Starship is Go...

louis · 2019-02-04 18:47:08

I got the impression from that NASA Mars landing location seminar (2017?) that the thinking was much more to find an area of (relatively dust-free, so elevated I guess in relation to surroundings) hard rock surface with less than 5% slope.

They seem to have found some such locations.

GW Johnson wrote:

I just took an approximate look at the Spacex BFS/Starship landing pad area required for rough-field capability on the moon, Mars, and Earth. The only way to achieve this, is with the folding-panel idea mentioned in post 143 above. The required total landing pad surface areas fall in the 30-45 square meter class, not the 2.5 to 3-something sq.m that Spacex currently shows in its illustrations.
Such large pad areas will have to fold out of the way. There is no way around that larger, folding landing pad requirement, if rough field capability on Mars and the moon (not to mention Earth) is desired. Otherwise you are restricted to reinforced concrete (or solid rock) aprons many feet thick.
See the posting "Designing Rough Field Capability Into the Spacex Starship", posted 2-4-19, on my "exrocketman" site. For those who don't know, that site is http://exrocketman.blogspot.com.
What I found is that handling what the bulk of Mars's surface seems to be like, is the critical design condition. What works for that is more-than-good for the moon, and for Earth, despite the variance in surfaces and properties and gravity. It's complicated, surprise, surprise! So what isn't?
GW

louis · 2019-02-04 19:17:39

Nice vid from Mic of Orion on the new Raptor engines:

https://www.youtube.com/watch?v=HGWSZixZLV8

Interesting question at the end - I didn't do the calculation but I guess he's saying is future Starships could lift several hundred tons into LEO, which would in turn reduce the cost of transit to Mars by a lot...who knows? 60%?

Anyone want to do the calculation properly?

SpaceNut · 2019-02-04 19:23:31

Since its the same model method as the Falcon 9 the non recovered first stage means a greater payload to orbit by expending the stage in full throttle use. The trouble still is the Starship stage must loiter in LEO until refuled before it can be used....

elderflower · 2019-02-05 04:12:15

Back to landing pads.
They must be fitted to the ship for landing on Mars but can be left behind on relaunch. They will not be needed for earth landing which will be on a prepared pad. If a ship is used as a hopper it must take its pads with it, but if not the pads might inject resin into the substrate for increased stability. Since they can be abandoned it wont matter if they have glued themselves onto a few tons of regolith. Indeed this may add ballast to oppose any overturning loads..

GW Johnson · 2019-02-05 10:22:46

Well, for airplanes, rough field capability is designed-in by adding more tires to the main gear. That reduces the bearing pressure under each individual tire. It's heavier, but that's just the "cost of doing business" on unprepared landing strips.

The Starship doesn't need this to land, the problem is at takeoff weight, which is very much larger in a high mass ratio-design stage. As a wild guess, using Spacex's own published numbers from its 2018 website presentations, the weight statement is 85 tons structure plus 150 tons payload (from Earth, 50 tons from Mars), plus 1100 tons propellant, for 1335 tons at second stage ignition going to Earth orbit, and 1235 tons at launch ignition leaving Mars. Those are metric tons.

Landing masses would be structure plus cargo, plus just a handful of tons of propellant residual. That would be 85 + 150 = 235 tons plus some propellant residual after landing on Mars. As a guess, call it 250 tons at most, 235 tons at the very min (a dry-tanks landing, very risky). That's roughly 6 times less weight at landing as at takeoff.

The factor 2 safety margin for dynamical impact is not really needed for takeoff, but is needed for touchdown. So the soil bearing strengths are still 3 times lower for landing as for takeoff. Takeoff from loose sand-with-rocks IS THE soil-strength-related design requirement for rough field operation on Mars.

If you add some sort of bearing area surfaces to achieve this, why design it to be left behind after taking off, in a reusable ship? The rough field airplane does't leave the extra tires behind. Besides, you might need it for landing on Earth. What if the entry trajectory doesn't bring you to the intended paved pad or steel-deck ship? You have to hit the nearby soft sand beach or soft grassy wetland, at full Earthly 1-gee gravity, instead? Happens all the time with airplanes. Ask the likes of Sullenberger about that.

As I said, the extra weight of carrying the increased touchdown pad area with you, is just the "cost of doing business" for rough-field operation. It's fundamentally the same as with airplanes, although the details are quite different.

