Landing on Mars

GW Johnson · 2012-07-23 14:30:18

Today's internet news: the IRVE-3 test was a success. Mach 10 speeds, temperatures to 1000 F. No other hard data could be gleaned from the story. Looks very promising, though.

GW

GW Johnson · 2012-08-02 23:57:06

This is very, very preliminary, but I've come to believe a conical lander of a shape similar to Apollo, but around 60 metric tons, will have a ballistic coefficient (beta) near 1200 kg/sq.m. From 200 km LMO, entry will start about 3.47 km/s at about 135 km, and about 1.6 degrees angle of depression. That requires a mere 50 m/s de-orbit burn. My back-of-the-envelope entry model suggests that this thing comes out of the hypersonics at M3 and around 7 km altitude. That would be about 720 m/s at 7000 m, angled about 2 degrees down.

That's only mere seconds from impact, so parachutes will definitely do no good. So, I'm thinking heavy rocket braking right from the end of the hypersonics, aimed at reducing the horizontal velocity quickly to zero, before one runs out of altitude. I will have to build a good model and develop it, but some very crude pencil and paper stuff says the thrust levels will be around 1000-2000 KN on that 60 ton vehicle, and thrust accelerations will be in the 2-3 gee range. The time from end of hypersonics to touchdown will be a minute or less.

That seems challenging but doable. It seems that my supersonics-to-touchdown heavy rocket braking model may be a worthwhile objective. This is with no rocket braking during the hypersonics, and no parachutes or ballutes. I will pursue this as time permits in the late evenings. Have patience, don't hold your breath.

As I get stuff done, I will post it over at "exrocketman". There's an update to the ballistic coefficient correlation over there right now.

GW
http://exrocketman.blogspot.com

"HA > H explains an awful lot of what's going on in this world"

Rune · 2012-08-03 03:23:55

I may not post much, these days (summer and a delightfully clever girl have much to do with that), but I am following this with close attention. Glad to hear that you seem to be converging to the same conclusion my gut told me should work a few pages back. So 720m/s at 7km for beta 1.2k? That does not sound even half as scary as I thought it would when I saw the 1200, and you could get to LMO from a very loose capture orbit through aerobrake, taking your time with little propellant expenditure, so that shouldn't be an issue. 720m/s also doesn't sound like a huge chunk of fuel (MR 1.27 with 300s isp). The limiting factor, really, is to provide enough deceleration force in the terminal stage, and rockets are good at T/W on demand. BTW, 2-3 earth G's are 5-7.5 martian G's... I love Mars for rockets, gravity losses are tiny.

"HA > H explains an awful lot of what's going on in this world"

I'm a bit thick right now and google doesn't help. Care to explain the explanation?

Rune. Keep up the good work!

GW Johnson · 2012-08-03 09:21:02

Hi Rune:

Ah yes. School, summers, and girls. I do remember, even after all these decades.

My calcs are for entry from LMO, not direct entry. Higher betas and higher entry speeds very quickly push you toward hypersonic impact with the surface. So does steepening the depression angle, which may actually be the most dangerous actor. It's not much problem from LMO, but could easily be a problem with direct entry. In those cases you must retro thrust during entry. There is no other option.

I've got the orbit mechanics all worked out, and the back-of-the-envelope entry model seems to produce reasonable results. I ran a exploratory spread of betas vs M3 altitudes last night, bracketing all the way from the small probes to 100 tons. As soon as I get it saucered-and-blowed, I'll post it.

What do you think of a 7.1 meter diameter cone about 7 me tall, for 60 tons? That's right at beta 1174 if you use an Apollo-like drag coefficient of 1.3. I get an outer envelope effective packing density of 654 kg/cu.m with numbers like that. Seems tight, but still sort of reasonable for at least a one-way lander. This does not take into account the possibility of an inflatable heat shield like the IRVE tests. That thing isn't "ready for prime time" yet.

