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#76 Re: Human missions » Landing on Mars » 2012-12-27 05:34:19
Hi Josh.. had a Merry and all too fattening Xmas 
Ok, lets step back a little and look at some assumptions and in particular the technology we may or may not rely upon.
Clearly, if we started with the technology that is available today, we'd either conclude we just don't know how to do it (particularly EDL) or we'd have to resort to some pretty heavy handed (and costly) techniques. Those that do say we can fly today, with today's technology are either making assumptions about what technology we can develop (ISPP etc) or openly declaring "this is forward work".
There are two technologies or strictly speaking capabilities I am fairly confident will be developed. One is the capability to launch into low Earth orbit at relatively lost cost ($1000-$1500/Kg). The other is the capability to launch and relaunch from Earth multiple times. Indeed, the two are actually sides of the same coin. Elon Musk figured this out and has promised both. And I think he will succeed, albeit I think he's going to prang a few rockets in the process. If he doesn't even I'll be impressed.
Right now he's testing his "grasshopper" because I think the hardest problem to solve is simply getting the thing to fly downwards and land itself. There is the related problem that Musk has, and that's getting rocket engines to operate over and over again, reliably, and in a way that is more like regular aircraft, with relatively few people on the ground doing the maintenance. Of all the things he has to do in order to get re-usability, I think the machinery is the least worst of his problems. And by the way, he's trying to routinely re-fly rocket machinery involving turbo pumps and the works. If you wee designing something that can be re-fired at Mars I doubt you'd go for that level of complexity.
So to put it bluntly, verifying that your hardware is re-usable on Mars means a visual examination, a systems check and a test fire. On Earth you may want to do other tests and measurements but then you're also probably dealing with much more complicated machinery.
Now, all of this is presumptive Josh. It all presumes something as if it exists now. We don't know. But we probably will in a time frame short enough that when it comes to someone actually flying to Mars, these capabilities probably will exist.
Another thing. If we get to the point where it is relatively cheap to send fuel into low Earth orbit, that is a game changer. A thousand tonnes of fuel yes? A billion dollars. Still a fraction of the overall development cost. Lets go in style. Lets not even worry about ISPP and instead send as much fuel as needed into orbit and simply brute force the problem. I really don't mind. And I'd like to cover this more later. Of course, its one thing to have cheap fuel but in general its not ok to waste it. But throwing more fuel at safety is a good idea in my book. In short you'd probably end up with an architecture that simply parks a bunch of fully fueled (methane/LOX) propulsion modules in Mars orbit and just use them as needed. Anyhow, that's going down another rabbit hole.
Now back to Mars landing/ascent vehicles. I'm not asking a vehicle to be re-usable dozens of times, I'm asking it to be re-usable at most a dozen times, and probably less than that if your basis of comparison is 3 missions. In a future where Mr. Musk does succeed in re-using his rockets dozens of times (and they will subjected to all kinds of stresses) we will probably know if my guess is right or not.
If not, you can blow me a rasberry 
"Your plan is to send two personnel ascent vehicles and two Fuel Ascent Vehicles to Martian orbit and leave one set there as a safety factor for every mission. Fully fueled? Or would the crew have to wait to refuel it, miss the launch window, and then return two years later? With what supplies will they last? How do you plan to store the hydrogen (yes, Hydrogen... Why are you recommending bringing methane? Insanity)"
Let me clarify this.
There are permutations but you can choose between the two basic forms.
One form is a landing/ascent vehicle that runs on CO/LOX sized to deliver oxygen to the transit vehicle and capable of docking with a manned capsule. In this form we start with two such vehicles. One remains as a fuel ferry and the other remains "permanently" docked to the capsule. In theory you could remove the capsule from one and transfer it to the other but yes that's logistically a last-ditch procedure. One advantage of this configuration is that the capsule would be capable of landing in its own right. So you have the capability of surviving failure of the ascent.
Correct me if I'm wrong, but does Mars direct have this capability or do they figure "it just has to fly" ?
Another form is more specialized vehicles optimized for scale. One is crew specialized. The other is pure fuel ferry. Benefit being commonality of design if not structure. You would have to consider how to make the crew version more robust in terms of failures on ascent but it could be made more survivable than an "all or nothing" alternative. In this case you could go belts and braces and simply deliver a second crew vehicle. That's probably overkill but the extra mass is small compared to the overall mission.
