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#1 Re: Meta New Mars » The great crash » 2012-08-19 03:15:12

We are going to get those posts back? Cool! Thanks for the hard work! 'Cause a lot of hard work went into writing them, and there are many threads I miss greatly. Hopefully it will also help getting people back to the site.


Rune. Keep up the good work!

#2 Re: Human missions » Landing on Mars » 2012-08-16 11:52:41

I AM going to answer to this, GW (and I expect I'll say mostly good things), but I'm afraid it will have to wait until I get back to Madrid, my own computer, and maybe a couple of exams.


Rune. Just logged in to let you all know... I will be back! wink

#3 Human missions » Commercial Crew integrated Capability (CCiCap) » 2012-08-06 12:28:47

Rune
Replies: 13

I am very surprised this isn't here already... too few people posting, or have you all really missed it?

NASA announces winners in commercial crew competition

After an intense competition, NASA announced contracts Friday totaling up to $900 million to be divvied up between three companies -- SpaceX, Boeing and Sierra Nevada -- to continue development of commercial manned spacecraft to ferry astronauts to and from the International Space Station.

...

SpaceX was awarded a $440 million contract to continue development of a manned version of the company's Dragon cargo ship, which completed its first test flight to the International Space Station in May.

...

Boeing won a contract valued at $460 million to develop its CST-100 capsule, The spacecraft will seat up to seven astronauts and fly atop a United Launch Alliance Atlas 5 rocket. The CST-100 will make a parachute descent to a ground landing. Barring technical problems or budget issues, the first manned test flight is expected in 2016.

...

Sierra Nevada of was awarded a $212.5 million control to continue developing the winged "Dreamchaser," a small spaceplane originally developed by NASA as a space station lifeboat. The Dreamchaser lifting body will seat seven and launch stop an Atlas 5. Like the now-retired space shuttle, it would land on a runway at the end of a mission.

So Boeing and SpaceX get the full awards, Sierra Nevada gets the half. Also interesting to note, SpaceX are the only ones that have two full-up tests (pad abort and Max Q abort) as part of the contracted milestones. It seems they are really further along than anybody else. They are also promising a first test crewed flight in 2015, but that's not in the contract, so they really haven't commited to that. To get the juicy details, you can check out the slides they used on the conference here: http://www.nasa.gov/pdf/672130main_CCiC … cement.pdf

Now let's discuss the hell out of it! Most interesting news in all summer ('cause MSL working as expected is not really news, it's confirmation).


Rune. How long before the first ticket is sold to a private citizen? big_smile

#4 Re: Human missions » Landing on Mars » 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.

#5 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2012-08-04 12:49:07

...And I'm sorry to say, you committed similar conceptual errors in both. Like assuming doubling the thrust in the core and taking our the rigid heavy boosters the whole thing hangs from does nothing to the structural weight in this last Arianne case, or like working out numbers assuming a 1.2mT payload weight to come up with a 7.5mT payload in the Falcon example. At first sight your analysis are full of correct data, but you should take care how you add and subtract things, because making changes to a subsystem impacts the whole system.


Rune. Look twice, then give it to someone to double check. You will probably miss something anyhow, that's my philosophy.

#6 Re: Human missions » Landing on Mars » 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... smile

#7 Re: Human missions » Landing on Mars » 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! big_smile

#8 Re: Human missions » Comparative Mission Archetecture » 2012-07-22 05:18:31

Impaler wrote:

This thread of for comparing detailed mission plans focusing on Propulsion methods.  GW has produced a modular NTR based mission with spin based artificial gravity

http://exrocketman.blogspot.com/2012/07 … icial.html

And I'm going to do a SEP counter proposal in equal detail, but first I'm going to try to get the ground rules established to allow as complete and apples-2-apples comparison.  I've looked over GW's mission and extracted what I think are the criteria that need to be replicated.


* Launch constraints on volume and mass of Falcon Heavy (~50 mt), with option to use other EELV's.

* Trajectory from LEO to LMO and back again bringing the transit habitat back to LEO but leaving the lander mass in LMO for it's mission to be conducted from.

* Offload 120 mt of lander/surface payload modules at LMO massing 30 mt each.

* Provide 50 mt of Habitat module/s during transit (reusable) and 50 mt of Consumable filled module/s each for out and inbound transit.

