Landing on Mars

GW Johnson · 2012-07-16 13:42:38

I just spent about 30 minutes roughing out a first-cut Mars mission for a crew of 4-6. It's not an optimal mission. It's crude, just rocket equation stuff. All docked modules of 25 metric tons each, 5 m dia and 15 m long, just like the biggest existing payload dimensions today. Two of these form a habitat module with about 50 cu.m free internal volume per person (spacious!). Two more are a storage place for life support consumables. There is a NERVA engine module that I assumed was 20 tons, for a big engine or pair of engines. Isp 900 at T/W 3.6 as demonstrated in 1973.

I assumed two landers made of two 25 ton docked modules, for 50 tons each. These are just allowances, no design. Hab plus storage plus landers is 200 tons dead-head payload. The propellant modules are also 5 m dia x 15 m long, and need cryo-coolers plus solar PV panels for LH2. I used 10% inerts. At LH2 densities, I get 9 tons propellant and 1 ton hardware per module.

Two ships, one manned, one unmanned, both Hohmann transfer from LEO to LMO (about 8 months one way). The manned ship returns to LEO, where it can be used again. The one-way delta vee total with plane changes is pretty close to 8 km/sec.

Outbound /manned: 36 propellant modules in a 4-parallel stack 9 modules long. NERVA at one end, hab /storage/landers at the other. My stack is about 195 m long, and masses 580 tons at departure. Essentially-dry tanks at Mars in LMO. There's about 95 m from the cg to the hab. That's 1 full gee artificial gravity at only 3 rpm, spun head-over-heels.

Outbound /unmanned: same 36 propellant module stack done 4 x 9, plus a NERVA module, and for the 200 ton dead-head payload, 20 propellant modules stacked 2 x parallel 10 modules long. That's the return propellant for the manned ship. That one is 295 m overall, same 580 tons at departure, and does not spin.

You will leave behind in LMO the two landers, one of the NERVA's, and the two 36-module propellant stacks that are empties. The hab+storage+the 20-module propellant stack, plus one of the NERVA's, comes all the way back to LEO for use again, if desired. That cluster is about 220 m long, and about 320 ton at departure for Earth. Hohmann again. It's about 100 m from cg to hab, so 3 rpm will do for one full gee artificial gravity, spun head-over-heels.

We could have done this, given a lander, in the 1980's except that they did not understand the zero-gee risks and radiation risks well enough back then (they probably would have neglected the artificial gravity and the shielding, guaranteed death/disability for a 2.7-year mission). By the 1990's, those risks were understood well enough to know that artificial gravity and some solar flare shielding was required. Period. No questions about either requirement. Again, given a lander, we could have done this in the 1990's.

And given a lander, we could have done it anytime since.

My other two points:

1. Artificial gravity is quite easy to do (!!!!!!) in any practical ship design. So there is no excuse not to do it. Provides safety, and makes all the life support jobs much easier (water, wastewater, cooking, ventilation, etc).

2. You can do this trip chemical, but the masses are very much larger. You'll have to stage off empty tanks between burns. NERVA (or a better nuke) makes a whale of a lot of sense.

Last point: the critical missing item all these decades is the lander. All the other tinkertoys have existed since the 1970's, and we have know how to design with them to those critical life support requirements since the 1990's.

I'm counting 4 Falcon-Heavies to send up the hab and storage modules, plus the two landers. There are 92 propellant modules , each of which could be launched by an Atlas-V 552 or by a Falcon-9. There are two NERVA modules, each of which could be launched by an Atlas-V 552, or both at once on a Falcon-Heavy. That's around 98 launches, most of which are $60-70M each retail. That's roughly $7 B in launch costs. Assuming launch costs are around 20% of program outlay, we're looking at near $35 B to send 4-6 men to Mars and make two landings while there. That's 2012 $.

GW

PS, if you were really smart, you would make the landers powered by NERVA's, and use them to push the return propellant supply to Mars. All 3 vehicle configurations get smaller when you do that. You get to pay less and make more landings while there.

Last edited by GW Johnson (2012-07-16 14:05:26)

NeoSM · 2012-07-16 20:19:11

GW Johnson wrote:

if you were really smart, you would make the landers powered by NERVA's, and use them to push the return propellant supply to Mars. All 3 vehicle configurations get smaller when you do that. You get to pay less and make more landings while there.

