Author Topic: MCT Speculation and Discussion Thread 4  (Read 504006 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1440 on: 01/23/2016 08:58 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.

I don't see why landing delta-V would be so high, if we can make a landing craft for Mars it's going to be capable of EDL landing on Mars at a minimum entry velocity of around 4 km/s (orbital velocity) with minimal propellant consumption, I'd say the landing Delta-V needs to be <1 km/s to be practical.

Now admittedly your entry angle is steeper on these re-entries but the velocities are considerably less so it seems reasonable that the vehicle can use lift to transition to a shallower entry and from then on the EDL would be just like any other landing.  So I see no more then 1 km/s for landing from any distance.

Likewise I can't see much gravity loss in this kind of hop, the atmosphere or Mars is so thin the vehicle should be able to start turning to the desired angle almost immediately upon launch and at 2g's acceleration you reach 2.5 km/s in only 2 minutes leaving virtually no time for gravity losses.

So for a 2.5 km/s launch and 1 km/s landing it looks to me that the 2500km range should be achievable with at 7 km/s vehicle.  And put half the planet within reach of one launch site.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1441 on: 01/23/2016 09:08 PM »
With a light payload, decent lift and ballistic coefficient, You shouldn't need more than 1km/s to land from orbit. It takes 4.5km/s or less to get to low orbit. 7km/s would be enough delta-V to land anywhere on the planet, maybe except Olympus Mons.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1442 on: 01/23/2016 09:26 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. 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.

(If they crash, they die.)



By the same logic, if they could land at all, at any point, then they could also go on to orbit. But if you are going to pick outlandish scenarios that somehow an abort makes sense I don't see why you restrict it to a certain distance from the base. From a mathematical standpoint you might be anywhere along a geodesic based on the launch direction.

Look, I started here by trying to point out the ΔV requirements of a "hopper" because a variety of people had suggested it for a variety of purposes. That was my point, what are the real numbers around the performance of a ballistic point to point vehicle on Mars and the fact is once you are travelling more than about 700km, unless you are going to another base with propellant supplies, you could design a more efficient vehicle that orbits refuels lands anywhere on the surface (from a polar or high inclination orbit for true all planet access) and then takes off again to orbit.

At orbital speeds there is some benefit from aero drag/lift reducing ΔV requirements, but at low speeds (range under 700km) there is little benefit, gravity loss on take off and landing will be higher.

With a light payload, decent lift and ballistic coefficient, You shouldn't need more than 1km/s to land from orbit. It takes 4.5km/s or less to get to low orbit. 7km/s would be enough delta-V to land anywhere on the planet, maybe except Olympus Mons.

4.1Km/s at the most efficient to orbit the way I see it and I agree with 6 to 7.5km/s to get point to point to anywhere on Mars. You can review my posts above and I had stated that. 

I disagree about 1km/s to land from orbit I say 2 or more particularly if it is partly to fully fueled at atmospheric entry, and more importantly if I am only travelling I need much more than from orbit because to get anywhere near 700km balistically for less than 2km/s launch velocity I have to launch at a high angle which means I only have a very short time to have aerodynamic forces slow me SO I need more retropulsion proportionally than the low angle of an orbital entry.

Impaler see my responses above - but tl;dr  lift won't work nearly as much as you are imagining (except for winged craft); ΔV for EDL is more like 2km/s than 1km/s; gravity loss at 2.5g would be under 10% on launch; 1g would be a little over 20% presuming you translate immediately from vertical to the most efficient angle for the distance (see the formulae I posted above); landing at full thrust(hover slam) would always be less gravity losses than take off but if you throttle down for precision/translation or any other reason you increase the gravity losses.
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!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1443 on: 01/23/2016 09:31 PM »
  According to my simulations even 500 m/s is sufficient for landing at low altitude   
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1444 on: 01/23/2016 09:36 PM »
  According to my simulations even 500 m/s is sufficient for landing at low altitude

can you give me your assumptions for this: altitude where you start calculating drag and lift, angle of flight at that altitude, velocity at that altitude, cross section area, mass
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 #1445 on: 01/23/2016 09:39 PM »
Are you assuming the crew would have enough supplies

By definition, somewhere between 3 and 9 months worth.

Pretty obvious we were discussing a bail out scenario so I wouldn't assume these supplies would be available, but paint the scenario any way you want. That's what everybody else is doing.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1446 on: 01/23/2016 09:54 PM »
  According to my simulations even 500 m/s is sufficient for landing at low altitude

can you give me your assumptions for this: altitude where you start calculating drag and lift, angle of flight at that altitude, velocity at that altitude, cross section area, mass
And can you give me the results of your sim calculation with the same threshold altitude, an angle of 40 degrees, the shape was a cone with a rounded base with the height and diameter of the cone being 10 meters, and the mass was 60,000kg?

EDIT - oops and the velocity is 1.6km/s
« Last Edit: 01/23/2016 09:55 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!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1447 on: 01/23/2016 10:18 PM »
No threshold, I use an exponential atmosphere model. I posted a link above. It contains all assumptions.
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 nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1448 on: 01/23/2016 10:23 PM »
No threshold, I use an exponential atmosphere model. I posted a link above. It contains all assumptions.

I have no experience in python, have you got it in Pascal?