* Transit times and total mission duration of 2-3 years with interplanetary transit times of 6-8 months

Good notion, we can move the discussion to where it's slightly less OT. When I finally stop partying like I am doing lately (holidays! ^^), I might take a stab at it and try to implement the various tricks to see the effects on each (by "tricks" I mean aerobrake, basing at high mars orbit instead of low, departing form a Lagrange point/HEO...).


Rune. I do have like 5 different threads open in tabs that I would like to write a reply to...

#9 Re: Interplanetary transportation » Reaction Engines » 2012-07-18 12:27:52

Just a quick de-lurk to post this, because it sounds quite important. Seems the folks at REL are finally taking their shot at getting the Skylon program started:

Europe’s Next-gen Rocket Design Competition Included Surprise Finalist

FARNBOROUGH, England — Astrium Space Transportation and OHB AG will lead two consortia to perform a design of a new heavy-lift launch vehicle for the European Space Agency (ESA) following a bidding competition that included a surprise third bidder in Reaction Engines Ltd. of Britain, ESA Director-General Jean-Jacques Dordain said here July 10.

The British bidder, a company that for more than a decade has been designing a spaceplane using a radical new engine design for atmospheric and orbital flight, was not selected for what ESA calls its New European Launch Service.

But ESA, whose Estec technology directorate in Noordwijk, Netherlands, has been monitoring Reaction Engines’ work for the past couple of years, was sufficiently impressed with the proposal to ask its launcher directorate to engage with the company starting this month.

Considering REL's only really equipped to deal with the heat exchangers and general design, and the actual effort would require a lot of additional contractors that could be spread geographically through ESA's member states, who knows, they may even have a shot. Rolls-Royce building the SABREs while the various EADS divisions design the airframe? And Astrium for the orbital systems so everybody is happy, except maybe the germans.


Rune. I know it's a beautiful impossibility, but I still wish they get the green light.

#10 Re: Human missions » International Space Station (ISS / Alpha) » 2012-07-06 07:35:41

JonClarke wrote:
Rune wrote:

And yet, historically, the price of boosters is about 30-50% of the total launch cost, no matter what launcher or payload you are talking about...

More like 5-30%.  Launch costs are largely a red herring.  It is the payload that costs.

With 5% for one-of-a-kind stuff like MSL and JWST, right? I said there would be exceptions. I'm referring more to the real space economy drivers, the ones that have to make some economical sense. The constellations in LEO and GEO, which is what 99% of space really is, these days, are launched by rockets costing 50-150 million bucks (though some estimates on the big deltas are closer to 250). The big GEO birds cost about a few hundred million, and LEO constellations, like GPS, are slightly cheaper (the best I can come up with for the cost of a single GPS sat is around 100-150 million), but they use cheaper rockets too.


Rune. Never forget, there have been a shitload of launches to date, in the thousands. Manned space and science is almost anecdotal, if not for the share of funding it takes.

#11 Pictures of Mars » Earth from ISS » 2012-07-05 05:43:55

Rune
Replies: 1

Not exactly Mars, but you can't argue it ain't pretty. 1080p video taken from the space station, as free as only government funded science can be, can be found here:

http://eol.jsc.nasa.gov/Videos/CrewEart … onsVideos/

It's even shorted by region, and you can download the pictures that make it! I hope you all enjoy it.


Rune. I am. And burning my DSL connection, too, trying to find my home in a night shot.

#12 Re: Human missions » International Space Station (ISS / Alpha) » 2012-07-05 05:22:59

Impaler wrote:

Come now GW you should be able to do your homework better then that.  At 1.5 Billion per Shuttle and assuming full utilization of 16 mt (largest segment shuttle ever delivered) and divided over 410 mt (approximate mass of US portion) is 25 launches.  Actual segment delivery shuttle launches totaled 27.  That totals only 40 Billion to LAUNCH the ISS, the reaming 60% of that 100 Billion price tag represents the actual cost of the segments and the systems that fill them and presumably crew training and other ground and logistical activities unrelated to the launch vehicle.

So sure would we have saved money by using an EELV to launch everything (or better yet the Proton) at 10% of the shuttles cost per pound, sure but it would only reduce the cost, by about a third of the total.  And of course their would also be the need to engineer each segment as an independent autonomous flying and docking ship as the Russians have always done with their station technology, their is some marginal cost in each segments performance in doing that (but it might still be worth it in the end for the vastly reduced EVA assembly hassle).