I'm in full agreement with you on the landers. A Mars lander powered by a NTR could obtain its own propellant for hops over the planet, or a boost back into Mars orbit via subsurface water depots. You could lower the overall cost for a return trip by refueling at mars. (Ofc this only works with NERVA designed engines - as mentioned above -, which would be orders of magnitude easier to refuel; trying to refuel chemical engines at mars without the infrastructure would be much harder)

Last edited by NeoSM (2012-07-16 20:30:04)

RobS · 2012-07-16 21:11:35

The only problem is that there's no facility on earth to re-develop a NERVA because it'd have to be enclosed to scrub any radioactivity from the exhaust (never mind there isn't practically any) and the fear/paranoia about nuclear engines would probably make it impossible to launch them into space. What about the use of inflatables?

NeoSM · 2012-07-16 21:50:30

RobS wrote:

What about the use of inflatables?

In terms of what exactly? If you mean inflatables inorder to land - currently, inflatable landing systems employ the approach of surrounding the lander with non-vented airbags where the lander bounces on impact a number of times until the impact energy is dissipated (this obviously wouldn't work for a human landing), and as i'm sure you know, there is also current testing on inflatable heat shields - NASA is launching one for testing on July 21st (I'm going to watch the launch as it's only 30mins away). Or do you mean inflating some sort of balloon structure while in the upper atmosphere using onboard hydrogen, then gradually releasing it to land? Does anyone know how much propellant it would take to inflate such a thing? You could take it a step forward and inflate yourself a large aerostat for more manueverability. Either way - it could be cost effective for landing depending on how it's set up, however it needs to be completely inflated and it would have to me massive - a hot air balloon sctucture would not work - but if it takes just as much hydrogen to inflate such a thing in the upper atmosphere as a NERVA would take to land, then I don't think it would be worth it because you would still sacrafice control with any inflatable (on mars anyways), unless we come up with a suitable air breathing propulsion system for it.

In terms of taking off theres still alot of problems asociated with getting it from its max altitude into orbit (rockets?), and then theres whether or not it would be able to carry a human payload up and dock with the "mother ship".

Last edited by NeoSM (2012-07-16 22:03:47)

RobS · 2012-07-16 22:02:53

No, I mean an inflatable hab for the interplanetary flight. They can be much larger and more comfortable and will fit in existing launch fairings. I gather you aren't planning to use aerobraking, right? It looks like you were talking about using a NERVA to Mars orbital insertion. If that is the case, an inflatable hab won't need a thermal protection system (though an inflatable heat shield appears possible, too).

NeoSM · 2012-07-16 22:18:48

Ah thats what you meant - makes a lot more since now, and correct on all accounts.

It's also a very good point - in the case with inflatable hab + storage modules, you wouldn't need all 98 launches that GW suggested; making it somewhat cheaper. If inflatable heat shield tech is perfected, then could aerocapture be used?(in conjunction with a rocket burn) as aerobraking at mars isn't efficient.

Last edited by NeoSM (2012-07-17 07:48:06)

RobS · 2012-07-16 22:21:20

Aerocapture into orbit from a Hohmann trajectory or a 6-month trajectory is not a problem, according to Zubrin's Case for Mars.

GW Johnson · 2012-07-17 07:38:52

Hey, I provoked a good run of conversation with that modular Mars mission post! (#226 above)! That's what it's all about.

I do think I need to re-run those calculations. I think I messed up reconciling available volume with contained mass for the LH2 propellant tanks. The concept is still "in the ballpark", though.

The inflatable habitat idea is just the thing. Bigelow comes to mind. I think it needs to be combined with two other things: (1) meteoroid "armor" made of alternating layers of foam and foil about 15 inches thick, and (2) the design constraint that access to the pressure shell be unobstructed for fast puncture repair.

Aerocapture at Mars is fine, once we've done it a time or two. Suspenders-and-belt thing. First time up, it might not work right. So, that should not be the manned trip. I'd prefer to stage the first exploration trip from orbit, anyway, so I could visit multiple sites on the one trip.

Still thinking about practical lander designs. I'm pretty sure retro thrust during the entry hypersonics will be needed. Not so sure a supersonic chute is worthwhile.

Testing nuke rockets down here requires a great deal of public education to undo all those decades of hysterical, nonsensical misinformation. I think a plume capture facility could be built (although hardly necessary), but it might be cheaper in the long run to test them on the moon. Good reason to go back there, actually.

