Author Topic: MCT Speculation and Discussion Thread 4  (Read 605495 times)

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1420 on: 01/23/2016 08:03 PM »
So just go into very low Orbit and deorbit. That'd take less than ~5km/s delta-v.

btw, are you relying on Earth figures, or are you recalculating them for Mars? The lower gravity makes a huge difference here.

...BTW, this is all off-topic. I just brought up the idea to counter the false (but oft-repeated) claim that abort would be useless for MCT because there'd be no way to get to them.
I was using 4.1km/s for surface to LMO

yes I was calculating for Mars for the point to point distances
care to show your work?

rm (radius of Mars) = 3,400,000m

Mars orbital velocity at height h = sqrt(3.75m/s * rm2/(rm + h))

or 3570m/s at the surface of Mars

ΔΦ is defined as the "range angle" of the difference around the circumference of mars which, if we presume Mars is a perfect sphere and that launch and landing points are the same distance from the centre of Mars, gives us a range of 3,400 km per radian or 60km per degree.

YL is velocity at launch in multiples of orbital velocity at the surface of Mars

θL is the launch angle which the further we go is closer and closer to perpendicular to the ground for maximum range at launch

So the optimal launch angle (presuming vacuum of course) is

θL = (π - ΔΦ)/4

and the optimal launch velocity is

YL = SQRT(2*sin(ΔΦ/2)/(1+sin(ΔΦ/2)))

So plugging that into a table in excel gives me

Launch V (in m/s) for a given range

741m/s for 150km at 44.37 degrees
1590m/s for 750km at 41.84 degrees
2596m/s for 2500km at 34.47 degrees

I rounded up to 1km/s for the first one for gravity losses both on launch and landing since this isn't a canon shell so the first one is a boost of 1km/s at 1g  take off and landing it could be reduced to 875m/s at 2g etc.

you can see what I did for the rest.

Notes anyone who wants to can read far more about the math for less ideal cases at https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM3752.pdf
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1421 on: 01/23/2016 08:04 PM »
The only reason the crew would be back on the surface is if the vehicle had a really bad day and the crew had to bail out, perhaps in a pod or ejection seats. Probably a pod, since it'd allow survival from Near Mars orbital velocity.

Then parachute down, with solid retrorockets to keep parachute size reasonable. The whole abort system could be relatively low mass. 2kg for the chute, 5-10kg for the rockets per passenger, plus whatever the suit would weigh and the pod to protect against the relatively modest Mars reentry (much more modest than. Typical Mars probe which comes in hyperbolically).
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1422 on: 01/23/2016 08:12 PM »
It stands to reason that if a hopper over 700km exceeds orbital delta-V, then an aborted crew would not be more than ~700km away.

HUH? where did you get that from? they could be anywhere from 0 to 10,000km away (any more than 10,000km away in one direction and they are closer on a different geodesic route) presuming that they had 7.5km/s of ΔV at launch they could have landed anywhere and survived.

That close, and a land-train of pressurized rovers would likely be able to meet them within a couple days of travel (or even 8-10 hours with a slightly prepared path, within EVA+contingency time). If the aborted crew aborted along with a rebreather system, they should be able to hang out in their spacesuits that long.

so you think that a path could be prepared that at .38g a rover train could motor along at 100km/h without rolling over?  I think over a prepared path a wheeled vehicle maximum speed will be on the order of 50km/h unless massively banked curves are put in, unprepared I wouldn't count on more than 15km/h average but obviously stretches of much faster could probably be practical.


There would likely be only one orbital track for return from Earth, so you could possibly prepare a route before hand.