Saying that the ISS or ANY space-venture comes down in total cost in direct proportion to reduction in launch costs is patently absurd.

And yet, historically, the price of boosters is about 30-50% of the total launch cost, no matter what launcher or payload you are talking about (although I am sure you can find some exceptions, generally speaking it's like that). Even the shuttle lies somewhere in there, a you have shown (and I had my doubts, shuttle is such a unique case in almost every aspect). There is a reason, too: you are not going to waste ~150 million bucks to launch a cheap satellite built for a couple grand, you go back to the drawing board, and cram enough tech and engineering hours in there to see the price raise into the hundreds of millions.

Cheap launchers would be a very strong incentive to launch cheap payloads. There may not be any physical law that says so, and yet I believe this trend of boosters costing slightly less than payloads will be a constant as long as expendable rockets exist, just because "humans work that way", if you will. You could also argue it's an economics thing, probably, but that's not my field.

Of course, ground support is more or less a fixed cost, and it works quite differently compared with payloads or boosters. It is mostly a function of mission complexity, I would say, at least at first approximation. And yet I look at the "mission control in a trailer" SpaceX started with (before they tried to approach the ISS and their red tape shielding), compare it to routine shuttle operations, and I see that the overall... ¿cost climate, shall we say? also has a hand in there. If you don't a have billions of bucks to build a rocket, you don't bother designing mission controls employing hundreds of people.

As to the later comments on "saddles", totally in agreement, only I would argue the saddle function changes with the market size: when you are launching tens of tons a year, a 10mT launcher makes sense. When you are launching millions of tons a year... see where I'm going? There is an alternate universe where Sea Dragon is a standard-size rocket.

And a shame what happened to the CAM. Not that that experiment couldn't have been performed much cheaper, and probably better, with an independent free flyer, like the one the Mars society wanted to try with tethers and small animals. The micro-gravity scientists complain enough of vibrations on ISS already.


Rune. Is that thing still going forward, BTW?

#13 Re: Human missions » Is it OK to talk about the secret missions? » 2012-07-03 18:21:53

This is better than the script from Iron Sky. Maybe a bit more lacking in internal consistency, but who cares. Seriously good sci-fi comedy can come out of this, I'm telling you.


Rune. Let's keep this guy as the official mascot!

#14 Re: Human missions » Landing on Mars » 2012-07-02 14:00:23

RobS wrote:

Zubrin says somewhere in The Case for Mars that you do not need to despin to do mid course corrections; you just use a series of small bursts when the engines or verniers are pointed in the right direction. The inflatable he proposed for the Dragon would have 180 cubic meters, if I recall. NASA says a Mars trip needs at least 90 cubic meters per astronaut, so it would be large enough for them to move around. They'd retreat into the capsule for solar flares and perhaps to sleep.

Exactly. It may be hell to model by hand, being a vary elastic system, but that is one of the reasons we invented fancy computers, and have plenty of geeks trained to come up with fancy realtime control systems. The part I don't get is how expended expendables shield you while coming back to earth.

RobS wrote:

I was struck by his statement that a Dragon would have a terminal velocity without a parachute of 340 meters per second (that's about 750 mph). Does that match your calculations? I think he was using a mass of 8 tonnes and a heat shield diameter of 4 meters. That's 12 square meters of heat shield and a Beta of 670, I think.

I think he blundered here. Terminal velocity assumes an infinite atmosphere of uniform density. The atmosphere of Mars is anything but. Can you brake fast enough so you hit terminal speed before you hit the ground? The charts I've seen seem to indicate you really can't with a beta anywhere close to what a 11mT Dragon would have, but then again 340m/s is quite supersonic on Mars, so he seems to be obviating the issue of supersonic propulsion too.

IMO, if you double the injected payload (assemble payloads with their propulsion stages in high earth orbit from pairs of FH), then the margin stops being such a ridiculously low one, and the whole thing looks way more feasible.