HA > H transliterates as "the number of horses' asses always exceeds the number of horses".

GW

TwinBeam · 2012-08-03 17:53:54

A hazy thought - what if one built an aerobrake cone out of a mesh, perhaps of tungsten, to increase the drag to mass ratio?

My (again, hazy) impression is that a mesh will have drag rather higher than the actual material surface area would provide if it were a solid sheet.

Unclear if that carries over from dense gas at low velocities to thin gas at hypersonic velocity, however. I figure the effect arises from creating a standing pressure wave over the gaps in the mesh, and there has to be some means to transfer the force back to the mesh itself - which might not apply in high velocity, low density air.

A two level mesh might work better - the first splitting the air and forcing molecules that didn't collide into a more compressed stream, which would tend to increase collisions with a second layer of mesh lying behind the gaps in the front mesh. The separation of the two layers might need to vary with velocity and density of atmosphere however? And that still may not work well in extremely thin air, as it relies on collisions between air molecules to form the compressed streams.

Something similiar might be done for parachutes to improve their efficiency - since for Mars the mass of the chute becomes a significant (though not major) component of the mass being braked. Since they'd be used at lower velocity and in higher density air, the increased drag effect might apply better.

Anyone have sufficient aerodynamics background to nix this?

Russel · 2012-08-04 05:13:29

GW,

I see no problem in landing a large mass ~60MT onto Mars with a bit of heavy rocket braking at the end. I just wouldn't want to be a passenger on that thing.

TwinBeam,

I've been toying with that mesh idea for a while. Only what makes sense to me is something that creates drag behind the vehicle. But yes, I suspect a mesh would present effectively more surface area and thus more drag per unit mass.

I still think though that it'll be one of those fine compromises (for non manned landings) where there is a trade off between the mass of fuel you need to burn (retros are unavoidable) and the mass of any structure that's there purely for drag.

Rune · 2012-08-04 11:06:56

Well, Twin beam and Russel, my aerodynamics teachers told me that Drag is a function of two things (apart from density and speed): Cd and area. Yes, Cd is a function of shape. But area is area, and even though vortex formation through shaping reasons can affect Cd, the area is what the area is. So it may sound way better than it is in real life, and you might find a birdie needs it's cone perforated to move the center of pressure away from the center of mass and thus exert more momentum with the same drag, not because the drag is greater that way. Also, if you are trying to increase area with the least amount of mass, you wouldn't pick one of the densest metals like tungsten is.

As to the 60mT lander described by GW: well, with MR >1.3, it may fit, payload-wise, riding empty on a Falcon Heavy launch to be fueled later... but that 7.1 diameter heatshield doesn't fit in any payload shroud currently in existence. You could design a bigger shroud, or make it ride "naked", but both things would need an aerodynamic analysis to make sure the launcher can handle it. Or you could use the hypothetical SLS and it's 10m humongous shroud. Or you could finish development of inflatable shields. Pick one, but note the cheapest (modify/recertify an existing launcher) is also the most limited, when you think of possibly even greater future payloads.

And another point, with >13 of those 60mT (MR>1.3) in the form of dense storable propellants, packaging density for the important part (pressurized capsule where humans and their supplies ride) looks much better. Same thing for the very dense rigid heatshield. What mass fraction would ypu assign to it anyway? 10-15% of braked mass like Zubrin?

As to LMO entry... well, as I said, even a flimsy unshielded orbiter with solar panels extended can lower it's orbit as much as it wants through aerobraking passes (from a suitably elliptic orbit) with minimal fuel expenditure. So starting entry from LMO does not necessarily exclude parking the return vehicle in a loosely bound elliptical capture orbit. The only extra requirement from that goes to the ascent vehicle, and it saves more same delta-v from the Earth return vehicle than it costs the ascent vehicle.

Rune. Summer! And here I am discussing geek stuff...