I sure would like to see some real work done toward inflatable heat shields. Intriguing idea. Some sort of flexible elastomeric ablative is needed for the outer layer, I guess. Nifty idea for one-shot use.

GW

RobS · 2012-07-17 07:52:04

Bigelow inflatables have a lot of armor because there's so much junk in low Earth orbit. Does anyone know the danger in interplanetary space? I bet its one or two orders of magnitude lower than in LEO. It may be that it'd mass less to send two light inflatables than one armored one, and switch if there's a puncture.

Regarding aerobraking; since the entire system needs to be tested before you send humans, I'd send the first wave of vehicles without people to serve as backups, and aerobrake them. There's no reason to assume it won't work if there are satellites in orbit measuring the atmospheric density and if we use aerobraking starting in the next few years with smaller probes and work our way up.

Yes, the moon would work better, or even Earth orbit, but I suspect that in our modern political culture where fund raising is more important than the good of the country, there will always be hysteria about launching anything radioactive into orbit, because people can make money off the hysteria. The Constitution doesn't just protect speech; it protects lying. So I am not optimistic about a resumption of nuclear engine testing. In my Mars novel, gaseous core rockets were developed in Mars and the testing was done on Deimos!

NeoSM · 2012-07-17 08:11:52

GW Johnson wrote:

I do think I need to re-run those calculations. I think I messed up reconciling available volume with contained mass for the LH2 propellant tanks. The concept is still "in the ballpark", though.
GW

The 20 ton NERVA module you stated factors in the idea that it would have to be a bimodal NTR correct? So when it's not on a burn, it's also generating power (also keeping the NTR warm so it can be burned at any time, without having to start up a cold reactor to get moving - incase an emergency manuever is required ect.) I also think the weight can be decreased and lsp increased by taking up a particle bed design for the NTR (if we are intent on using proven, previously tested designs) - bringing the lsp up to around 1000 with a thrust:weight ratio of 30:1; this also allows the particle core to be dumped after full use, reducing the shielding required. Also, pebble bed reactors are inherently safe, active safety systems aren't needed as there is no active piping in the core (this would the case for a pebble/particle bed NTR (the core would spin, pushing the pebble-like fuel to the edges, allowing a clear path for the hydrogen)), and the cooling system uses a fireproof inert gas, making it impossible to explode (this would also help make a case for on-earth testing)- I imagine an active cooling system for the pebble-bed NTR's (even though a cooling system isn't really needed for a NTR as all waste heat is carried away - not the case for bimodal NTR, which would benefit from one), could be employed throughout the length of the ship, which could double as solar radiation protection... then again heat radiators would still be needed for a bimodal NTR to take away any excess heat (they wouldnt have to be that big) - (right? - they are needed anyways since humans generate alot of heat) adding to the weight - the particle bed NTR itself would be much lighter than the NERVA, so adding the heat radiators could possibly cancel it out. Then again you have to wonder the effect aerocapture has on heat radiators, and what interplanetary debris has on them - which brings us to the next point:

RobS wrote:

Bigelow inflatables have a lot of armor because there's so much junk in low Earth orbit. Does anyone know the danger in interplanetary space? I bet its one or two orders of magnitude lower than in LEO. It may be that it'd mass less to send two light inflatables than one armored one, and switch if there's a puncture.

There are alot of interplantary dust particles, from comets and asteriod collisions that could possibly be a problem, however I imagine it wouldn't be as bad as it is around earth (although interplanetary particles would be moving much faster than the space junk around earth). So I second this question "Does anyone know the danger in interplanetary space?"

Bigelows were designed only to function while in a planets magnetic field, so additional shielding is needed anyways to protect it from the added thermal stress of interplanetary flight, and cosmic radiation - however as you said they do have a lot of armor - much more protected than the ISS, still don't think it will be quite enough though; yet I could be wrong.

Last edited by NeoSM (2012-07-17 10:01:10)

Impaler · 2012-07-18 01:10:35

GW I recommend you start a new thread on your NTR based mission profile as it's not really on topic and it is getting some interest as you pointed out. I might make a counter proposal with SEP which is think blows NTR out of the water on both performance and feasibility grounds but I'll need to get your exact figures to make an apples-2-apples comparison. The Delta-V's are quite a bit different without Oberth effects as you know, can you break down your Delta-V values a bit further for me?