Rune. Other than that, no problems with the idea wink

#15 Re: Science, Technology, and Astronomy » Nuclear airliners » 2012-06-29 07:41:41

That's a nice SR-71 trivia bit I never heard about either. The first thing that popped through my mind is that the old Blackbird would have made a hell of a delivery system for a KEV without any additional rocket stages. Would make a very responsive and cheap system to take out satellites in LEO, but I guess if you are worried about stopping salvos of ballistic missiles you would stick with missiles as delivery systems like they ended up doing with the upgraded Standards on Aegis destroyers (SM-3, is that the one?).

Terraformer wrote:

GW: at what Mach number does it make sense to transition to a Ramjet?

Well, a Ramjet will work (efficiently) from M>1.something to M~6, depending on design (a ramjet optimized for M2 flight won't do well at M6). Since a jet typically maxes out at about M2.5 (you can get faster, maybe 3.5 but you are pushing it and you will pay the price in T/W), I would say at about that speed, from M2.5-3 to M5-6, is when Ramjets make the most sense. Don't forget that to get a single engine to perform adequately over a big Mach range, you would have to use tricks like variable geometries, and those things complicate stuff, and you have to get into specifics to find out how they would turn out. But sometimes technological solutions complicate these analysis a lot. I'm thinking about SABRE, for example, where their magic heat exchanger changes the design parameters greatly (they make a freaking turbojet fly at M5.5!).


Rune. Sometimes I get surprised that armies make the rational decision when buying weapons. Sometimes. ^^'

#16 Re: Human missions » Landing on Mars » 2012-06-28 09:44:38

GW Johnson wrote:

(2) this url you are talking about,  could that be some sort of tag for the images I have been posting at exrocketman?  If I could figure out what those are,  could I use that link to make an illustration there appear here?

Exactly, it's the internet address for that file/page. You know, what appears in your address bar when you are looking at an image. For the images in your blog, it goes something like this:

http://1.bp.blogspot.com/-Cwfu4JCrSDA/T … rofile.bmp

... but it looks better when you put it between tags like this (check it out by quoting this post if you want to):

Mars+T+profile.bmp

You can get the address of any picture on the internet, not just yours, by right-clicking on it, clicking "see image" (or whatever looks similar in your browser) and copy-pasting the address of the picture that comes up. Oh, and .bmp is a very wasteful file archive. Paint or any other image program can change them into .jpg (or save them as such from the beggining), which takes much less data for each image and looks just as good.

GW Johnson wrote:

(1) you found better reports than I did in my simple google search.  Thanks.  I printed those out.  I think I can revise my Mars atmosphere model to be good enough to serve,  at least for rough feasibility calculations.  Based on a quick read-through of your links,  it appears the hypersonic deceleration at Mars is inadequate (you end up at M3 too low),  and the available aero-decelerator supersonics are way short of adequate,  for anything of a size and ballistic coefficient that we might really be interested in.    I'm thinking retro thrust all the way down really is the answer.  Low thrust hypersonics,  and high thrust supersonic to touchdown.  Aero-decelerators may or may not be feasible at all.

Yup, you either can get your beta in the vicinity of 150-200, or you can forget about chutes, that's also my take. Inflatable heatshields might take us there, but then you can get very low betas, and then you don't need chutes. If you have to take a very big ballistic coefficient... yes, then you just have to go with rockets all the way. But of course, I want to see numbers before I say some method is the best!

So int he end the lesson I take from all of this is that supersonic retropropulsion and inflatable heatshield should both be two very high-priority technology developments. Which they aren't right now.


Rune. Methinks that, at some level, someone doesn't want to run out of excuses not to go.

#17 Re: Science, Technology, and Astronomy » Nuclear airliners » 2012-06-27 03:54:56

There are arcjets running on hyrogen, and there are arcjects running on ammonia. Being an electrical engine, they can run on any fluid suitable to be heated by electrical discharge. You could call them thermal electric engines if you prefer. They use hydrazine now, I guess, to run monopropellant RCS from the same tanks, or for fuel commonality, or for some other similar reason (like hydrazine breaking down into hydrogen and ammonia on the engine through the use of catalysts). The figure of merit for propellant is molecular weight: the lower it is, the higher the isp and the lower the thrust.

As to fancy nuclear reactors, fancy chemical engines get to T/W about 150, as SpaceX has just shown with the Merlin 1D. Show me the NTR that, even on paper, gets close. I remember Timberwind as being about 30. Add the airbreathing gizmos, and see T/W decrease again.