Russel · 2012-08-05 05:43:27

Rune,

The motivation behind having a mesh surface to form drag actually has more to do with controlling the overall stream, and stability. So instead of having one solid area where the heated gas spills over the edges, it instead flows through in a controlled manner. As I said earlier it may pay to have a profile.. more density of mesh here.. less there.

Tungsten wasn't my idea. Rather What I want to see is figures that give me a rough idea of how big you'd need to build something to bring the temperature down into a range where you can use less exotic materials - even to under 500C.

Here's a half baked idea. Has anyone thought of what amounts to a water balloon? Its towed and as it heats, the water turns to steam, thus inflating it. Not sure of the materials but this would have some interesting properties as to where exactly it kicks in. Interesting question as to whether it would still have lift nearer ground level - in other words how fast does the steam cool.

Just a wild thought there

Does anyone have any idea when SpaceX will start testing its new thrusters on the Dragon? That'll answer so many questions about how to retro thrust into a hypersonic stream - at least if they test it in that regime (and I would think they'd have to).

RobS · 2012-08-05 07:02:02

I don't think there's any plan to use the Drago thrusters hypersonically. Dragon has parachutes to slow it to subsonic speeds.

GW Johnson · 2012-08-05 15:28:03

I haven't posted my stuff yet, but I did estimate a range of ballistic coefficients for a 6 ton (metric) dragon, as 380 to 453 kg/sq.m, depending upon whether you believe the hypersonic drag coefficient is closer to Apollo's 1.3 vs closer to Mercury/Gemini's 1.55.

I ran the old 1956-vintage "back-of-the-envelope" entry model reported by Justus and Braun, in their "typical" Mars atmosphere, at a nominal 500 kg/sq.m, and got a terminal (Mach 3) altitude for the entry of 15 km, from a circular 200 km orbit entry. A capsule entering at interplanetary speeds would penetrate even deeper.

My estimates for ballistic coefficient of Mercury, Gemini, and Apollo range from 246 to 374 kg/sq.m at entry, with Gemini the lowest and Apollo the highest. That's for hypersonic drag coefficients of 1.3 for Apollo, and 1.55 for Mercury and Gemini, based on the the hypersonic drag data reported in the old Hoerner "drag bible". The recent Mars landers all cluster around ballistic coefficient 100 kg/sq.m, with Curiosity near 115, and most of the rest nearer 63 to 90-something.

The problem with Mars entry is the parachute phase. Once you come out of hypersonics at about Mach 3, the parachute terminal velocity is still supersonic in the Mach 1 to 2 range. The 200 mph stuff I see on the internet news is just BS. It takes finite time to deploy a supersonic chute, and then slow down with it. If your terminal altitude is too low, the chute does you no good, you might as well simply rocket brake, only.

My guesses for a Dragon equipped with Super Dracos and landing legs says it comes out of the hypersonics somewhere near 15 km altitude, with no retro thrust during entry. I have yet to run numbers, but I'd think it could rocket-brake from there to touchdown pretty effectively, since the Super Dracos are very "thrusty". You just need enough on-board propellant to carry it out. That propellant mass will displace deliverable cargo.

I'm getting entry decel gees near 0.7 from circular. And braking gees near 3-ish. Nothing final yet. But rocket retro thrust can simply be on the capsule. You just need T/W enough to fly "smartly" upward on Earth, maybe a tad more. That's what the Super Dracos were designed to do, for launch escape.

A "skycrane" is not the only answer. Although it might have been the lightest answer for Curiosity. And the rocket-braking gees are not all that bad.

It'll be worse still for a big lander in the 60 ton class. But not that bad. We could easily ride it.

GW

Rune · 2012-08-05 17:16:02

RobS wrote:

I don't think there's any plan to use the Drago thrusters hypersonically. Dragon has parachutes to slow it to subsonic speeds.