In any case its not 2 of each.
As to the next question. You always have a second fueled vehicle capable of crewed ascent into orbit ready before your launch window. And just to be specific I'll break it down for you into the possible cases.
Case 1: You've got yourself two nearly identical landing/ascent vehicles. One is your fuel ferry and the other is notionally your crew vehicle. Before the first manned launch you've arrived at a situation where both of these vehicles are fully fueled and the orbiting transit vehicle is also fully fueled - time is on your side here. The crew arrive, do their thing on Mars, and also check out the crewed ascent vehicle. It is found to be flawed. What do you do? Ok, that depends on the nature of the failure. Could be anything from avionics - in which case you cannibalize your fuel ferry (or more likely you have some spares. It could be that it has a major structural weakness. The last-ditch resort in this case would be to use the resources at your disposal. And this probably means first of all arranging a platform for the capsule to be undocked onto. Then you command the fuel ferry to make a short flight, unmanned, so that it parks itself over the capsule. Then using simple technology you remount the capsule to the fuel ferry. Now, that's pretty out-there, but its a capability you don't have with other architectures.
Case 2: You've got yourself two specialized landing ascent vehicles. No more. In this case you've got one crewed vehicle only. In this case before the first manned launch that vehicle is fully fueled and tested (flown a short distance). When the crew arrives you again have the opportunity for further systems checks and inspection. Again, in some cases you have the opportunity for cannibalization. But in this case you focus more heavily on spare parts and even some limited repair capabilities. Yes of course the same things are available in most cases in other architectures, but the nice thing here is you have a relatively light vehicle and its not quite so costly to test fly it.
There are orbit and trajectory options that give you a few days if not a couple of weeks to recover. Most likely what you'd want to do is to progress to a highly eccentric orbit a week out of your optimal launch window and from there you have similar options that you have from the surface.
Case 3: As above but now you've brought a spare crewed vehicle. My preferred option here is to have the spare vehicle fully fueled before the first manned launch from Earth. Depending on your risk analysis it might be ok to defer fueling the second vehicle whilst the crew is on the surface.
In option 1 an abort from ascent is relatively straightforward and you could recover.
In option 3 an abort from ascent depends on the nature of the problem and the robustness of your design. But assuming you can abort recovery is very straight forward.
In option 2 an abort from ascent an abort from ascent means you've a limited time to fix the problem (and transfer fuel) or else you're on short rations. That's why I'd avoid it, but its here for completeness. In short my original idea (option 1) has its attractions - not the least of which is every vehicle is identical.
Its not clear what your reference to hydrogen entails.
To clarify the landing/ascent vehicles are all CO/LOX powered. That entails on their very first landing there has to be some fuel that is shipped from Earth. Not a big deal. After that its over to ISPP.
The transit vehicle is methane/LOX fueled. The methane always comes from Earth. But the bulk of the mass (oxygen) is supplied from Mars. Its precisely because of the hydrogen storage problem that I've arrived at these two fuels.
As for the last question. If I were attempting to save 10 tonnes I'd say it aint worth it. Obviously I'm attempting to save hundreds of tonnes of mass (LEO). That occurs over 3 missions. I'm using that baseline because firstly NASA is targeting that. Secondly *anyone* would be crazy not to think in terms of at least 3 missions, all costs considered, and thirdly I think 3 missions is about right in that time and lessons learned will eventually make whatever technology you started with redundant.
Obviously cheaper fuel into low Earth orbit is a game changer here, but for us to make any progress on this we need an agreed basis of comparison. What masses we're actually sending. How many missions etc. Then we can look into both re-usability and overall costs.
Anyhow I'll throw this back to you.
Several things bug me about Mars direct and other related missions.
Firstly, there is a limit to how well you can check out the ERV (or its equivalent). Its got to work and its not clear to me what happens if faults develop that you simply cannot test for - especially if you have no option to test-fire, either because of the sheer scale, or because what you've designed has limited cycles built into it be design. There is a reason why Earth ascent vehicles have abort capability. They designed it out of the space shuttle and that wasn't such a good idea, and I'd be loathed to do without it on Mars.