GW

Last edited by GW Johnson (2019-02-05 10:45:49)

GW Johnson · 2019-02-05 17:29:25

I updated the rough-field analysis at "exrocketman" to include more traceable weights, and all 6 possible cases investigated. No real change to the answer: what Spacex shows on its website for landing pads is about factor 20 too small to provide landing capability on anything remotely like the surface of Mars. As shown, it requires reinforced thick concrete landing aprons only. Or else it crashes. Period.

GW

SpaceNut · 2019-02-05 18:02:47

Here is the information from the Apollo Lunar Lander
https://en.wikipedia.org/wiki/Apollo_Lunar_Module

https://www.hq.nasa.gov/alsj/LM_Landing … 010151.pdf

The footpad, which is attached to the lower end of the inner cylinder by a ball-joint fitting, is approximately 0. 91 meter (3 feet) in diameter and is designed to support the LM on a 0. 69 N/cm2 (1. 0 lb/in2) bearing-strength surface as well as to maintain sliding capability after having impacted rocks or ledges during touchdown.

Sensing the hover to ground was meantioned in the document to shut off the engines.

louis · 2019-02-06 02:42:44

Just in case people haven't come across this before, this NASA Human Landing Site Workshop link is a mine of useful information:

https://www.nasa.gov/feature/mars-human … sentations

louis · 2019-02-07 05:23:13

A really good video from Everyday Astronaut on the stainless steel/heatshield issues:

https://www.youtube.com/watch?v=LogE40_wR9k

Space X seems to have got it right again, finally. I find this encouraging.

Just noticed he's got nearly half a million views for this video. Shows that a quality product will attract interest.

Also - the Raptors are "Go"!

https://twitter.com/elonmusk/status/1093423297130156033

Last edited by louis (2019-02-07 08:17:32)

GW Johnson · 2019-02-07 10:01:15

Understanding the Apollo lunar lander surface bearing pressure data requires context. Most folks on these forums are too young to remember that context, coming as it does from the science of the late 1950’s and engineering of the early and mid 1960’s.

At that time, the scientific consensus of what the moon’s surface was like was no consensus. It ranged from rock to very fine dust. If dust, the worst case, the bearing strength was so low a man in a suit would sink out of sight. That’s the lunar weight of man plus suit, spread over the area of two bootprints. They guessed bearing pressures lower than that to support the design of the Surveyor probes and the Apollo LM. What to do about “snowshoes” for astronauts was unresolved.

Surveyor 3 landed and showed the weak dust surface problem did not exist, it was more like fine sand. (That’s a great testament to the value of ground truth, by the way. So, the Apollo LM was OK as-designed, and no astronaut “snowshoes” would be needed. The rest is history. The Apollo landings and every probe since have shown the “loose fine sand” assumption to be correct at widely-spaced sites all over the nearside of the moon, and now with China’s probe, the farside.

We already have ground truth for Mars from the various landers and rovers. For the majority of the surfaces experienced, the same “fine loose sand” assumption applies. There are some stronger exceptions, but they are a minority, and mostly only slightly stronger. That would be more like natural beds of medium/coarse sand mixed with gravel. Spaceship designers simply have to deal with that, if they intend to land on Mars.

GW

Last edited by GW Johnson (2019-02-07 10:02:59)

SpaceNut · 2019-02-07 18:04:15

Thanks for putting the LM into perspective for what was not known at the time....

GW Johnson · 2019-02-09 10:25:35

That 1 psi under the LM footpad converts to 0.0069 MPa or 0.072 US tons per sq.ft, if you want to compare to the usual reported units for Earthly soil bearing capacities.

In comparison, "fine loose sand" is usually at least rated at 1 US ton/sq.ft = 0.1 MPa. That larger "fine loose sand" value is a fair approximation to the bearing strength of the lunar surface, and most of Mars, too. That's what the ground truth (so far) says.

It took landers and rovers to determine that ground truth, both places. That's just not something one can evaluate from afar. And, inconvenient as it is, that's still just as true today, as it was in 1963 before Surveyor 3 landed on the moon.

GW

Last edited by GW Johnson (2019-02-09 10:27:08)

RGClark · 2019-02-09 16:46:10

SpaceNut wrote:

https://www.universetoday.com/141328/sp … igh-winds/
https://www.universetoday.com/wp-conten … .14-AM.jpg
4 to 8 weeks to make repairs if all goes well
~1750K is peak heating expected on about 20% of Starship for LEO entry, ~1600K on 20%. Rest drops below 1450K, so no heat shield needed. Radiative cooling at T^4 takes care of 60% of the ship.
A dragon hits higher temperatures for the same location for return to earth and it has a heatshield....

