-
#80
by
mmeijeri
on 25 Apr, 2009 16:20
-
Mass? Volume.
Current state of the art is, what, 5m PLF? That allows about 3t entry mass. IIRC that's pretty much what MSL will be, so the current system is maxed out.
So is that the real reason for the size of Ares V?
-
#81
by
Kaputnik
on 25 Apr, 2009 16:21
-
Sure there is 2 J246s per mars direct component
If 10m payload fairing on a j246 limits a landable mass to 12mT and Zubrins idea of a Hab was 25mT then the current state of the art may not be good enough.
Was 25mT landable? or total?
It was the landed payload. The EDLS is extra.
Interestingly, historical payload fractions for Mars payloads have been less than a third of entry mass! Ouch.
-
#82
by
Kaputnik
on 25 Apr, 2009 16:33
-
...
Using current technology limits, the entry vehicle cannot have a 'density' (mass/heatshield area) of more than about 150kg/m2, of else it hits the surface before it has slowed enough to deploy a parachute. So for something using a 10m PLF this would mean an entry mass of less than 12t.
Unless your heat shield is launched to LEO in 2 or more pieces. The it can be arbitarily large.
IIRC, Mars Drive were suggesting a hinged heat shield. 10m PLF would allow almost 20m diameter. 4 times the area, 48t? 12m PLF gives almost 70t
A segmented rigid heatshield is certainly one possibility. It would have to weighed up against an inflatable or a biconic system though. IMHO an inflatable system has the most promise. I could envisage a rigid folding system being quite heavy because you'd need to build the shield more heavily around the joint, with overlaps to ensure no plasma leakage. I also don't like the idea of having to choose between a) having the shield exposed all the way to Mars or b) not finding out there's a deployment problem until you get there.
-
#83
by
Spacenick
on 25 Apr, 2009 17:24
-
Who said we are going to land on arachutes anyway, it might easyly be lighter to drop the remaining ground speed and altitude with rockets. If the Russians really go the no parachutes route with PPTS here on earth that would really push this design idea.
Another possibility I could imagine is to use a rocket propelled rotor for landing.
-
#84
by
kevin-rf
on 25 Apr, 2009 19:38
-
Another possibility I could imagine is to use a rocket propelled rotor for landing.
In the thin air of mars? Pucker factor as you wait for the rotors to deploy aside.
-
#85
by
Kaputnik
on 25 Apr, 2009 22:15
-
Who said we are going to land on arachutes anyway, it might easyly be lighter to drop the remaining ground speed and altitude with rockets. If the Russians really go the no parachutes route with PPTS here on earth that would really push this design idea.
Another possibility I could imagine is to use a rocket propelled rotor for landing.
The 'remaining' speed is hypersonic- unless you have a gigantic heatshield which will be a far higher mass solution than a parachute of the same area.
You cannot fire up your landing rockets whilst travelling hypersonic. The interaction of the plume with the airstream is a bit of a mystery, apparently, and the real-life tests needed to determine just what happens would stack up to a massive research effort. This money would all be spent in the knowledge that propulsive descent will be less mass-efficient than parachutes, so in at least one aspect, inferior. Couple that with uncertainty over whether hypersonic retro-propulsion would even work, and it seems like a bit of a dead end to go down, IMHO.
-
#86
by
butters
on 28 Apr, 2009 09:34
-
I don't see why we want to put the bulk of the mission payload down on the surface at the bottom of the gravity well. Stage everything as a primary mission base in low mars orbit. Then send robots, cargo, and crew down to the surface in a reusable surface shuttle in pieces that are manageable given the physical limitations of the entry vehicle.
This is what worries me about mars mission planning. If we haven't figured out how to use LEO as an effective space base, then we're likely to get wildly off course in planning for mars exploration. And the Mars Direct proposals seems to actively badmouth LEO development as a distraction from the self-proclaimed objective of putting man on mars.
It's no easier to live in a life support vessel on the martian surface than it is to live in a life support vessel in mars orbit, but it's a lot harder to get back from the surface than it is to get back from orbit.
These days, space exploration usually only makes the evening news when something goes wrong. One of the few exceptions was the pair of mars rovers, which substantially exceeded expectations and whose stunning images touched the particular demographic that needs to be engaged in order for space exploration to succeed: those who are mindful of the need to fund open-ended scientific discovery but skeptical of the payoff from space exploration in particular.
The decisive scientific and political success of the mars rover program should be in the back our minds as we plan further mars missions. The vocal minority in favor of colonizing mars is blinded to a variety of other mission profiles that are more likely to generate positive results and popular support.
