Red Dragon Discussion Thread (1)

[Nathan]

Whilst Red Dragon is presumably a ballistic capsuile, it could have a parafoil in place of its standard parachutes.  This would allow a degree of guidence during most of the descent phase.  Any idea of how big a foil a fully loaded Dragon RV would need for Mars's atmosphere?

All I have seen on Red Dragon is a "no parachute" EDL. Current chutes on Dragon would be ineffective in Mars atmosphere and no redesign is planned for different chutes (wouldn't fit anyway). The concept is a large skip maneuver to bleed off most energy, then a terminal descent and firing of abort thrusters in final phase. Sporty, but could be cheap science mission...

Skip entry sounds right.

[Robotbeat]

BTW, while scooting around the Internet, I noticed that Mars landing delta-v for a small payload descending at Mach 3 (i.e. without chutes) is about 1250 m/s or so. Not surprising, but there was a figure...

BTW, welcome to the forum, RocketJack! Your input is appreciated!!!

[Kaputnik]

BTW, while scooting around the Internet, I noticed that Mars landing delta-v for a small payload descending at Mach 3 (i.e. without chutes) is about 1250 m/s or so. Not surprising, but there was a figure...

Is that 'Mach' as a shorthand for 340m/s, or does it mean a true value on Mars, and at what altitude?

1250m/s is a pretty big number, when you are trying to achieve it using pressure-fed engines firing at an oblique angle.

If SpaceX would just stick a 6.5m heatshield on the proposed lander, many problems would go away.

[AstronauticalJoe]

An interesting discussion going on here 

Thought I'd add some input, I'm only 14 so there could be a slip-up.

I used the formulas from Werhner von Braun's 'Project Mars'  and simulated an EDL for a 8400kg Dragon, assuming the standard 4m aeroshell and decending from LMO (apoapsis of 105km, periapsis lowered to 65km for entry).

I used this: http://www.grc.nasa.gov/WWW/k-12/airplane/atmosi.html
for atmospheric density info.

Note that vertical speeds were estimated from the entry angle and the spacecraft's mass so won't be completely accurate, but can used as a rough guide for discussion.

The results are attatched below. The blue line is the results from the sim, the grey line is the predicted results based on the formula of the curve so far.

Assuming the LAS Draco motors can slow the Dragon down by 500 m/s (optimistic for an off-center firing), the Dragon would reach that point about 5km below the martian 'sea level'. I therefore think that the only place such a vehicle could safely land would be the hellas impact basin, about 6.5km below 'sea level', would give the dracos 1.5km of space to land the dragon.

-Joe

[MP99]

It's not really that different, unless you have only a single engine pointing directly upstream. I'm not going to trivialize what needs to be done, but you seem to have only a very superficial understanding of hypersonic retropropulsion. Dragon's landing thrusters would be on the side, like crewed Dragon, and thus wouldn't be directly in the airstream... And what simulation/experiment that has been done on supersonic retropropulsion (it's not likely to be hypersonic for an unmanned spacecraft) has shown that thrusters to the side like that work quite well, keeping the vast majority of the drag. And actually, hyper- and/or supersonic retropropulsion HAS been done on Earth (remember reading about it), just not operationally, since there's exactly no reason to do it operationally at Earth since the atmosphere is far, far denser than at Mars.

I admit I am not an expert on super/hypersonic retropropulsion. But I know that is something that is considered pretty hard to pull off, and virtually impossible to test in a lab. I would suggest that SpaceX are not experts in this field either, nor is this operational environment the principle design goal for the Super-Draco system, which is already fulfilling two radically different purposes.

And this can actually be tested to a certain extent (though not all the aerothermal aspects of entry will be the same, at least you can validate your models of supersonic descent control) by SpaceX if they wanted to in the upper atmosphere of Earth. Might not be a bad idea, actually, though it could be expensive if they don't find a way to integrate it into some other mission.