GW Johnson · 2012-07-18 01:37:33

OK, I revisited the calculations and did a bit of convergence here and there with a spreadsheet to help me out. I'll post that stuff over at "exrocketman" in the next day or two. Lots of figures and spreadsheet images. It'll take me a while just to get it done. Everything is built out of multiple examples of common modules for hab, supply storage, propellant and engine. The landers are just a mass allowance. I haven't added up the final launch costs, but it's 6 men, and two separate landings 3 men at at time.

I built up two vehicles in LEO, one manned, one not. The manned vehicle returns to LEO using propellant modules sent to LMO by the unmanned vehicle. Everything else gets left behind in LMO. The manned vehicle spins head-over-heels at 3 rpm for 1 full gee artificial gravity both ways. My Hohmann transfer delta-Vees are pretty crude overestimates, intended to cover plane changes and propellant boiloff: I made the vehicles capable of 8 km/sec (1-way) LEO to LMO. I used 8.5 months 1-way, and a 27 month round trip to estimate supplies. I was using about 15 kg food+water+makeup O2 per person per day, but that's just a wild guess.

I used the old NERVA data because that's the only design that ever got any significant testing, to the best of my knowledge. It was essentially flight-qualified when they shut it all down. There were other better ideas, but none got the massive testing that NERVA got, to the best I know. Any better nuke just makes things look better. My finished vehicles are closer to 800-something tons than the 580-ish I originally thought. The NERVA modules are smaller and lighter, too. I sized for 0.05 gee ignition acceleration or better, same engine all three configurations at the same thrust.

Can't test nuke engines in orbit, must test on a celestial body somewhere. Rocket development testing cannot be done hanging weightless in space where every test is a flight test. You must have a stable thrust stand that does not move. Fact of life. If we can't do it here at home, then the moon is close enough to reach without nuke power.

GW

orionblade · 2012-07-18 01:58:26

Would an aerospike engine configuration get around the into-the-wind issue?

GW Johnson · 2012-07-18 02:08:27

If you are referring to retro thrust during supersonic or hypersonic flight, then, no I don't think an aerospike would help provide directional stability to the plume. But cant angle would. Aerospike (and the other semi-free expansion designs) merely match expansion to a varying backpressure. Stability has to do with which way the plume turns when it reverses. That process induces moments and side loads on the vehicle. If the plume is flip-flopping around unsteadily, you could tumble the vehicle.

GW

orionblade · 2012-07-18 08:44:17

I was simply thinking the aerospike's spike/plug could be utilized to set up a bow shock, thus stabilizing the plume, or at least reducing the velocity of the incoming air to subsonic, at which point the exhaust plume would dominate the interaction, instead of the other way around.

NeoSM · 2012-07-18 08:48:31

Impaler wrote:

GW I recommend you start a new thread on your NTR based mission profile as it's not really on topic and it is getting some interest as you pointed out.

Agreed - this thread is supposed to be specifically addressing how to land - that being said, I find discussing GW's mission profile much more interesting.

Impaler wrote:

I might make a counter proposal with SEP which is think blows NTR out of the water on both performance and feasibility grounds but I'll need to get your exact figures to make an apples-2-apples comparison.

For a manned Mars mission? A manned SEP ship wouldn't be feasible for anything further out in the solar system, and in the inner solar system it would take too long to get anywhere; not to mention how heavy it would be - especially with all the extra radiation protection equipment it would have to have from the multi-day soak it would need to take in the Van Allen Belts, before it gained enough speed to break out of orbit.

Edit: didn't see there was another thread on SEP - however I would like to see some comparison stats addressing the concerns I mentioned, and how it compares to other propulsion methods. I want numbers!

GW Johnson wrote:

I used the old NERVA data because that's the only design that ever got any significant testing, to the best of my knowledge.

The pebble bed NTR project (Project Timberwind) ran for 6-8 years (mid 1980's - early 1990's). Various prototypes were under construction (that's why I used that design).

DoE nuclear/lh2 rocket engine. 2451.6 kN. Development ended 1992. Isp=1000s. Used on Timberwind launch vehicle.
Thrust (sl): 1,912.300 kN (429,902 lbf). Thrust (sl): 195,000 kgf. Engine: 8,300 kg (18,200 lb). Thrust to Weight Ratio: 30.1204819277108.