With no depot that is not at all correct, if there were a depot then the tracks would have to match the depot and would be fairly close.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1423 on: 01/23/2016 08:16 PM »
BFS should be able to get to orbit in a jiffy. They would only abort if they couldn't make it to orbit (even with secondary thrusters). It doesn't take 10,000km to get to orbit in a BFS with high thrust.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1424 on: 01/23/2016 08:17 PM »
Impaler for the shorter distances aero lift and braking will not make any significant difference and have much less impact than gravity losses. Once you go above 2km/s in launch speed then it is possible that you can either use one of or  both lift and drag to somewhat reduce the amount landing ΔV required. But you will still need a significant proportion of your launch ΔV to land even at the antipode.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1425 on: 01/23/2016 08:18 PM »
No evidence of a depot in LMO. Vastly more likely to launch directly to Earth or possibly visit a depot in high orbit like MSL-2.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1426 on: 01/23/2016 08:23 PM »
No evidence of a depot in LMO. Vastly more likely to launch directly to Earth or possibly visit a depot in high orbit like MSL-2.
Obviously there is no evidence of a depot in orbit around Mars yet, but there are no people, hoppers or vehicles returning to Earth. There is evidence that depots in LMO are a practical solution to a number of issues of the logistics of settling and exploring Mars. If you read the various threads on this forum there is as much evidence of depots as of other systems discussed here.

BFS should be able to get to orbit in a jiffy. They would only abort if they couldn't make it to orbit (even with secondary thrusters). It doesn't take 10,000km to get to orbit in a BFS with high thrust.

HUH again, if we are talking about a failure that causes an abort the failure could take place anywhere between launch and when the final orbit is achieved. So if that failure takes place at 95% of orbital velocity just 100km down range from the launch site, momentum would take them halfway around Mars.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1427 on: 01/23/2016 08:24 PM »
Yes, I think an average speed of over 70km/hr is feasible for a properly designed rover (huge wheels, low center of gravity) with a minimally prepared path I required. A much more leisurely rate (20-30km/hr) for an unprepared path. Eugene Cernan was able to tool around at nearly 20km/hr in half as much gravity and a rover designed only for 13km/hr.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1428 on: 01/23/2016 08:27 PM »
No evidence of a depot in LMO. Vastly more likely to launch directly to Earth or possibly visit a depot in high orbit like MSL-2.
Obviously there is no evidence of a depot in orbit around Mars yet, but there are no people, hoppers or vehicles returning to Earth. There is evidence that depots in LMO are a practical solution to a number of issues of the logistics of settling and exploring Mars. If you read the various threads on this forum there is as much evidence of depots as of other systems discussed here.

BFS should be able to get to orbit in a jiffy. They would only abort if they couldn't make it to orbit (even with secondary thrusters). It doesn't take 10,000km to get to orbit in a BFS with high thrust.

HUH again, if we are talking about a failure that causes an abort the failure could take place anywhere between launch and when the final orbit is achieved. So if that failure takes place at 95% of orbital velocity just 100km down range from the launch site, momentum would take them halfway around Mars.
If you're that close, thrusters could get you to orbit. You just said that more than 700km would take more than orbital velocity, a claim I still have seen no calculations for that include lift.
« Last Edit: 01/23/2016 08:30 PM by Robotbeat »
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1429 on: 01/23/2016 08:31 PM »
If you're that close, thrusters could get you to orbit. You just said that more than 700km would take more than orbital velocity, a claim I still have seen no calculations for.

It is above! You can also do the math yourself see the link I quoted above along with my calculations:

https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM3752.pdf

I said that it would take the same ΔV as going to orbit to launch and land 700km down range. You only need half orbital velocity to launch and crash 700km down range.

« Last Edit: 01/23/2016 08:32 PM by nadreck »
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1430 on: 01/23/2016 08:32 PM »
The only reason the crew would be back on the surface is if the vehicle had a really bad day and the crew had to bail out, perhaps in a pod or ejection seats. Probably a pod, since it'd allow survival from Near Mars orbital velocity.

Then parachute down, with solid retrorockets to keep parachute size reasonable. The whole abort system could be relatively low mass. 2kg for the chute, 5-10kg for the rockets per passenger, plus whatever the suit would weigh and the pod to protect against the relatively modest Mars reentry (much more modest than. Typical Mars probe which comes in hyperbolically).

Not sure if you're arguing for or against a rescue hopper here. Are you assuming the crew would have enough supplies and be close enough to base that only a rover would be needed.?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1431 on: 01/23/2016 08:36 PM »
I'm arguing that a rescue hopper is probably not required for 95% of abort scenarios.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1432 on: 01/23/2016 08:37 PM »
Ok Robobeat you modified your post to say calculations with lift, that was not there when I replied. Lift is pretty irrelevant at lower speeds especially since to get efficient ballistic distances from your launch velocity you have to launch at an angle greater than 40 degrees (which means as you approach the surface (and lift can only play a role in the last 10 km or so) you are coming in at the same angle.  So lift is not going to increase your range significantly.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1433 on: 01/23/2016 08:38 PM »
If you're that close, thrusters could get you to orbit. You just said that more than 700km would take more than orbital velocity, a claim I still have seen no calculations for.