Actually, on earth, Dragon turns subsonic with no parachutes involved just fine. Our atmosphere is just that great for aerodynamic deceleration. They will retain a single chute, but that is a backup in case either the engines fail, or you spend the landing fuel to escape an exploding rocket. Right now, the parachute system is triple-redundant, since just one of them would be enough for a safe terminal velocity. Again, and just to be crystal clear, a nominal land landing of a crewed Dragon would not involve deploying chutes at any point. Which is great in principle at least, because you don't have to repack it. So theoretically it's just a quick refuel, and you can stick the capsule on another booster and use it the same day.

Rune. I've seen that misinterpretation too often not to point it out.

RGClark · 2012-08-06 02:39:33

TwinBeam wrote:

A hazy thought - what if one built an aerobrake cone out of a mesh, perhaps of tungsten, to increase the drag to mass ratio?
My (again, hazy) impression is that a mesh will have drag rather higher than the actual material surface area would provide if it were a solid sheet.
Unclear if that carries over from dense gas at low velocities to thin gas at hypersonic velocity, however. I figure the effect arises from creating a standing pressure wave over the gaps in the mesh, and there has to be some means to transfer the force back to the mesh itself - which might not apply in high velocity, low density air.
A two level mesh might work better - the first splitting the air and forcing molecules that didn't collide into a more compressed stream, which would tend to increase collisions with a second layer of mesh lying behind the gaps in the front mesh. The separation of the two layers might need to vary with velocity and density of atmosphere however? And that still may not work well in extremely thin air, as it relies on collisions between air molecules to form the compressed streams.
Something similiar might be done for parachutes to improve their efficiency - since for Mars the mass of the chute becomes a significant (though not major) component of the mass being braked. Since they'd be used at lower velocity and in higher density air, the increased drag effect might apply better.
Anyone have sufficient aerodynamics background to nix this?

My background is in math rather than aerodynamics but I like the logic of your argument. That standing wave you mentioned happens when the "stagnation pressure" is reached. See description here:

Stagnation pressure.
http://en.wikipedia.org/wiki/Stagnation_pressure

I'd like to see this tested in a hypersonic wind tunnel. But before you send it to NASA perhaps you should look up if the idea has been patented before and patent it yourself if has not: http://www.google.com/patents?hl=en

Bob Clark

Russel · 2012-08-06 02:47:06

"My guesses for a Dragon equipped with Super Dracos and landing legs says it comes out of the hypersonics somewhere near 15 km altitude, with no retro thrust during entry. I have yet to run numbers, but I'd think it could rocket-brake from there to touchdown pretty effectively, since the Super Dracos are very "thrusty". You just need enough on-board propellant to carry it out. That propellant mass will displace deliverable cargo. "

GW, I tend to agree with you there, that a Dragon could land humans on Mars, even without a parachute. Only when the fuel is taken into account there won't be much room apart from the humans, some spare life support and a mini rover. So yes, you don't need to test the super dracos into a hypersonic stream to get that.

My interest though as far as that is concerned is much larger, unmanned landings where retro thrust later on in the hypersonic stage might be a necessary evil (its either this or some extra drag somewhere). And again the best vehicle to actually do the experiment with is the Dragon. If we learn how to pull of this trick then it will scale well.

I've even envisioned a landing system that starts off more or less as a blunt body but once you get past peak heating, it unfolds or extends retros. You could even control the angle starting with high cant, and then moving more towards vertical.

Going back to a manned landing I now realise there is a tradeoff of risks. On the one hand you can use a first pass aerocapture to burn off some speed, but that has its risks, Or on the other hand if you have a direct entry that puts you closer to the ground before you can use retro braking (again, assuming you can only do this below a certain speed). Interesting tradeoff that one.