Rune. Not really a fast reactor either, I think... that means fast neutrons are dominant, not anything about the energy density of the core (which on Timberwind is incredibly high, BTW).

#18 Re: Science, Technology, and Astronomy » Nuclear airliners » 2012-06-26 17:28:38

Terraformer wrote:

I started this by asking if you are ready to accept a nuclear airliner. I guess the answer is no. That means you will experience ever increasing air fare, and a dramatic jump when the US federal government no longer pays for airports.

Not necessarily. I can easily accept nuclear power on the ground, and manufacturing renewable fuels for airliners using the energy. As long as the total cost of manufacturing the jet fuel remains the same as it does to extract and refine it now, the airfares shouldn't increase.

I was going to reply to say that, but it would be repetitive now. So +1. As to flying fast, why would you want scramjets if you have a nuclear thermal engine: run it with atmospheric air until there is either not enough or it's coming too fast (/too hot) if you wanna get fancy, then switch to onboard propellant. Or just use a VTOVL NTR, Isp is enough. As to arcjets, they run on anything but have very bulky power needs. Nowhere near T/W>1.


Rune. I doubt safe NTR's can approach chemical engines' power to weight ratios anyway.

#19 Re: Interplanetary transportation » Falcon 1 & Falcon 9 » 2012-06-26 10:32:17

Merlin 1D completes full mission duration test fire.

Hawthorne, CA – Space Exploration Technologies (SpaceX) announces that its Merlin 1D engine has achieved a full mission duration firing and multiple restarts at target thrust and specific impulse (Isp).

The engine firing was for 185 seconds with 147,000 pounds of thrust, the full duration and power required for a Falcon 9 rocket launch. The tests took place at SpaceX's rocket development facility in McGregor, Texas.

"This is another important milestone in our efforts to push the boundaries of space technology," said SpaceX CEO and Chief Designer Elon Musk. "With the Merlin 1D powering the Falcon 9 and Falcon Heavy rockets, SpaceX will be capable of carrying a full range of payloads to orbit."

Wonder what the new payload turns out to be. I imagine they didn't have in mind creating a 150 vacuum T/W engine when they first posted the payloads on their site. BTW, biggest T/W ever, and biggest mass ratio ever, all for one of the cheapest first stages to develop ever. Impressive, most impressive.


Rune. Video on the link.

#20 Re: Human missions » Landing on Mars » 2012-06-26 10:18:44

RobS wrote:

If the rest were argon, which doesn't create the bends, one could use lower pressure counterpressure suits as well and have easy in and out access.

Just one quick comment more... that is an awesome idea! Not many people look into that kind of problems, and having to wait every time you do an EVA like in ISS is horribly time-consuming, and risky.

#21 Re: Human missions » Landing on Mars » 2012-06-26 10:10:42

GW Johnson wrote:

I used the data from the Glenn Research Center links you sent me to put together a sort of model of the Mars atmosphere for entry purposes.  Extending the Glenn model to 100+ km altitudes is not very good,  because the lapse rate is most likely wrong way up there.  But the pressure is probably ballpark correct,  and the density profile I got is conservatively too dense.  Below around 28 km altitudes,  it's probably not too bad a model.

I've had a look, and from that inconsistency you found with temperature I assume my help wasn't that great. Try looking at the atmospheric model here:

http://www.ssdl.gatech.edu/papers/confe … 6-0076.pdf

The charts are already made, and especially the one with mach numbers looks very right to me (page 7). For more stuff on reentry (I'm documenting myself, so why shouldn't you all share the results of my google-fu):

Sizing of an Entry, Descent, and Landing System for Human Mars Exploration

Atmospheric Environments for Entry Descent and Landing (not limited to Mars)

The silly NASA charts, useful for the references, mainly.