What information could they gain by doing test firings of the "non-super" Draco during the 12-ish CRS re-entries?

cheers, Martin

[Ben the Space Brit]

Nice bit of science there AstronauticJoe.  I have to say that I'm twitchy about still going comfortably north of 500m/s at mean Martian surface radius, even though there are several target landing sites where that is above local surface radius by some margin.  Any of the brain trust want to offer a way that Red Dragon can be decelerated further?

Suggestions:

1) Steeper descent due to longer de-orbit retro-fire (losing more energy before atmospheric interface);

2) Fold-out aero-brake panels to increase Red Dragon's braking area.

[AstronauticalJoe]

Suggestions:

1) Steeper descent due to longer de-orbit retro-fire (losing more energy before atmospheric interface);

2) Fold-out aero-brake panels to increase Red Dragon's braking area.

In regard to 1), I reckon a steeper descent isn't the way to go. The vertical speed would be higher, so the entry would be shorter, so you wouldn't have as much time to decelerate. Also, using more of the Dragon's fuel up on the deorbit burn would leave less for the super Dracos to kill the remaining 500m/s or so of airspeed and land (which is a close call already).

I personally would love to see more development into inflatable heatshields, despite their current flaws. If the technology from this experiment: http://www.nasa.gov/pdf/378699main_NASAFacts-IRVE.pdf was scaled up from it's current 3m diameter to 6.5m (and packed into the dragon's trunk) then things would get a lot easier for EDL. Once entry was over, we'd have a comfortable terminal velocity of 343 m/s for the super dracos to kick in and do the rest.

Although, using new tech isn't playing SpaceX's strength of taking existing technology and reducing cost and reliability. However, if Red Dragon is going to be SpaceX/NASA joint mission, then this could be a viable option. Plus, if inflatables for Mars are developed properly here then we're also one step closer to the EDL requirements for manned flights.

-Joe

[Zed_Noir]

Nice bit of science there AstronauticJoe.  I have to say that I'm twitchy about still going comfortably north of 500m/s at mean Martian surface radius, even though there are several target landing sites where that is above local surface radius by some margin.  Any of the brain trust want to offer a way that Red Dragon can be decelerated further?

Suggestions:

1) Steeper descent due to longer de-orbit retro-fire (losing more energy before atmospheric interface);

2) Fold-out aero-brake panels to increase Red Dragon's braking area.

Additional suggestion

Equipped the Red Dragon with a pop off hypergolic propellants drop tank on the nose to increase the Super-Draco burn time. I guess about 2 to 3 times additional burn time with the combined onboard and drop tank propellants.

But we are just guessing until some performance figures is available for the Super-Dracos. Even then, there might be Mars specific Super-Dracos that's different from the Dragon Earth LAS Super-Dracos.

[Kaputnik]

Good work, Joe, welcome to the forum.
We should bear in mind that the atmospheric density on Mars is not constant, but varies considerably with temperature (and perhaps other factors too?)- so enough margin should be considered to allow a safe landing.

The ideas about major modifications to Dragon such as fold-out or inflatable deccelerators go against the notion of a cheap and cheerful mission.

My opinion is that this concept is going nowhere. It is supported only by a small number of people at Ames who, it seems, have not fully thought out how it is supposed to work.

If SpaceX wanted to offer a cheap lander that would actually work, they would stick a big PLF on FH, and use a big heatshield coupled with Super-Dracos in a more conventional lander design.

[Kaputnik]

One other point- any 'drop tanks' or attempts to enter with the trunk attached will completely invalidate the entry characteristics of the vehicle.

And of course we know very little about the all-important Super-Dracos. Given that their primary function is as an abort motor, they must be totally reliable, high thrust, and operate instantaneously. That all points to pressure-fed monopropellant, but perhaps bipropellant will be considered acceptable and provide higher isp. But it should be borne in mind that high isp is not a design criterion. They almost certainly will not be pump fed.