Status: Development ended 1992.
Unfuelled mass: 8,300 kg (18,200 lb).
Diameter: 8.70 m (28.50 ft).
Thrust: 2,451.60 kN (551,142 lbf).
Specific impulse: 1,000 s.
Specific impulse sea level: 780 s.
Burn time: 493 s.

You are correct though in that unlike NERVA, it wasn't flight ready.

Also...

GW Johnson wrote:

I used 8.5 months 1-way, and a 27 month round trip to estimate supplies. I was using about 15 kg food+water+makeup O2 per person per day, but that's just a wild guess.

This may help you with estimating consumables:
http://www.5596.org/cgi-bin/mission.php

Last edited by NeoSM (2012-07-19 07:05:33)

RobS · 2012-07-18 11:22:35

Here's your inflatable heat shield!

http://www.space-travel.com/reports/Inf … h_999.html

The article was put up today on spacedaily.com

orionblade · 2012-07-18 17:28:13

That test is going off about 80 miles west of where i live. I'm driving ~30 mins down to Virginia Beach (Chicks Beach to be more accurate) and I'll try and get some photos. If they turn out, i'll post here. Its ground track should take it almost directly overhead, beginning its reentry phase just out to sea.

This test goes to nearly twice the altitude and speed of the last test. Slowing 600kg down from mach 6 with a 10 foot diameter inflatable that packs into a 22 inch diameter canister... ain't too shabby.

orionblade · 2012-07-18 18:59:02

Also, regarding the Bigelow units... I've got some kevlar-based armor that's performing quite well against handgun rounds with only 4-6 layers of fabric, so we well might be able to shrink the foam-and-foil down by a solid factor of 2 or 3. I do, however, realize that meteorites have kinetic energies about 2 orders of magnitude higher than even a rifle round, but our technology is something that can be integrated into the structural/pressure shell material of the inflatable hab itself, resulting in a much more resilient hull, and that translates to lower weight, which in this case, is everything.

Another thought - if the first mission is unmanned, perhaps it could go nearly-fully-loaded, and provide something for the second mission to dock with, doubling Mars-orbit accomodations in the short term, prior to EDL of the components? Seems like over the course of two launches, you could have a space station in orbit around Mars that's larger, in terms of habitable volume, than the ISS, providing a long-term safe haven for astronauts, should things go wonky with surface systems - they can get up to the station, dock, and sit tight, and still do at least some science from orbit, while they wait for the third mission to provide a way back home (assuming ISPP can't cook up enough juice for the whole trip before they had to bail from the surface habs?)

GW Johnson · 2012-07-19 18:07:07

OK, as promised above, I refined that impromptu Mars mission study, and published it over at http://exrocketman.blogspot.com, as the article dated 7-19-12. The vehicle designs are "reasonable", and I accomplished my purpose of showing how easy it can be to incorporate 1 full gee of artificial spin gravity.

Scroll on down and I have posted articles on a simplified entry model, and good atmosphere data for Mars and Titan. There's some gravity data, too. Enjoy.

Now I can go back to my real project: roughing-out the design of some big but practical Mars landers.

GW

NeoSM · 2012-07-19 19:41:37

GW Johnson wrote:

I accomplished my purpose of showing how easy it can be to incorporate 1 full gee of artificial spin gravity.

First, I went over, and read through your mission design - very impressive; interested in seeing your lander designs (maby then we can keep the thread on-topic), any questions - please ask.

Second; I would have posted this as a reply on your site, however - was on my phone and didn't have time to create a typepad (or any of the other accounts needed to post) - i'll do that later when i'm at work and feeling lazy. (I think you'll find this of interest)

GW Johnson - Rough-Out Mars Mission with Artificial Gravity // exrocketman wrote:

All you need is a radius exceeding 56 m from the center of gravity of a long, slender orbit transport to the habitat at one end. The gee level achievable this way is 1 full gee at 56 m for a “fuzzy” maximum rotation rate speed limit of 4 rpm. Longer radii allow lower, even more tolerable, rotation speeds.