GW Johnson wrote:

I am still defeated by the notions of posting illustrations on this forum.  I know those links you gave me are in English,  but I do not share a dictionary with the writers of those instructions.  So,  I posted the model with the curves I got over at my "exrocketman" site:  http://exrocketman.blogspot.com

And I am determined to make you learn, whether you want to or not! wink Here is a very Inception-like example:

thisisatest.jpg
The url in the middle of the image is a popular image hosting site I in no way endorse, but still use 'cause it's the first I remembered off and had an account. In this case (and you can check out how I made this post by cliking the quote button, in the following screen you will see a plain text version of what I wrote between quote tags, [ quote] and [/ quote], so you can check out with a little time and the preview tool how I did each trick I used here, from named links to pictures). The main thing is, change the url in the image for any url (also called "link") that takes you to a picture, in this case: "http://img24.imageshack.us/img24/5959/thisisatest.jpg", and when written between the [img ] and [/img ] tags, the forum will display it as an image (I've added a couple spaces to foul up the tags and make them visible in this case).

...get it now? I know I explain myself like shit, but it really isn't anything complicated... and the "preview" button can catch any mistakes you made, like leaving open a tag.


Rune. This is the informational post. It required, by the way,  several dozen tags opened on firefox, more than a few minutes of google, opening of paint, and waiting for the forgotten password to my image account to arrive to my mail, so I'll leave the opinions for some other time, k? I am still in exams after all...

#22 Re: Human missions » Landing on Mars » 2012-06-26 09:09:29

RobS wrote:

Where pressurizing space is concerned, keep in mind that a pressurized space is like an upside down suspension bridge. If you are pressurizing to full atmospheric pressure, the upward force is 10 tonnes per square meter. If you are using 1/10th atmosphere pure oxygen, it's 1 tonne per square meter. All that upward force has to be opposed through the edges of the dome, so you need some huge piles of regolith or, more likely, pile-driven pylons frozen into place to anchor the dome.

It's cool weight works the other direction and you can balance both, then. Imagine an arbitrarily big hole sealed with a 25mT/m2 (10/0.4=25) homogeneous cover, pressurized at 1atm and... net load on the structure is zero and you can use bubblegum to build it, no matter the size. wink


Rune. Totally off-topic, too, but I'll get back on it on the next post... I've got a lot of stuff to reply to. big_smile

#23 Re: Human missions » Landing on Mars » 2012-06-24 09:35:08

Re: RGClark:
Exactly the inflatable test I was thinking about. And yeah, that seems to be the way to go for manned payload: big non-rigid heatshield to get beta under control, and terminal rocketry. Precision would also be greatly increased, since both methods provide active steering. The big source of error on reentries is the time falling on chutes, when you are at the mercy of wind.

As to vehicles that are designed for lifting off again from the surface... they pretty much have all they need already, right? Including low densities, since they are mostly tank for the ride back up. They could brake harder and longer than required for landing, just because they have to lift off afterwards. Well, unless they have to land the ascent fuel, then things change, but I don't think that is the smarter way to go about it.

In summary... I would correct you a bit: we have a pretty good idea as to how to go about it. Only no one has put real money to prove or disprove the current ideas.

Re: Matthew Heins:

Welcome! You have the right attitude smile

Re: Louis & Space nut

As to "go with what we have now"... like Impaler says, what we have now is approximately capable of landing one a half tonnes (MSL). Building the ISS for 25mT and 5mT chunks took a crew of three the better part of 10 years, with long stretches of time unmanned at first, and very frequent resupplies. Not to mention a shitload of EVA's and constant telerobotic operation of most of the station systems form the ground. Just to put things in perspective, there are just not enough man-hours in a whole martian campaign with several missions to set up all the stuff the first mission will require to be operational as soon as they land or shortly afterwards. That is why mission planners want to land one-piece independent habitats that can work as a standalone through the entire mission and do so from the minute it touches down, and that is why that mass kind of drives the landing system requirements. Oh, and most landers/ascent vehicles capable to handling the crew are approximately the same weight, at least within the same order of magnitude, so the EDL requirements are quite similar too. The ballpark is about 50mT at entry, it may be half if you are an optimist, or twice that if you are a pessimist, but that is the approximate order of magnitude of all I've seen. It is also way beyond the capabilities we have "right now". But it wouldn't be too hard to improve those capabilities, if a serious effort was initiated.


Rune. MSL essentially copies technology derived form Apollo after all (Viking), with a fancy LIDAR on top and no landing struts to call it new.