GW, in this post I discussed that at a wing loading of 10 psf, a spaceplane would require no thermal protection:

http://newmars.com/forums/viewtopic.php … 31#p142931

More generally what would the ballistic coefficient have to be for the max reentry temperature to be below say 1450K?

Bob Clark

louis · 2019-02-09 18:20:02

Another great update from Cloudlicker - v. detailed. I like it!

https://www.youtube.com/watch?v=JifMioInL8U

GW Johnson · 2019-02-10 10:59:39

Bob:

I'm not sure what the answer is to your question about a 10 psf wing loading for a spaceplane. Most spaceplanes will have to "fly" at AOA near or above stall for reentry deceleration purposes. Something in the 30-45 degree range, but not much higher than that, or else risk aerodynamic breakup. Broadside winds tend to rip off wings of structures actually light enough to fly.

That's lifting flight, but not lift = weight equilibrium flight. I'm not sure a winged vehicle has something you can really identify as "ballistic coefficient", but it wouldn't be much different from the wing loading, in any event. If wing loading is low, so also would be "ballistic coefficient" be low, by any sort of definition.

I going to have to think in terms of capsules with ballistic coefficients here, because the only tool I have is set up for that. That would be my crude spreadsheet implementation of the old mid-50's warhead entry ballistics. Given a shallow entry angle, low ballistic coefficient items hit peak heating and peak deceleration gees higher up, because the drag forces are larger in proportion to the mass. That lowers peak stagnation heating rates somewhat, but increases deceleration gees somewhat.

Decreased heating is not necessarily decreased material temperature, though. The rule of thumb for entry speeds 10 km/s or less is that plasma radiation heating to the surface is less important than simple convection heating to the surface. Above 10 km/s, the reverse is true, with radiation quickly dominant and exponentially so with speed.

The material receives lots of heating, and that heating must balance in some way. This is true whether you ablate or not.

The surface can cool by conduction inward, and by radiation to the environment, plus on a transient, it can increase the energy stored in its own heat capacity. Conduction inward must be dealt with, usually by storage in the interior structure's heat capacity (this works for the mere dozens of seconds of entry, but NOT for sustained hypersonic flight, by the way, because you so very quickly fill up the available heat capacity). Conduction inward varies linearly with surface temperature.

Radiation cooling is different. It varies as the 4th power of the surface temperature. The hotter you allow the surface to get, the more heat can be transferred to the environment by radiation. But there are obvious limits. This is substantially influenced by the thermal emissivity of the surface, which is why "black" surfaces are preferred. High absorptivity at IR wavelengths is high IR emissivity.

With ablation, some fraction of the heating to the surface is proportional to the material destruction rate, sort of like a latent heat of phase change (except this is a sort of latent heat of ablation). This is generally a significant effect, which is why ablatives are still being used more than half a century after the first spacecraft went up.

The other thing to think about is some sort of liquid cooling, whether open-cycle like transpiration cooling, or closed cycle like regenerative cooling, or something in-between. As the liquid performs its function, it absorbs heat and gets hotter. If it vaporizes, it is far less effective as a coolant, so you cannot allow that to occur, especially in regenerative designs. That means high liquid pressures to prevent boiling, or a high-boiling liquid (like a liquid metal) or both.

Point is, getting to a material surface temperature is a complicated balance that depends utterly upon a whole slew of design choices. All I can suggest is a rough upper bound (which is independent of wing loading). The material surface temperature cannot exceed the driving effective plasma temperature. Numerically, that temperature in degrees K equals flight speed in meters/second. The actual designs and characteristics result in near-equilibrium surface temperatures far below that upper bound.

All that being said, the last time I saw a bare-metal spaceplane proposed was the X-20 Dyna-Soar, which featured metal-reinforced (ablative) graphite for its nose cap and leading edges. That was a 1958-ish design that was cancelled in 1963, with the first 3 test examples nearing the end of the production line. It never flew. (Lateral heating is less than stagnation, that we know, except that the belly of a spaceplane is not all that far from stagnation, it being the heat shield face.)