GW Johnson · 2012-08-07 08:38:37

OK guys, I've run one Mars entry study using a range of ballistic coefficients with the old 1956-vintage scale-height method. Those are posted over at "exrocketman", with a gob of spreadsheet images and plots. The net outcome shows end of entry nearer 20-30 km altitudes in the 100 kg/sq.m class like the Mars probes, but under 10 km when in the 1000-1500 kg/sq.m class, more like a manned lander. This is for entry from LMO circular orbit, not direct entry. Direct penetrates deeper even at the same angle, and that's likely to be steeper as well.

I'm going to refine the density scale height correlation for the lower altitudes, and try this again, centered on about 1200 kg/sq.m. That's just about what I'd expect a manned lander to be, at around 60 tons entry mass. I think it'll be pretty close to end-of-hypersonics (at Mach 3), pretty close to 7 km altitude. That's too low to deploy a parachute, there isn't time to get it open, much less slow down significantly with it, in that thin "air".

It's looking like rocket braking-to-touchdown to me. These numbers are for no thrust during the hypersonics, BTW. Just the supersonics.

As for brainstorming configurations, here are two notions I'd like to pitch into the punchbowl and see who screams: (1) who says a heat shield has to be round? (2) who says we have to drop the heat shield to land, if it actually serves a purpose landing?

Those are in addition to the idea I threw in the other day, that you can fire a rocket through a hole in a heat shield, if the compartment containing the rocket is sealed gas-tight to prevent throughflow through the hole.

Anybody else come up with some hare-brained configuration ideas, with those three crazy items in the mix?

GW

"exrocketman" is http://exrocketman.blogspot.com

Last edited by GW Johnson (2012-08-07 08:40:34)

Terraformer · 2012-08-07 09:38:55

Mmmm, retrorockets first, to take off 0.5-1km/s of the velocity before entering the atmosphere?

GW Johnson · 2012-08-07 15:03:41

Terraformer:

I had 3.455 km/sec for 200 km circular orbit, and 3.405 km/sec for the apogee speed on a transfer ellipse that just hit the surface. That's 50 m/sec to de-orbit, something an attitude thruster can do. Using the Justus & Braun report, atmospheric interface occurs at 135 km on Mars. That was at 1.63 degrees below horizontal, and 3.469 km/sec entry speed (it increased as we descended).

From there I used the 2-D Cartesian model described in Justus & Braun of the old 1956-vintage density scale height analysis. It took me from 3.6 km/sec to 0.6-0.7 km/sec at varying altitudes, depending upon ballistic coefficient. The range of altitudes was 29 km at beta=100 kg.sq.m, down to 3 km at beta = 2000 kg/sq.m. Nonlinear curve. Not too steeply bent. I never saw peak gees above about 0.7.

The landers (including MSL/Curiosity) cluster around beta 100, being 63 up to about 115 for MSL. A manned lander is probably beta 1200 kg./sq.m, in the 60 ton entry mass class. Plus or minus maybe 500 kg/sq.m.

I haven't run data for a stronger de-orbit burn. That will certainly lower the entry interface velocity, but it will also steepen the trajectory, for sure. Steeper path tends to makes it dive deeper before the Mach 3 point. Interesting question, I need to look at that.

GW

SpaceNut · 2012-08-07 17:05:09

The SuperDraco is part of Dragon’s launch-escape system, however the engines may gain additional roles, including the ability to land Dragon propulsively on land.

However, Earth isn’t the only landing destination for Dragon, with SpaceX holding ambitions of landing on the Moon and more notably Mars. Nicknamed “Red Dragon” – SpaceX have made no secret about heading to Mars, even publishing a graphic of their spacecraft touching down on the Red Planet.

louis · 2012-08-07 18:07:58

I think Space X are being a little diplomatic at the moment. They don't want to show up NASA too much - and they will probably need NASA's comms to get to Mars, so why make enemies?

Once the commercial programme for NASA is fully under way, then I think we will see a much higher profile on the Mars mission.

