Quote from: rklaehn on 09/02/2009 12:30 pmhttp://www.archimedes-ballon.de/index.php?id=ENThanks for the link. Basically this is what I meant - some kind of combination of aero-braking and use of buoyancy. The project website seems to be a bit dated (they speak of a launch this year...) but it quite interesting.
http://www.archimedes-ballon.de/index.php?id=EN
But then you still need HLVs with 10m fairings ... unless you go for some sort of lifting body design, which may be a bit tricky ... since you would enter the atmosphere "head first", slow down, deploy your chute, turn around, fire your engines. With blunt decelerators the vehicle always faces in the "correct" direction for reentry, parachute deployment as well as rocket motor deceleration.
Quote from: simon-th on 09/02/2009 07:49 amWhatever works is good enough. We don't need to go "exotic" just for the sake of doing it. If conventional, proven technology works, then we should use it. However, absent technology development of a folding technique (or inflatable heat shield add-ons) for Viking-shaped heatshields and parachute technique, we ain't going to see >30mt to the Martian surface.There's still powered descent or a hybrid approach.
Whatever works is good enough. We don't need to go "exotic" just for the sake of doing it. If conventional, proven technology works, then we should use it. However, absent technology development of a folding technique (or inflatable heat shield add-ons) for Viking-shaped heatshields and parachute technique, we ain't going to see >30mt to the Martian surface.
The need for 10m (and bigger) fairings is also driven by the lander stage and the habitat module dimensions, among others.
And to be honest, HLVs with 10m fairings sound like a pretty standard configuration to me.
Quote from: Xentry on 09/02/2009 01:11 pmThe need for 10m (and bigger) fairings is also driven by the lander stage and the habitat module dimensions, among others.Horizontal landers don't need large fairings. Habitats don't strictly need them either, look at the ISS. Inflatable habs sound more practical though.
Quote And to be honest, HLVs with 10m fairings sound like a pretty standard configuration to me.Could EELV Phase 1 have 10m fairings? Current EELVs could support 6.5m, but I vaguely remember Crawley saying something about even Atlas Phase 3 having smaller fairings than an SDLV.
Well, then you'd have to build a new type of lander which you haven't done before. Hey I'm all for it, in fact I think it's the most practical solution, but it would sure add to the development costs. As a lifting body would.
My view is that you'll either have HLV or you can forget about Mars.
Also I think that despite the problems NASA is facing right now there will be an HLV. I respect your point of view, but I think that it greatly complicates an already technically difficult mission to make (meaning, you'd need to add technology development costs that you wouldn't need in case you had an HLV, to the point where avoiding to build an HLV would be unreasonable).
In any case the Atlas Phase 3 would apparently have a cryogenic upper stage 7m in diameter, on top of which one could eventually imagine a ~10m fairing could be placed.
Well, I certainly respect your point of view too, but I don't see how not having HLV complicates things very much. Propulsive braking might be expensive (much less so if rklaehn is right it's the last 2 km/s that is the problem), but not complicated.
You don't need 50m diameter heatshields (I am not even talking about the structural problems you get into by using a 50m heat-shield...). 15-25m does perfectly well. And even for the unreasonably-sized 50m inflatable (or foldable) heath-shield, you don't require a 10m PLF. A 20m heat-shield folded up fits perfectly well into a 6m PLF.
So the conclusion is that you want your vehicle to have a large T/W and to do the propulsive braking as late as possible.
Quote from: Xentry on 09/02/2009 01:39 pmWell, then you'd have to build a new type of lander which you haven't done before. Hey I'm all for it, in fact I think it's the most practical solution, but it would sure add to the development costs. As a lifting body would.I wonder if that's true. Such a lander could be single stage, which would be a simplification. And initially it wouldn't have to do true landings, assuming the Deep Space option is chosen. What would make a horizontal lander difficult to design?
Quote from: Xentry on 09/02/2009 01:39 pmMy view is that you'll either have HLV or you can forget about Mars.Why is that?
Well, I certainly respect your point of view too, but I don't see how not having HLV complicates things very much. Propulsive braking might be expensive (much less so if rklaehn is right it's the last 2 km/s that is the problem), but not complicated. Hypergolic in-flight refueling is a proven technology and it's good enough for the foreseeable future.
To be honest I'm not at all enthusiastic about anything bigger than EELV Phase 1, since it is hard to imagine how that could lead to a high flight-rate.
Propulsive braking is not a problem, it just has to be accounted for. You also don't need to brake all the way from 2km/s (about Mach 6), just from Mach 0.8 according to Braun.
I don't understand how a delta-v of 2km/s can lead to a mass fraction of 0.2. That's about what I'd expect with the vertical descent option using hypergolics.
Not to speak about assembling this massive vehicle in space and launching it towards Mars and then braking into on orbit.
I'd rather put my money on a HLV than that scheme.
Quote from: Xentry on 09/02/2009 02:23 pmPropulsive braking is not a problem, it just has to be accounted for. You also don't need to brake all the way from 2km/s (about Mach 6), just from Mach 0.8 according to Braun.In the table 6 "SRP Performance for 5 t Robotic Cases." in the paper linked by mmeijeri, the initiation M∞ is between 2.96 and 1.82. The M∞ is the mach number at initiation of the supersonic retro propulsion. Even in the case with the highest T/W, the initiation mach number is well above one.Just for those who do not want to read the paper in detail: the proposed configuration uses a reentry shape similar to the mars science laboratory (MSL) with a diameter of 4.5m and very low-performance (but reliable) hydrazine monopropellant propulsion. It manages to land 5t on mars using between 1.0t and 1.5t of propellant depending on T/W.
Ok, but is an initiation Mach number over 1 really surprising for a paper which title is "Performance Characterization of Supersonic Retropropulsion for Application to High-Mass Mars EDL"?
Quote from: Xentry on 09/06/2009 01:19 pmOk, but is an initiation Mach number over 1 really surprising for a paper which title is "Performance Characterization of Supersonic Retropropulsion for Application to High-Mass Mars EDL"?No, of course not. I know that the standard approach is to use various aerodynamic devices such as supersonic parachutes to decelerate to subsonic speeds. I just think that the supersonic retropropulsion option is very attractive. You need rocket propulsion in any case for a soft landing. So why not use it earlier and accept a bit lower mass fraction. You can use much higher ballistic coefficients, and you do not need something like 30m diameter supersonic parachutes.