GW

Last edited by GW Johnson (2019-02-10 11:42:54)

elderflower · 2019-02-11 08:39:46

On the subject of bearing loads on soils:
The presence of fluids between small particles on the earths surface has a lubricating effect on those particles, enabling them to move relative to their neighbours. This has the effect of greatly reducing the angle of repose and the bearing capacity, until sufficiently loaded to squeeze out the lubricant. The loading to do this is quite high as the surface films are often bound to the particles and are very difficult to displace compared to the bulk of the pore fluid. This does not apply to particles on the moon as they are subject to high vacuum which has removed all the volatiles (but see Arthur Clarkes "A Fall of Moondust" for a good story). I'm not sure how much effect this phenomenon would have on Mars.

SpaceNut · 2019-02-11 18:16:40

Ya having one sink in a softer muddy spot would be a bad thing for the BFR as it surely would topple over in that its so top heavy.

elderflower · 2019-02-12 03:40:51

I don't suppose there will be fuel enough for the rockets to take the weight of the vehicle whilst it sets piles, but with a hovercraft skirt arrangement maybe the load could be spread out enough to support it using rocket exhaust to generate a few psi under the skirt. Then perhaps a few vibro piles could go in and some kind of support structure but you would still have to be very quick. That might not be a good plan on an ice bound substrate as the heat would soak into the ground and turn it to mud.
Perhaps by deliberately toppling the first ship against some air bags and with thruster (big thrusters) control a belly down, stable position could be achieved. That would mean it wouldn't be returnable. It would have to carry the site prep equipment and the return fuel manufacturing system ready for the second ship.

Last edited by elderflower (2019-02-12 03:43:43)

GW Johnson · 2019-02-12 10:15:30

I've already run the numbers and posted them over at "exrocketman". The problem is less about touchdown and more about filling up for launch, at a weight ~ 5 times that for touchdown.

If you don't address this correctly, you get a "leaning tower of Pisa" effect that topples the ship, or at least sticks its landing legs into the ground like tent stakes. You cant't take off, and may be destroyed. This is really serious.

To land unrestricted by surface characteristics on Mars, you plan for "fine smooth sand". At Spacex's reported masses, that takes about 45 sq.m of pad area in contact with the surface, at takeoff weight (figured on Mars at Mars gravity). Period. It's just physics, math, and real data. You cannot hem-and-haw your way around that.

Three rounded pads on the fin tips, about a meter diameter each, are what Spacex shows. These total about 2.4 sq.m area. They are factor 20 short. It means they can only land on hard ground, something rare on Mars, as the rovers have shown.

You do not get a factor 20 increase in landing pad area at fixed geometry, in anything that would otherwise function aerodynamically at all, upon ascent from Earth (or Mars). That means you fold it to stow it, like aircraft landing gear. There is NO way around that in a reusable ship (remember it has to land on Earth, too).

I also put a concept for exactly how to do that on "exrocketman" as well. It would work on Mars, and it would work pretty much anywhere on Earth for an off-site emergency landing. What more could you possibly want from a design?

GW

elderflower · 2019-02-13 03:24:08

You will need large pads for Mars, but they can be left behind. You only need small ones for Earth landing on a prepared position or on a steel ship. For a water landing you don't need any pads.
Landing on Mars the rocket fuel is depleted. Once down you can extend the bearing surface before refilling the tanks, provided you have some structural material.
We need a civil engineering site survey before the BFSs go.

SpaceNut · 2019-02-13 18:38:29

Yes the large pad extended size could be left behind but they need to be in place before you land as we will not be able to put them on afterwards....

RobertDyck · 2019-02-14 00:35:42

GW Johnson wrote:

At Spacex's reported masses, that takes about 45 sq.m of pad area in contact with the surface, at takeoff weight (figured on Mars at Mars gravity). Period. It's just physics, math, and real data. You cannot hem-and-haw your way around that.
Three rounded pads on the fin tips, about a meter diameter each, are what Spacex shows. These total about 2.4 sq.m area.

Using your numbers, dividing pad area by 4 because it'll have 4 feet, that works out to 1.89234939 m radius. That's 3.78469878 m diameter. That's 12 feet, 5 inches. Kinda big, but doable. Are you sure? We are talking about dry sand.

GW Johnson · 2019-02-16 11:03:11

Robert:

I’m pretty sure.

I used Spacex’s own published figures for the mass of the BFS/”Starship” weight statement: 85 tons structure, 50 tons of return payload, and 1100 tons of propellants, for a total of 1235 (metric) tons at ignition for Mars departure.