GW Johnson · 2012-08-08 13:36:07

Louis:

I think you are quite correct. Spacex is capable of sending small cargoes one-way to Mars very quickly with Dragon, equipped with Super Dracos, sometime later this year, with nothing more than Falcon-9, perhaps the upgrade with the new Merlin D's. It might take a bit more than the current 1290 kg allotment of thruster fuel in Dragon, but I dunno, haven't checked that. Given the right cargo, that could be a tiny sample return mission.

With Dragon/Falcon-9, I think we're looking at low-angle direct entry, hypersonic drag deceleration to about M2.5 to 3 at about 3-5 km, then direct rocket braking on the Super Dracos to touchdown. No chute, there isn't time. The Super Dracos could even be used during entry if needed.

They cannot afford to piss NASA off by upstaging them too quickly, because they (like everybody else) needs nuclear thermal for the manned mission to Mars. It cuts launched mass and costs by a factor of 3 to 4. Nuclear-anything is a government monopoly in nearly every country, including the USA. Spacex needs NASA to get those nuclear rocket engines. With them it's 6 men to Mars, easily, with technology and hardware we have right now, and a high probability of safe return, and for less cost than the ISS.

On the other hand, NASA needs Spacex to push it off dead center. But Spacex has to make that look like NASA's own idea. It's a face-saving thing, typical of western bureaucrats long before we met any of the oriental cultures. Silly but true.

NASA-JPL is doing great things with the unmanned probes, and is quite content to do more great things in perpetuity, without ever a man going to Mars. In point of fact, they are the best-functioning part of NASA. Can't piss them off either, their expertise will be needed for the manned trip's lander, and all the robots that need to go with the men.

Once again business holds government's little hand, and drags it kicking and screaming and dragging its heels into the future. We've done it that way for centuries. I wish it was easier, but it is not. Right now there is only one big, credible company (Spacex) willing to lead. ULA seems quite content to lie there and suck on the government's tit. That sow shows no willingness to stand up.

Now you understand the problem of going to Mars, or anywhere else, even just the moon.

GW

louis · 2012-08-08 16:33:26

GW Johnson wrote:

Louis:
I think you are quite correct. Spacex is capable of sending small cargoes one-way to Mars very quickly with Dragon, equipped with Super Dracos, sometime later this year, with nothing more than Falcon-9, perhaps the upgrade with the new Merlin D's. It might take a bit more than the current 1290 kg allotment of thruster fuel in Dragon, but I dunno, haven't checked that. Given the right cargo, that could be a tiny sample return mission.
With Dragon/Falcon-9, I think we're looking at low-angle direct entry, hypersonic drag deceleration to about M2.5 to 3 at about 3-5 km, then direct rocket braking on the Super Dracos to touchdown. No chute, there isn't time. The Super Dracos could even be used during entry if needed.
They cannot afford to piss NASA off by upstaging them too quickly, because they (like everybody else) needs nuclear thermal for the manned mission to Mars. It cuts launched mass and costs by a factor of 3 to 4. Nuclear-anything is a government monopoly in nearly every country, including the USA. Spacex needs NASA to get those nuclear rocket engines. With them it's 6 men to Mars, easily, with technology and hardware we have right now, and a high probability of safe return, and for less cost than the ISS.
On the other hand, NASA needs Spacex to push it off dead center. But Spacex has to make that look like NASA's own idea. It's a face-saving thing, typical of western bureaucrats long before we met any of the oriental cultures. Silly but true.
NASA-JPL is doing great things with the unmanned probes, and is quite content to do more great things in perpetuity, without ever a man going to Mars. In point of fact, they are the best-functioning part of NASA. Can't piss them off either, their expertise will be needed for the manned trip's lander, and all the robots that need to go with the men.
Once again business holds government's little hand, and drags it kicking and screaming and dragging its heels into the future. We've done it that way for centuries. I wish it was easier, but it is not. Right now there is only one big, credible company (Spacex) willing to lead. ULA seems quite content to lie there and suck on the government's tit. That sow shows no willingness to stand up.
Now you understand the problem of going to Mars, or anywhere else, even just the moon.
GW

Fortunately Musk is I think a big enough man not to be motivated by petty vanity. He is keeping his eyes on the prize - the biggest step since the neolithic revolution made us settled farmers I think he calls it and I think he is right...humans on another planet. He knows it will be his name that will go down in history if he achieves his objective.