-
#87
by
Analyst
on 28 Apr, 2009 10:43
-
Very well said.
Analyst
-
#88
by
Jim
on 28 Apr, 2009 13:18
-
It's no easier to live in a life support vessel on the martian surface than it is to live in a life support vessel in mars orbit,
Incorrect. ISRU is feasible on the surface
-
#89
by
butters
on 28 Apr, 2009 14:53
-
It's no easier to live in a life support vessel on the martian surface than it is to live in a life support vessel in mars orbit,
Incorrect. ISRU is feasible on the surface
Perhaps, but locating the main base in orbit doesn't preclude landing a Sabatier reactor module on the surface and lifting the propellant back up to the orbiting base in several trips with a reusable surface shuttle.
We don't want to land a massive return vehicle. Even if the propellant is produced on the surface, we still don't want to send the return vehicle down the to surface to fuel up. We want to bring the propellant up to the vehicle stationed in orbit.
Also, conditions on the surface can be challenging and variable, so it would be nice for the surface crew to be able to retreat to the orbiting base in any number of contingencies without committing to earth return.
The fact that mars has an atmosphere is convenient for propellant production but decidedly inconvenient in most other ways, from aerodynamic loading to dust storms.
-
#90
by
Jim
on 28 Apr, 2009 14:56
-
The "reusable" shuttle is the hole in your plan.
-
#91
by
Kaputnik
on 28 Apr, 2009 22:55
-
I don't see why we want to put the bulk of the mission payload down on the surface at the bottom of the gravity well. Stage everything as a primary mission base in low mars orbit. Then send robots, cargo, and crew down to the surface in a reusable surface shuttle in pieces that are manageable given the physical limitations of the entry vehicle.
It depends which DRM you look at. A 'semi direct' plan leaves the MTV and TEI stage in Mars orbit and only sends the surface habitat, rover, and ascent vehicle to the surface.
It's no easier to live in a life support vessel on the martian surface than it is to live in a life support vessel in mars orbit, but it's a lot harder to get back from the surface than it is to get back from orbit.
Absolutely incorrect. On the surface, mars can provide you with gravity, materials fort radiation shielding, water, oxygen, propellant, even a medium for growing food in the long term. If you are in space, you must be 100% self-reliant.
The decisive scientific and political success of the mars rover program should be in the back our minds as we plan further mars missions. The vocal minority in favor of colonizing mars is blinded to a variety of other mission profiles that are more likely to generate positive results and popular support.
I think you have some preconceived notions at work here. Some people do advocate 'colonising' by building up surface resources at a single location from the first mission onwards. But most people, myself included, would opt for a series of initial missions visiting different locations.
One idea which I have looked into personally is a 'traverse' model where the crew would deliberately land thousands of km from their ascent vehicle, using mobile habitats to slowly wander across the surface over the course of eighteen months. There are some major technical hurdles with this idea though.
-
#92
by
Kaputnik
on 28 Apr, 2009 23:01
-
Perhaps, but locating the main base in orbit doesn't preclude landing a Sabatier reactor module on the surface and lifting the propellant back up to the orbiting base in several trips with a reusable surface shuttle.
Your 'reusable shuttle' would need a collosal development effort to make it a reality. We're talking a leap in technology akin to STS. You're also proposing a model which has yet to be demonstrated as viable here on Earth- RLVs supplying a propellant depot.
We don't want to land a massive return vehicle. Even if the propellant is produced on the surface, we still don't want to send the return vehicle down the to surface to fuel up. We want to bring the propellant up to the vehicle stationed in orbit.
This was the flaw in the original Mars Direct. Having the ERV ascend from the surface meant that it was unfeasibly small. More recent proposals leave the MTV in orbit as a return vehicle which allows the ascent vehicle to be a smaller and simpler affair.
Also, conditions on the surface can be challenging and variable, so it would be nice for the surface crew to be able to retreat to the orbiting base in any number of contingencies without committing to earth return.
The fact that mars has an atmosphere is convenient for propellant production but decidedly inconvenient in most other ways, from aerodynamic loading to dust storms.
If I understand you correctly, though, you are proposing an orbit-based model where the crew make multiple trips to and from the surface in a reusable vehicle. So they pass through atmospheric entry and landing several times, not just once. You are over-exposing them to the two most hazardous phases of a Mars mission.
-
#93
by
clongton
on 28 Apr, 2009 23:12
-
Is the top of Olympus Mons still in the atmosphere?