On Earth, that would be 12.11 MN of weight, but on Mars it is only 4.619 MN of weight, again, at ignition. (These weights are ~ 5 times the weights at landing.)

According to my 1987-vintage copy of Marks’ Mechanical Engineer’s Handbook, the safe bearing capacity of “loose fine sand” is only 0.1 to 0.2 MPa = MN/sq.m. That sort of design data has not changed in decades, nor will it.

Prudence dictates using the lower number, until you have real field test data justifying using a higher figure. But even the best “fine loose sand” rarely exceeds the higher figure of 0.2 MPa. That’s why the published range is what it is.

Since for this scenario, bearing pressure is weight divided by the area that supports it, and because prudence demands we use the lower figure for capacity, the required landing pad area is 4.619 MN/0.1 MN/sq.m, or 46.2 sq.m. There’s just no way around that! If that’s too big to fly with fixed pads extended, then you fold them. No way around that, either, if you build something intended to be reusable.

The last-published Spacex design shows three round tip-mounted pads each about a meter diameter (they look to be half the dimension of the man in the Spacex illustrations with a human figure alongside for scale). That’s about 2.4 sq.m, which is about factor 20 (20 !!!) too small to launch from the bulk of Mars.

If you put the launch weight onto 2.4 sq.m pad area, the required safe bearing capacity is near 1.92 MN/sq.m = MPa. A minority of Mars resembles “loose medium-to-coarse sand”, which ranges in safe bearing capacity from 0.15 to 0.38 MPa, or “thick beds of coarse sand and gravel”, with safe bearing capacity of 0.38 to 0.48 MPa. None (none !!!) of these are even remotely (!!!) feasible for takeoff, even from a minority of Mars.

How reasonable are my equivalencies between these Earthly surfaces and what we see on Mars? Well, we lost Spirit stuck in “fine loose sand”, didn’t we? And the tracks made by Opportunity and Curiosity sure look like something no better than some sort of sand. The hardest surfaces have loose rocks as well, but these are just loose rocks laying about. They act more to damage metal tires, than to support them, as we learned with those idiotic aluminum tires on Curiosity. I’d say I have it about right.

The only feasible natural surface in my handbook’s list is a “solid ledge of hard rock”, at safe bearing capacity 2.4 to 9.6 MPa. Not even “medium rock, requiring blasting for removal” is feasible, at a safe bearing capacity of only 0.96 to 1.43 MPa. And just where on Mars do surfaces like this exist?

The only other feasible surfaces are reinforced concrete multiple feet thick, like a commercial runway, or well-supported thick steel decks (marine engineering usually uses 1 to 2 inch plate for ship hulls). So where on Mars do surfaces like that exist, prior to major human occupation and construction?

GW

tahanson43206 · 2019-02-16 11:57:17

For GW Johnson #173 above and ...
For RobertDyck #172 above

I'll start with what I believe is true, before asking your opinion.

I believe it is possible to fit special devices to the landing fins of a Mars bound vehicle while it coasts in LEO.

And, I'm pretty sure (without looking it up) that ablative shields have been used successfully to slow payload packages during descent to Mars surface.

So now, my question:

Could strong back ablative shields be fitted to the small landing pads of the proposed SpaceX vehicle, to assist with slowing during descent, AND, to serve as discardable landing platforms. Assuming the the shield/landing pads serve their purpose, they could be unsecured from the small landing pads just before launch from Mars.

The size and number of the shields would total to the 45 square meter area given by GW Johnson as needed.

A question that worries me about this suggestion is that the proposed shields might well protrude into the exhaust stream of the rockets. On the other hand, for a landing on Mars, not all of the rocket engines may be needed, similarly to the use of a small number or rocket engines of SpaceX Falcon 9 boosters when they land on barges or on land.

A related question is whether the proposed shields would protrude into the exhaust stream of rockets that would be fired to leave LEO, or to brake at Mars if that is needed.

Thanks for considering this inquiry!
(th)

Last edited by tahanson43206 (2019-02-16 11:59:04)

SpaceNut · 2019-02-16 13:02:41

The pure touch down equation is such that we have near zero acceleration second, 1/2 seconds before touching the surface but the reality is that there is still some to which that increase that touch down force which requires the larger pads as well on the feet making contact.
Landings for mars starts here on atmospheric entry where its needs a heat shield due to the friction of it against the ship.

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