Whether it requires nuclear thermal I remain doubtful, but I am sure Musk is highly focussed on what is the (comfortable) minimum required to achieve the task (that always seems to be his focus).

Russel · 2012-08-09 03:31:58

I remain agnostic about nuclear thermal. Partly because anything thermal is going to give you only a 2 fold increase in Isp.

Now I could be proven wrong. Maybe a hybrid of nuclear thermal plus limited magnetic confinement might get the temperature up a bit further. But its not easy either.

My attitude to getting to Mars revolves around that fact that chemical launches are getting cheaper and there's good physical reasons why that should continue for a while. If we can get the cost of launching fuel into LEO down to around $1000/Kg then its not a big deal to just simply launch another 200 tonnes of fuel into orbit and make good use of it.

In other words, its the least of our worries.

I also have the attitude that if we go to Mars, we should do it right and do it in style, with plenty of margin.

Anyhow, it remains to be seen - we may go through a generation of nuclear thermal rockets in the medium term but I suspect the first real mission to Mars will be chemical.

And in the medium term its just possible we might actually leap frog to aneutronic fusion and have both the power and the Isp we need to go even further.

GW Johnson · 2012-08-10 07:57:57

Terraformer:

I did look at the bigger retro burn, just like I said I would. It lowered velocity at entry, as expected, but it also steepened entry, as I feared. The dynamics were more sensitive to the steeper entry angle than the entry velocity. 200 km circular 3.455 km/s minus 50 m/s put 135 km entry at 3.469 km/s at 1.63 degrees. At 1200 kg/sq.m, you come out of hypersonics at M3 at about 8 km altitude. Plus and minus 500 on the ballistic coefficient lowered and raised terminal altitude by about 2 km.

If from the same orbit you de-orbit by 500 m/s, then entry is at 3.029 km/s, but at 6.4 degrees. At the same 1200 ballistic coefficient, you are still doing 4.7 Mach when you impact the surface. Entry isn't over yet.

Both entries were figured with a "typical" Mars atmosphere, and that 1956-vintage 2-D Cartesian "back-of-the-envelope" entry model. The orbital mechanics part is pretty standard ellipse stuff, I just did it pencil-and-paper.

GW

GW Johnson · 2012-08-10 18:26:26

OK, guys, I have just finished posting all of my results from the big Mars lander entry sensitivity study. It is the latest in a series of articles about, or related to, landing on Mars. All of these are recent postings over at http://exrocketman.blogspot.com. List follows:

Date    title    subject
8-10-12 Big Mars Lander Entry Sensitivity Study impacts of beta and de-orbit burn size
8-5-12 Ballistic Entry from Low Mars Orbit    impacts: beta from 100-2000 kg/sq.m
7-25-12 Rough Correlation of Entry Ballistic Coefficient scaling from <1 ton to 100-ton sizes
vs. Size for “Typical” Mars Landers
7-14-12 Gravity Data on All the Interesting Worlds    traceable data for surface gee, etc
7-14-12 “Back of the Envelope” Entry Model    2-D Cartesian estimator, updated
6-30-12 Atmosphere Models for Earth, Mars, and Titan traceable “typical” data
6-3-12 Deceleration by Drag Devices (and more)    concepts for retro thrust during entry
on Mars

I still need to explore what happens after entry ends, because a lot of these altitudes are single-digit km, not the 20-30 km stuff NASA-JPL has been seeing in its designs. This is seconds-to-impact stuff. No time for a chute, just massive rocket braking to touchdown. I don’t have a model built, but at least it is just straight supersonic-to-subsonic aerodynamics and vehicle dynamics. I know how to do that.