The research stating that anything over 3 RPM is intolerable for humans (some even getting sick at 3 RPM) is outdated (as i'm sure you know) - (in the trials, subjects went from 0 RPM to 3 RPM immediately), however, more recent research suggests that rotation rates up to 10 RPM are possible, with no adverse effects- As long as the RPMs are incrementally increased, to allow adequate adaptation.

http://www.ncbi.nlm.nih.gov/pubmed/1102 … t=Abstract

Small excerpt from the abstract:

"Early studies suggested that 3 rpm might be the upper limit because movement control and orientation were disrupted at higher velocities and motion sickness and chronic fatigue were persistent problems. Recent studies, however, are showing that, if the terminal velocity is achieved over a series of gradual steps and many body movements are made at each dwell velocity, then full adaptation of head, arm, and leg movements is possible. Rotation rates as high as 7.5-10 rpm are likely feasible. "

Last edited by NeoSM (2012-07-19 23:10:31)

Russel · 2012-07-21 01:44:42

Hi guys.. been away for a while.

When I originally posted I raised the issue of a trailed drag system for entry to Mars. The analogy I gave then was like a deep sea anchor.

You could call it a hypersonic parachute if you like, but calling it a heat shield doesn't work because its actually trailing behind the capsule.

What interests me about this idea is that there essentially nothing hard about the problem. You just need to come up with something that gives you the most drag for the least mass and is heat resistant enough. And more to the point, whatever you trail is by nature stable.

I'm sure you could get somewhere with ballutes that are basically an extension of the regular blunt-body heatshield. But there's a limitation on how big you can make it without running into stability issues. Whereas if you trail something you can in theory just keep adding more drag by extension.

The other consequence is that whatever you do that adds more drag, also lowers the requirements and thus the mass of the main heat shield that still has to protect the capsule.

Anyhow, I'm wondering if anyone here has given that some thought, or would like to. Its one of several technologies I'd like to see tested before it makes any sense to design a mission.

The others being being able to retro thrust at hypersonic speed and more attention to shielding. To be honest I see a lot of speculations in the "lets get to Mars" problem that are too focused on mass reduction when the majority of the mass problem is simply getting fuel into LEO and that's going to be solved as an economic issue anyhow.

Now, what I'd love to see is some clever ideas as the the form and materials you could use as a drag system. And also speculation on where its best used.

Oh and btw as I said a while back, I still choke on ideas that land people along with large masses. It just makes more sense to me to land people in smaller craft where you've got more margins.

Cheers

GW Johnson · 2012-07-21 09:52:42

Lessee, NeoSM post 246 has seen some actual studies that suggest up to 7.5-10 rpm may be tolerable in stages. The old 3 rpm figure was definitely too conservative. My 4 rpm figure was an impression, an educated guess, from the old centrifuge studies for pilot gee tolerance, plus experiences playing on a merry-go-round as a child. Nice to see it was roughly correct.

I'd also suggest my orbit-to-orbit transport designs are more appropriately included under the interplanetary transportation topic, or here under human missions as a separate thread.

That last one I posted over at "exrocketman" was more of a demonstrator for "easy" artificial gravity than a proper mission design, although the vehicles so sized resemble those I got for the paper at last year's convention. The key to artificial gravity is modular spacecraft design. That allows one to adjust the form factor at any given mass to get the radius you need to use 3-4 rpm for 1 full gee. With 1 gee, you arrive fit at both ends of the trip.

Russel post 247 has an interesting idea: the trailed drag item, like a long windsock or streamer. I have done a thing like that before, and supersonically, too. It worked, but it's not a lot of drag. There's more drag the longer it is, but not as much as you think, because it's immersed in the vehicle's wake, where there is a dynamic pressure deficit (that persists to very far distances behind the vehicle). What we did was grocery store grape bag netting, in tubular form, as the stabilizing drogue on a towed ribbon decoy. It worked in the wind tunnel to Mach 1.4. We switched to a ribbon chute drogue for packaging reasons.

For entry hypersonics, different forms and materials would be required. That wake zone is not only dynamic pressure-deficient, it is also incandescently hot. A stream of intensely-ionized plasma, as a matter of fact. I don't know what form or shape to use, but a long ribbon of something could be streamed out for kilometers behind. It would have to be made of some kind of ablating sacrificial material that was also very flexible.

Hmmmmm. I wonder if the inflatable heat shield cover fabric isn't canvas impregnated with something like DC-93-104, the silicone hard char-forming rubber? That kind of thing might work for a trailing ribbon, too.