#24 Re: Human missions » Landing on Mars » 2012-06-22 07:35:56

TwinBeam wrote:

Assuming a L/D of 0.2, a capsule starts falling when drag force goes below about 2g's (4m/s^2 / 0.2 = 20m/s^2) - no point using rocket before that, as you get free lift to stay in the air and get free braking.
Assuming a 30T, 8m diameter vehicle, and getting low enough that air density is around 0.01kg/m^3, at 2g's you'd still be clipping along at about 1430m/s.
If you're going to rocket decelerate at constant 2g's beyond that point, you need another 72 seconds with average rocket deceleration of 1g (+ avg 1g aerobraking) to kill horizontal velocity - about 720m/s deltaV equivalent. 
Assuming an average of 2m/ss falling acceleration (4m/ss minus average 2m/ss aerodynamic lift), you'd be heading down at 144m/s if you did no vertical braking.  So rocket braking at 2g (net 1.6g after gravity, 16m/ss), you'd need to slow the fall for another 9 seconds - 180m/s more.  Total 900m/s effective delta-V. 

If on the other hand you use the rocket purely for lift from 2g drag (20m/ss) down to 0.4g (4m/ss), you'll have shed all but 450m/s horizontal velocity, or 980m/s shed before it's cheaper to use the rocket to brake.   Average aerodynamic lift of 0.2*(12m/ss avg drag accel) = 2.4m/ss average lift, so average of 1.6m/ss rocket acceleration needed to stay at a fixed altitude.  With average of 12m/ss avg deceleration, that's 980/12 = 82sec at 1.6m/ss for 130m/s rocket deltaV.  Then rocket brakes at 1.8g avg (2g with 0.2g avg drag) for another 23sec - 400m/s deltaV.  And it's falling at 90m/s after that, so another 5.6sec at 2g (net 1.6g) to kill that, for 110m/s more deltaV.  Total net of 640m/s rocket effective deltaV.

Or about 30% less rocket acceleration required.

First, to vindicate GW and me a little bit, I'll say your idea is not really at odds with our comments: you turn on the engines when you can't get enough "umph" form the air alone. Having said that, I love it when someone shows me numbers to support their idea, and the result is not what I expected. Going by gut feeling, I would have said you were just paying bigger gravity loses, but apparently I would have been wrong.

As to the scenario you have painted in particular, that is close to 600 beta, so the descent trajectory is going to look different than the ones I got my hands at. I assume you got the terminal velocity from drag and density? If so, what altitude-density is that? 'Cause you have to have enough space and G's to slow down from there, and if you don't have the room to do it in vertical, that is more gravity losses ('cause a vertical descent takes the least time, and gravity losses are a function of time, but I have a feeling you know all this already). Also, consider that if the landing craft is meant to be refueled and launched (it's a classic MAV with ISRU on the surface), asking 10G's or even more out of it when it is mostly empty of fuel is not really that much. Changing the G's changes the analysis you did back there a bit, but I don't think much.


Rune. The more I think about it, the more useless the chutes look.

#25 Re: Human missions » Landing on Mars » 2012-06-21 18:37:04

Sorry if it sound like I'm teasing you but... have you read the link I provided? There's you rational way of getting a big surface for aero-deceleration, and it cuts the useful payload of an Ares V to 60mT apart from the massive, heat-shielded shroud, which masses 50mT in itself. As to shape, that only affects L/D, not the ballistic coefficient (well, it's not the dominant factor, to be more precise, Cd also plays a part), so it makes no difference one way or the other, and a delta wing is more wasteful of structural weight than a conic section close to a circle. Just... trust me, as long as you don't intend to fly, just decelerate in a controlled trajectory, then you are better served with more-or-less oval shapes. There's a reason they are the ones used and suggested.

As to sidemount configurations, while they may make a lot of sense for clustering first stages around second stages (like Angara, Delta IV, Falcon Heavy... they must be in fashion), they are not so nice for payloads. Too close to the business end, and it works very bad from a structural standpoint  (all those torques on the primary structure take weight to handle).

Parashields /inflatable heatshields may be an ever better solution than shrouds, with regards to weight fractions. Or so their researchers claim, at any rate. I would put money in certifying both for interplanetary reentry speeds so at least one pans out and we can skip shroud limitations completely, while potentially adding reusability. I think some inflatable test has actually flown, but at much lower suborbital velocities.


Rune. Sadly, no real money goes to the useful stuff being developed... this problem first appeared decades ago, and the solutions suggested then, I'm sure.

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