-
#94
by
butters
on 29 Apr, 2009 03:35
-
Yes, the surface shuttle is certainly the long pole in my proposed mission profile, and yes, we should absolutely gain experience with propellant depots in LEO before we attempt this in mars orbit.
That's my point: we have to substantially expand development of LEO as an orbiting spaceport and substantially refine the performance, economics, and capacity of mass transport between surface and orbit.
We have to get it right on earth before we can do it right on mars. COTS-C/D is a step in the right direction, but we have a ways to go. Low earth orbit is the gateway to interplanetary space travel.
We put men on the moon, and then we left. We squandered a whole lot of developmental momentum, and we haven't been back since. Wherever man sets foot in the universe, we should establish and maintain an orbiting spaceport. That way, we never regress, and we gain footholds from which to stage missions to more distant worlds.
-
#95
by
butters
on 29 Apr, 2009 04:18
-
The first mission to mars should be a preparatory unmanned mission that carries the initial module of the mars orbiting habitat and a surface shuttle loaded with a Sabatier reactor. After the habitat is orbited, the shuttle would descend, land, deploy the reactor, refuel, ascend, and re-dock with the habitat.
This would be the proof of concept mission to qualify the shuttle and reactor for a manned mission. Because there's no crew to support and nothing to return to earth, smaller or fewer launch vehicles would be required and/or a larger habitat can be delivered.
I think that demonstrating the production of propellant from the martian atmosphere would go a long way to impress the kinds of people we need to engage in order to protect and expand funding for space exploration, and this is a short path to that milestone.
-
#96
by
kkattula
on 29 Apr, 2009 10:51
-
Is the top of Olympus Mons still in the atmosphere?
No, even at 27 km, the atmospheric pressure at the top is at least 5% of the surface pressure. The lower gravity on Mars allows its atmosphere to extend much higher. Pretty thin but enough for serious heating at orbital velocity.
Anyway, if you were going to slow to sub-sonic velocity relative to the surface above the atmosphere, then land, would 27 km less make that much difference? Maybe 400 m/s delta v for a powered descent.
-
#97
by
clongton
on 29 Apr, 2009 11:13
-
Is the top of Olympus Mons still in the atmosphere?
No, even at 27 km, the atmospheric pressure at the top is at least 5% of the surface pressure. The lower gravity on Mars allows its atmosphere to extend much higher. Pretty thin but enough for serious heating at orbital velocity.
Anyway, if you were going to slow to sub-sonic velocity relative to the surface above the atmosphere, then land, would 27 km less make that much difference? Maybe 400 m/s delta v for a powered descent.
I'm thinking the summit of the mountain should be looked at as a possible landing place
-
#98
by
Kaputnik
on 29 Apr, 2009 11:38
-
Landing at higher altitude is a difficulty, not something that makes the process easier. The paper on EDLS explains this.
If you aim for the lowest points on Mars, e.g. Hellas Basin, you can land the largest payloads. This is because your aerodynamic decellerators have the best chance to work.
The only way landing on Olympus Mons could be 'beneficial' would be in an all-powered descent, which would be horribly costly in delta-v.
-
#99
by
Kaputnik
on 29 Apr, 2009 11:48
-
The first mission to mars should be a preparatory unmanned mission that carries the initial module of the mars orbiting habitat and a surface shuttle loaded with a Sabatier reactor. After the habitat is orbited, the shuttle would descend, land, deploy the reactor, refuel, ascend, and re-dock with the habitat.
This would be the proof of concept mission to qualify the shuttle and reactor for a manned mission. Because there's no crew to support and nothing to return to earth, smaller or fewer launch vehicles would be required and/or a larger habitat can be delivered.
I think that demonstrating the production of propellant from the martian atmosphere would go a long way to impress the kinds of people we need to engage in order to protect and expand funding for space exploration, and this is a short path to that milestone.
For a long term plan, perhaps your model makes sense. But I think that we accomplish exploration of Mars using more proven systems first. I want to see a Mars landing in my lifetime.
I do think, though, that your reasoning may be off. Mars orbit is not really a safer place than the surface. You aren't really any closer to getting home since you still have to wait for a launch window.
Let's look at worst case scenarios- some major malfuction to a propulsion system strands you at Mars.
If you're on the surface, you can continue to grow food, generate oxygen, mine water ice, even manufacture new propellant in case you suffered a leak and lost the first batch. You might have to sit tight till the next launch window but you'll basically be alright.
If you're in orbit, and a failure loses you the launch window, then you need either a 100% closed ECLSS or a huge margin of reserve consumables to tide you over while you wait on a rescue mission at the next window.
I know where I'd rather be.