So far, none of this requires rocket braking during entry. But that topic needs exploration, too. And I still have to figure out exactly how I am going to do that.

GW

GW Johnson · 2012-08-12 21:59:23

OK, guys, I did a pencil-and-paper analysis of post-hypersonic trajectory dynamics, and rough-sized a family of chemical landers for it. The manned lander has an ascent vehicle for return to LMO with crew of 3. There's no plane changes allowed. Descent and ascent use the same engines. The unmanned lander is a one-way device, where more cargo mass replaces the ascent vehicle, in an otherwise overall-the-same vehicle.

I'm showing end of entry at local Mach 3 at 8 km altitude, no retro thrust during entry. The 2-ton heat shield gets blown off explosively in pieces. Retro thrust kills horizontal speed by about 3.4 km altitude, then kills vertical sink, then does a tail-sitter "shtick" from about 200 m. No chutes or ballutes, there isn't time to deploy them or for them to be effective. I used 10-degree cant for retro plume stability.

There's extendable landing legs, and the aeroshell segments get used as unload ramps. Depending on the mass of the crew cabin capsule, the manned version carries 2 to 6 tons cargo to the surface, but nothing significant back up. The unmanned version carries about 25 tons one-way to the surface. Both are 60 tons at entry. About 7 m diameter, short and squat, tall slender single-stage ascent vehicle, hypersonic CD 1.30, beta = 1200 kg/sq.m. MMH-NTO engines. Built in Earth orbit from pieces, including a segmented heat shield.

I posted the whole thing over at "exrocketman" minutes ago. Let me know what you think.

If I can come up with this, based on no more sophisticated tools and analyses than I have used, then NASA bloody well had better have already come up with something similar. They have the people and the analysis tools to put this on a firmer footing than I can do.

Landing big stuff on Mars turns out not to be all that hard, as long as you are unafraid to look at things besides those things you have already done before.

GW
http://exrocketman.blogspot.com

Rxke · 2012-08-12 23:34:33

25 tons, wow, that does open possibilities.

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#251 2012-07-23 14:30:18

Re: Landing on Mars

#252 2012-08-02 23:57:06

Re: Landing on Mars

#253 2012-08-03 03:23:55

Re: Landing on Mars

#254 2012-08-03 09:21:02

Re: Landing on Mars

#255 2012-08-03 17:53:54

Re: Landing on Mars

#256 2012-08-04 05:13:29

Re: Landing on Mars

#257 2012-08-04 11:06:56

Re: Landing on Mars

#258 2012-08-05 05:43:27

Re: Landing on Mars

#259 2012-08-05 07:02:02

Re: Landing on Mars

#260 2012-08-05 15:28:03

Re: Landing on Mars

#261 2012-08-05 17:16:02

Re: Landing on Mars

#262 2012-08-06 02:39:33

Re: Landing on Mars

#263 2012-08-06 02:47:06

Re: Landing on Mars

#264 2012-08-07 08:38:37

Re: Landing on Mars

#265 2012-08-07 09:38:55

Re: Landing on Mars

#266 2012-08-07 15:03:41

Re: Landing on Mars

#267 2012-08-07 17:05:09

Re: Landing on Mars

#268 2012-08-07 18:07:58

Re: Landing on Mars

#269 2012-08-08 13:36:07

Re: Landing on Mars

#270 2012-08-08 16:33:26

Re: Landing on Mars

#271 2012-08-09 03:31:58

Re: Landing on Mars

#272 2012-08-10 07:57:57

Re: Landing on Mars

#273 2012-08-10 18:26:26

Re: Landing on Mars

#274 2012-08-12 21:59:23

Re: Landing on Mars

#275 2012-08-12 23:34:33

Re: Landing on Mars

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