Seriously I am not familiar with your software, you asked me to show you my work, you might be willing to come up with the numbers for your assumptions for me.
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 Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1449 on: 01/24/2016 06:55 AM »
I should point out that their are MANY possible reason why we might wish to abort to surface other then loss of engines, anything which endangers the long term life-support capability, or compromises the heat-shield which would make Earth entry in that vehicle dangerous.   So I can't agree that any abort to surface is necessarily coming down with no propulsion and must therefore be a bailout.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1450 on: 01/24/2016 02:40 PM »
I should point out that their are MANY possible reason why we might wish to abort to surface other then loss of engines, anything which endangers the long term life-support capability, or compromises the heat-shield which would make Earth entry in that vehicle dangerous.   So I can't agree that any abort to surface is necessarily coming down with no propulsion and must therefore be a bailout.
Abort to orbit in most of those situations would be much safer since you're not risking landing in a compromised vehicle. The only reason you'd want to be on the surface rather than in orbit is if you literally can't get to orbit.
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 BSenna

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Re: MCT Speculation and Discussion Thread 4
« Reply #1451 on: 01/24/2016 05:14 PM »
Hi everyone, I just made a design concept, I'm a product designer, not engineer.

Newest version: http://imgur.com/a/15fO2

Initial concepts and idea development: http://imgur.com/a/EtH8F

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1452 on: 01/24/2016 05:58 PM »
I should point out that their are MANY possible reason why we might wish to abort to surface other then loss of engines, anything which endangers the long term life-support capability, or compromises the heat-shield which would make Earth entry in that vehicle dangerous.   So I can't agree that any abort to surface is necessarily coming down with no propulsion and must therefore be a bailout.
Abort to orbit in most of those situations would be much safer since you're not risking landing in a compromised vehicle. The only reason you'd want to be on the surface rather than in orbit is if you literally can't get to orbit.

It depends on how the vehicle is compromised, if were in doubt of the vehicles thermal protection then aborting the launch immediately and landing propulsivly puts the vehicle through the least thermal load, less even then reaching orbit and then landing again.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1453 on: 01/24/2016 07:14 PM »
I should point out that their are MANY possible reason why we might wish to abort to surface other then loss of engines, anything which endangers the long term life-support capability, or compromises the heat-shield which would make Earth entry in that vehicle dangerous.   So I can't agree that any abort to surface is necessarily coming down with no propulsion and must therefore be a bailout.
Abort to orbit in most of those situations would be much safer since you're not risking landing in a compromised vehicle. The only reason you'd want to be on the surface rather than in orbit is if you literally can't get to orbit.

It depends on how the vehicle is compromised, if were in doubt of the vehicles thermal protection then aborting the launch immediately and landing propulsivly puts the vehicle through the least thermal load, less even then reaching orbit and then landing again.
No. Aborting to orbit puts the vehicle through the least thermal load. They would be rescued by another vehicle. That was the plan for Shuttle post-Columbia, and it'd likely be the case for BFS at Mars.
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 JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1454 on: 01/24/2016 07:46 PM »
Hi everyone, I just made a design concept, I'm a product designer, not engineer.

Newest version: http://imgur.com/a/15fO2

Initial concepts and idea development: http://imgur.com/a/EtH8F

Nice pictures! Gives some idea of the sizes of craft that may be necessary!

Offline meetsitaram

Re: MCT Speculation and Discussion Thread 4
« Reply #1455 on: 01/24/2016 07:59 PM »
Why are all the rocket tall and cylindrical? What are the disadvantages of launching disc like space ships?
Is it possible to build a single stage spaceship that is capable of launching from earth and landing on mars?


Offline BSenna

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Re: MCT Speculation and Discussion Thread 4
« Reply #1456 on: 01/24/2016 08:16 PM »
Higher costs for designing and building, basically.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1457 on: 01/24/2016 08:50 PM »
Why are all the rocket tall and cylindrical? What are the disadvantages of launching disc like space ships?
Is it possible to build a single stage spaceship that is capable of launching from earth and landing on mars?


Basic rocketry 101 doesn't really belong here, but respectively:

*Because it's the best way to defeat air resistance and the easiest to build.

*Drastically increased drag losses, massive logistical difficulty processing large flat sheets, increased weight from stiffening up large flat sheets (depending on construction techniques).

*Not really.  It's barely possible to build one to get to a 9km/s orbit, and although the math suggests SSTO is practical on an expendable rocket if you're satisfied with a low payload mass fraction, a reusable rocket stage which can survive reentry would be much heavier.  It costs more like 13km/s to get to Mars (*before* entry, descent, and landing is taken into account), which at 450s Isp is the difference between an 8:1 mass ratio and a 20:1 mass ratio.  There is a large segment of space enthusiasts that fixate on the dream they were sold in the 90's (and to some extent earlier) of reusable SSTO spaceplanes or VTVL craft, but now that we have reusable first stages, it basically always makes more sense to mount any such craft as an upper stage that only needs ~6km/s to reach orbit (a mass ratio at 450s Isp of 4:1).
« Last Edit: 01/24/2016 10:05 PM by Burninate »

Offline meetsitaram

Re: MCT Speculation and Discussion Thread 4
« Reply #1458 on: 01/24/2016 11:21 PM »
I appreciate that Burninate. I will follow up in the 'Basic Rocket Science Q&A' thread.

Offline Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #1459 on: 01/25/2016 12:17 PM »
Hi everyone, I just made a design concept, I'm a product designer, not engineer.

Newest version: http://imgur.com/a/15fO2

Initial concepts and idea development: http://imgur.com/a/EtH8F
I appreciate your excellent work to create superb renderings.

A rocket engineer would require some changes to the model based on his/her calculations and experience. You should be aware that a team of persons in the L2 section of this forum are also creating renderings based on expert opinion and hints from SpaceX. It would be worth your joining L2 just to review the large amount of information and the history behind their work.

I have also proposed some concepts on this forum that have received good reviews and critiques, but I have mostly relied on sketches. I would love to provide some ideas to you for your review and your rendering machine.
« Last Edit: 01/25/2016 02:40 PM by Ionmars »
* Mars' orbit: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

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