Secondly, I really don't like options where you land people inside something that weighs several tens of tonnes. The reason is simple. Given that what fixes the ballistics is the size you can launch from Earth, something that weighs under 10 tonnes is probably going to get you to around Mach 3 at a much higher altitude than something weighing tens of tonnes - even if you resort to landing belly first (multi-conic) NASA has at least given that some thought. Mars direct are still squibbing it a bit.
In the end given the choice, I'd rather land in a simple capsule with more heat shield area relative to mass, than in a much bigger structure that ultimately has to push the limits harder.
I can handle aerocapture of a crew inside a "space-hab" or "transit-vehicle" more easily than I can do that
Thirdly. The criteria of minimal launches from Earth is something that gets people into all kinds of strife. We either end up with brand new super-heavy launchers or we end up cutting corners on various systems. the first is ugly and expensive. The latter is taking all kinds of risks. I'd rather both NASA and the private groups put a whole lot more thought into this.
Fourthly. I've avoided producing methane on Mars. For the simple reason that if you want something complex to fail in unexpected ways, its a chemistry lab operating on another planet. For this reason, I'm not all *that* keen on simply mining oxygen either - and I'll get to that later -. But if you *have* to rely on Martian resources, its oxygen that is the heavy bit. Both Mars Direct, and NASA, and others have underestimated in my view the amount of time it will take to demonstrate the technology and I do believe that it should be demonstrated, with people present but where its not essential to the mission.
Like I said before.. cheap fuel... reusability.. and ISPP. All of these have yet to be developed and tested. So what we're doing now is playing what-ifs. Allow me some latitude since I also extend that to others who also plan things based on stuff that has yet to be proven.
Cheers
May not be here till after the New Year hangover.. have fun
#77 Re: Human missions » Landing on Mars » 2012-12-25 01:40:14
The ERV has more payload than both your Fuel Ascent Vehicle and your Personnel Ascent Vehicle. However, even ignoring the reusability issues of your architecture Mars Direct (and derivatives) have a major benefit that yours doesn't: Fewer rockets and fewer transfers. Each engine firing is a chance for something to go wrong. When your crew and all of your payload are separate in two rockets, the failure of either rocket results in the death of the crew. This approximately doubles the chance of a loss of crew (technically, it is [1-(chance of one rocket succeeding) x (chance of another rocket succeeding)] ). The use of two rockets where one can be used is done not once but twice in your architecture, both in sending things to Mars and in launching things from Mars to LMO. While you try to compensate by suggesting that the fuel rocket could carry crew to orbit (without suggesting a way to mount the capsule atop the rocket), separating the rockets does not enhance crew safety because without fuel for return the crew is also dead.
When considering safety issues you have to consider the time scale and sequence. If for instance you were to climb into a rocket designed to launch from Mars surface and go directly to Earth, you'd have to be pretty sure that one rocket will work. Of course you can build in redundancy and even testing - but something of that scale is a little hard to test fire.
If you stage it and instead rely upon two rockets to get you home - one to Mars orbit - then of course you've got two vehicles you have to rely upon. But its a bit more complex here.
Firstly the vehicle parked in orbit has a twin. A fully fueled and provisioned twin. Its a consequence of re-usability that you can afford to do this. In the worst case scenario you've got yourself a few days to fall back onto the spare. Likewise with the ascent vehicle. The reason I am still fond of a vehicle with commonality of design is that either your fuel ferry can be a spare or can be sacrificed or cannibalized in other ways.
Now, if you really, truly, want belts and braces you have 3 vehicles. One ferry, two crew. Again, assuming re-usability over a number of missions that's still a huge saving of fuel in LEO terms. I don't know if the extra spare is really needed. But the point here is that by the time you've climbed into you ascent vehicle you have had the opportunity (2 years on the surface) to verify/inspect/service these vehicles.
Also, being smaller vehicles, and vehicles that are better suited to repeat soft landings its not as difficult to test fire them either. Yes, you can test fire a "direct to Earth" counterpart but that begs the same questions about reliable repeat firings, doesn't it.
So in short, time is on your side when on Mars. Use it.
Now, going back to the amount of fuel. What we need here is some simple, albeit rough basis of comparison.
What I'd suggest is that if we were to cast aside all thoughts about mass that has to be landed on Mars, and will stay there, that will simplify the comparison.