Russel is probably right in these early days , in his preference for landing crew and cargo separately. That eases the entry vehicle design problem. It does incur the precision landing problem, which in Mars's super-thin atmosphere will require active guidance and steering during entry hypersonics. There is no time or cross-range capability left once the hypersonics are over, you are simply too low.

With enough propulsion, you can do entry retro thrust to terminate hypersonics at much higher altitudes. The low thrust level simply "offsets" some of the mass, making the ballistic coefficient "appear" lower. You're not going to do much of that with NTO-MMH at 280 sec Isp. But later on, with a nuke rocket in the 900-1000 sec Isp class, well, big single-stage fully reusable "landing boat" designs start to look really good. 60 ton vehicle, 6 ton dead-head payload, down and back up on one fill-up. That sort of thing.

I just wish we had a working nuke rocket that used water instead of LH2. Sure would make in-situ refueling easier.

GW

Last edited by GW Johnson (2012-07-21 09:56:43)

Russel · 2012-07-21 11:22:37

GW,

Where I actually came from was originally a design that put a large payload (30 tonnes) on an even larger platform. One that's relatively low mass per unit area and tends to become more porous as you head towards the edge (Think 30-40m dia). To that I needed to add a drogue for stability. Then the more I thought about it the more I thought that the whole thing is better placed behind the vehicle.

Thus evolved a concept that's less like a trailed line and more like a very large "racket". A structure with a stiff edge but between that something more akin to a fabric, again with deliberate porosity this time increasing toward the center.

That then became an inverted cone. So yes, something not unlike a parachute but obviously built out of high temp materials. And when we say high temp, you build it big enough you're then dealing with 1000C not 1500C.

That then begged the question, do you need just one.. or could you build a string of these things and trail them.

And of course hangs the question, mass for mass, is a simple trailed cable a good solution. And then of course there's a million variations, including inflatable parts. I wish I had the means to do the rough sums on this, but that's why I'm mentioning it here.

My gut feeling says it can be made and made out of existing materials, and be worth the mass. One thing I'm not sure about is how it will behave as you detach it and where its likely to land. Again the question has to be asked, could a simple cable then simply be reeled in. I don't know.

My take on retro thrust into a hypersonic stream, gut feeling says it has to be angled, and it has to be well controlled. Fortunately someone's already built the right experiment and that's the dragon. Once they test that with its full abort from orbit capability, we'll have enough data. However, my original take on this was to put the thrusters far enough out on their own jib so that the control system has enough of a time constant to work with. Whether that's too hairy an idea I don't know.

Of course one thing we do know is that you have to have to mix and match technologies. So the question is, would a simple drag system get you far enough to then enable retro thrust sooner.

GW Johnson · 2012-07-21 12:44:42

Russel:

What you describe as trailed bodies sounds sort of like a series of badminton birdies connected together as a linear string. These are flying in each other's wakes, in regions of ever-increasing dynamic pressure deficit as you look downstream.

Try it with actual badminton birdies connected with string, and film it with a high-quality camera. I think you will find on close analysis of the photos, there isn't much tension in the string. It'll likely go completely slack. In the limit of hypersonics, I think these things will want to rear-end each other, as only the lead "birdie" sees most of the drag.

You can offset the drag deficits looking downstream by going to extremely low mass. But, very lightweight structures usually cannot survive the heating. There's exceptions: that IRVE series of inflatable heat shields is one. They seem to have some sort of sealed Kevlar "inner tubes" as the inflation members, some sort of a low-density insulating layer, and what they describe as an elastomer-impregnated fabric surface layer as the actual ablative surface. I have no idea what the low density insulating layer is, and very little idea what the flexible ablative elastomer/fabric combination is.

IRVE is essentially all one big badminton "birdie" as an extended heat shield surface for a fixed mass, to lower the ballistic coefficient. To do your trailed thing this way, there must be some sort of size and mass distributions that put the higher-drag lower mass items further out to the rear. The only way I really see to do that is to make the ones further downstream bigger and bigger, yet lighter and lighter. That way, at least a part of the object pokes out into undisturbed slipstream at full dynamic pressure. That's the only way I can think of to keep tensions in all the towline segments.

So, your train of drag shapes is starting to look like a train of carefully sized inflatable cones, perhaps porous, with the larger ones further behind. Might work, but how one might actually build such a thing, I dunno.

GW

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