will this cause damaging debris when a rocket exhaust hits it' is even harder, given our models of the martian surface are laughably incomplete compared to what we know on earth.

The weightbearing capability - especially with relatively long legs on the BFS is likely a secondary concern.

A hypothetical lander on FH would also tell you a little about the effects of rocket impingement on the surface at the location you happen to be in, for ground truth.

[KelvinZero]

How about something between a carbon fibre honeycomb filled with ice and a waterbed mat that you roll out and inflate with a portion of the water for your propellant ISRU? Sort of like an expensive but light to transport high tech version of https://en.wikipedia.org/wiki/Pykrete

There was a whole thread on constructing mars launch pads somewhere.

[Negan]

BFS to LMO is harder than getting it to the surface possibly, even with aerobraking.

Why? Aerobraking has been successful with Mars orbiters. What about the other orbiters that are there? Those insertion techniques are not possible?

So it sounds like, with a slow transfer, BFR could drop off some payload in LMO and still land.

[Lar]

... either that or land the payload, refuel, and deploy in desired orbit. We all like the idea of dropping stuff off on the way but they might only do that once, or never.

[speedevil]

The first two ships will be uncrewed, so just land them in a non-cratered and boulder free area. More likely than not they will stay upright. Get a remote controlled/autonomous grader out and scrape a suitable pad. Fit the grader with a geotech probe that assesses load bearing capability of the substrate and if it checks out, land the next one on the prepared surface.

Do that PLUS sinter the surface and you've got a pretty good pad I think.  Making a flame trench can come later

https://www.nature.com/articles/s41598-017-01157-w#Tab1

Upon a high-pressure compression, Mars-1a particles form a strong solid at ambient, with resultant flexural strengths exceeding that of typical steel-reinforced concrete or many in situ resource utilization (ISRU) created materials formed by adding binders

The pressure involved is high - 600 bars.

[docmordrid]

Sounds like a job for robotic debris removal and microwave surface melting, perhaps a square meter at a time.

[gin455res]

Just want to see if I understood SpaceX's entry simulation properly:


https://www.youtube.com/watch?v=tdUX3ypDVwI?t=2132


Is the BFS doing a negative-lift (and hyperbolic) entry from 7.5km/s to about 3.5km/s, whereupon it rotates into a positive-lift entry?

[hkultala]

Just want to see if I understood SpaceX's entry simulation properly:


https://www.youtube.com/watch?v=tdUX3ypDVwI?t=2132


Is the BFS doing a negative-lift (and hyperbolic) entry from 7.5km/s to about 3.5km/s, whereupon it rotates into a positive-lift entry?

Yep. When it's approaching mars at speed which is higher than mars orbital speed, it needs the negative lift to stay in the atmosphere and to not fly out of it. But when the velocity becomes lower than mars orbital speed, it needs positive lift to keep flying instead of falling to mars surface too quickly.

To bleed maximum amount of horizontal velocity by aerobraking, it has to stay in the atmosphere for as long as possible.

[RoboGoofers]

They'll use the sea level raptors for landing, I assume. How 'damaging' is the underexpanded exhaust? Mvac impingement doesn't seem to hurt falcon boosters.  If a sufficiently flat spot was cleared, would a heat resistant tarp be enough to protect the surface?