[MikeAtkinson]

Accordig to the document "Low Density Supersonic Decelerator Technology Demonstration Mission":

MSL aeroshell diameter is 4.7m, LD is 0.24 and entry mass is 3300kg. At parachute deployment it is going at mach 2 and at about 9km altitude (above landing site), with deceleration down to about 3m/s.

Red Dragon is going to have a greater velocity at 9km because it is over twice the mass.

I don't see how a fully impulsive landing is possible, there isn't room for that much fuel in the current tanks and if there were it would push the entry mass higher which in turn would mean more fuel were required.

There is a gap between the 1000m/s or more which is the speed Red Dragon would hit the ground with no propulsion and the 500m/s delta-v from its super-draco propulsion system.

I can think of a dozen ways this gap might be closed or at least narrowed, many of which have been mentioned up thread. One which hasn't is a redesign leading to a much lower dry mass. There is no need for parachutes or their attachment points, berthing adaptor, windows, nor is there need for a pressure vessel, this in turn might allow a much lower mass structure, this feeds through into a thinner heat shield, etc. The end result would be something with the outer mold line and many systems in common with Dragon, but really a quite different vehicle.

[Kaputnik]

Or, even better, use the Viking/MPF/MER/MPF/MSL outer mould line, push it out to the 6.5m maximum size that can be launched by FH, and get SpaceX to build it. This will work out better than trying to use a capsule that is designed for a different mission.

Also, Dragon is not only much heavier than MSL, it also presents a smaller cross sectional area (about 60%). The ballistic coefficient works out at about four times worse for Dragon than MSL- and MSL is already considered to be at the limit of what is possible.

[ugordan]

push it out to the 6.5m maximum size that can be launched by FH

I thought the 5.2 m fairing is already at the limits given the core diameter?

[Robotbeat]

push it out to the 6.5m maximum size that can be launched by FH

I thought the 5.2 m fairing is already at the limits given the core diameter?

Not a limit... I believe more hammer-heading is possible. Analysis is, of course, needed.

The 3.8m diameter Atlas V is supposedly doable with a 7m fairing. Given that, 6.5m should be doable with the 3.66m diameter core Falcon Heavy.

[ugordan]

Even so, 6.5 is only the outer diameter while the allowed inner diameter would be less than that.

At one point you have to ask yourself: when does all this stop being a Dragon and a stock Falcon Heavy and relatively cheap in the first place.

[Kaputnik]

Even so, 6.5 is only the outer diameter while the allowed inner diameter would be less than that.

True, calling it 6.5m is a bit optimistic.

At one point you have to ask yourself: when does all this stop being a Dragon

I stops being a 'standard' Dragon the moment that it leaves LEO.
When it is asked to carry a large scientific payload and a drill, it stops even looking like a Dragon.
And the moment it is tasked with landing on Mars, it stops being feasible.

[baldusi]

Does somebody have the Red Dragon presentation? I have it, but on a different computer and cannot access it.

Could you upload it? I can't find it.

[Blackstar]

I found it. Here is the presentation.

[Robotbeat]

I found it. Here is the presentation.

Ah, thanks!

Looks like Dragon can land a ton on Mars, as long as you pick a low-altitude landing site. This answers the question of why Dragon can land as much on Mars as MSL's EDL system but with a smaller heat shield and no parachutes (because Red Dragon doesn't have parachutes, it doesn't need to slow down to Mach 2 before initiating the next phase in landing--it can fire its thrusters at Mach 3 or higher).

That, along with the fact that the mass of Dragon on entry is higher, so there's a lot of room for more landing propellant than MSL will have. MSL's EDL system is designed to make high altitude sites feasible for directly deploying large payloads on the surface while Red Dragon is shooting for low elevation sites with near-surface ice.

[baldusi]

I found it. Here is the presentation.

Thanks! What's very interesting is this:

Fire launch abort motors supersonically and use them for remainder
of descent.

They are stating that they can fire their SuperDracos while in supersonic flow? Wasn't that completely out of question?