It is above! You can also do the math yourself see the link I quoted above along with my calculations:

https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM3752.pdf

I said that it would take the same ΔV as going to orbit to launch and land 700km down range. You only need half orbital velocity to launch and crash 700km down range.
What delta-V are you assuming is needed for landing?
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1434 on: 01/23/2016 08:39 PM »
If you're that close, thrusters could get you to orbit. You just said that more than 700km would take more than orbital velocity, a claim I still have seen no calculations for.

It is above! You can also do the math yourself see the link I quoted above along with my calculations:

https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM3752.pdf

I said that it would take the same ΔV as going to orbit to launch and land 700km down range. You only need half orbital velocity to launch and crash 700km down range.
What delta-V are you assuming is needed for landing?
Equal to take off, particularly on hops that are less than 2km/s launch velocity since they are also really high angle
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1435 on: 01/23/2016 08:39 PM »
Ok Robobeat you modified your post to say calculations with lift, that was not there when I replied. Lift is pretty irrelevant at lower speeds especially since to get efficient ballistic distances from your launch velocity you have to launch at an angle greater than 40 degrees (which means as you approach the surface (and lift can only play a role in the last 10 km or so) you are coming in at the same angle.  So lift is not going to increase your range significantly.
Boost-skip is one strategy to significantly increase point to point range. Lift is a big component of that.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1436 on: 01/23/2016 08:41 PM »
If you're that close, thrusters could get you to orbit. You just said that more than 700km would take more than orbital velocity, a claim I still have seen no calculations for.

It is above! You can also do the math yourself see the link I quoted above along with my calculations:

https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM3752.pdf

I said that it would take the same ΔV as going to orbit to launch and land 700km down range. You only need half orbital velocity to launch and crash 700km down range.
What delta-V are you assuming is needed for landing?
Equal to take off, particularly on hops that are less than 2km/s launch velocity since they are also really high angle
Poor assumption. Lift changes the optimum angle significantly, can allow you to reach near terminal velocity.

Also, gravity losses would be quite small since Raptors will give BFS much, MUCH greater than T/W=1 on Mars.
« Last Edit: 01/23/2016 08:46 PM by Robotbeat »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1437 on: 01/23/2016 08:44 PM »
I wrote a fairly simple simulator which allows you to take lift into consideration. It's currently setup for seeing max gee-loads during aerocapture, but can also be used for this sort of calculation:

https://repl.it/BXvp/15

(feel free to make changes and save. It will save it in a new spot by incrementing the number)
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1438 on: 01/23/2016 08:50 PM »
lift for skip is not significant at 2km/s it will not extend the range, if you launch at a 20 degree angle instead of 40 you don't go nearly as far ballistically so the net effect is to lower your range.  If you think angling a conic or biconic shape will allow you to fine tune your landing within a 30 or 40km radius at 700km ballistic range, I will say that is possible, if you say you will 'glide' a biconic shape 100km or more at 2km/s from where-ever you consider that enough lift starts above the Martian surface that is way off, even a winged vehicle would not get much further than that.
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Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1439 on: 01/23/2016 08:55 PM »

Nadreck, an abort scenario has to be realistic. If the BFS is able to be used as its own abort vehicle, it needs to be able to land. (If they crash, they die.) That means at least some engines are functional. So anything over half orbital velocity and your mission profile would be abort-to-orbit, then land at the base in the normal way once your orbit aligns with the base again.

During the first couple of minutes, you're close enough and slow enough to RTLS. After 5-6 minutes you default to abort-to-orbit. So there's a narrow window where you can't make orbit, but can land, but are too far from the base to RTLS. That's not going to be 10,000km.

Are you assuming the crew would have enough supplies

By definition, somewhere between 3 and 9 months worth.
« Last Edit: 01/23/2016 08:56 PM by Paul451 »

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