But I'll grant you that the landing/ascent vehicles that I'm talking about do need to get there, at least once. So we can go into more detail about their mass and and the required fuel.
One other wrinkle here is this. What's going on in the background is the thought I discussed many pages back, and that is if you can draw a line between the mass you need to send to Mars just once, and the mass that you have to send with your crew, you then have the freedom to think about propulsion options.
For instance, its not impossible that you could send all the heavy plant and equipment (the Mars, hab, the landing/ascent vehicles, rovers.. etc.. etc) in the most fuel efficient (but slow) manner you can think of. Think as an extreme example, ion propulsion.
But even better, think about a relatively small nuclear plant - a MW or so. Useless for a fast crew mission, but very efficient for just this purpose.
(And there's lots of other alternatives in the pipeline)
If you can solve the problem of minimizing fuel for the bulky/heavy non manned part of the mission, then you're onto a winner because you have more freedom to send more mass and thus you can afford to add things that in short add up to more comfort and more safety.
Having done that, you're then left with the crewed vehicle. And again my approach (as you might see if you read back a number of pages) was to avoid having un-necessary mass travelling with the crew - and that's where not having to travel with your lander comes in. So I'm basically down to the transit vehicle and its modest propulsion system - again you don't have to blast your way out of the galaxy, you just need a few KN of thrust. Again, mass saving.
As a rough guide, we're talking about 20 tonnes of transit vehicle, give or take, all up. We can argue the toss about he exact amount but lets have a fair basis of comparison.
But at that mass level, and with methane/LOX you basically need to double your mass with fuel to get back from Mars orbit to Earth. (Assuming aerocapture, which this thing has to be designed for).
Oh and btw.. other missions that assume aerocapture end up with a lot of extra baggage which then simply adds to the task of successful aerocapture, and your mass starts climbing - rather like a bad car design
So, and again we can haggle, 20 tonnes of fuel equates to roughly 4.5 tonnes of methane (the oxygen comes from Mars).
Going back a step you've arrived in Mars orbit with a vehicle weighing (its not quite empty) about 27 tonnes.
Going to Mars we'll assume a delta V of about say 4.5Km/s (I could have cheated and boosted this thing to a high Earth orbit first but lets keep it simple). Again feel free to haggle over the numbers.
So a mass ratio of 3.2 and total vehicle mass of 86 tonnes. About 60 tonnes of fuel, of which about 13 tonnes if methane, not including the 4.5 tonnes you need to keep in Mars orbit.
That's the rough starting point. And again, you can easily fiddle with that by assuming more exotic things like a higher Earth orbit, solar powered tugs etc.
Anyhow.. I guess if we go further we need some basic assumed masses etc. So feel free
Xmas dinner! cya!
#78 Re: Human missions » Landing on Mars » 2012-12-25 00:59:27
To address what is, I think, the most glaring error in your post:
Whether [multiple launches of a Mars Ascent Vehicle are] a reliability issue or not has yet to be established.
While you later suggested that each mission would only involve one launch and one entry, subsequent missions would reuse the same craft. So what you would have is a six month flight to Mars, followed by a long period of stasis (either in LMO or on the Martian surface). There will be an atmospheric entry at some point, followed by a 4 km/s launch. After two years of stasis, repeat. Two more years of stasis, repeat again. With an LMO orbital period of two hours that's hundreds or thousands of thermal cycles for each rocket, followed by a high thrust launch (even though the Martian gravity is lower, it's still pretty high thrust and high powered). After this launch is an aeroentry followed by rocket braking and vertical soft landing.
Your quite correct in saying that it would be challenging to design something that unattended could last for years - particularly long periods of stasis.
But, your assumptions here are wrong.
Firstly these vehicles don't spend more than a fraction of their lives in orbit. Most of the time they're parked on the surface.
Secondly, they're not unattended. They will be inspected and if needs be serviced. Just like anything else we design here on Earth that is intended for years of service.
Even the fuel ferry will have the opportunity for inspection and service.
Its the same with the transit vehicle. Its not going to be re-usable without some level of human intervention. But re-usability is an inevitability.
Yes, you can make the transit vehicle a throw away item. But the price to be paid is having to loft it with every mission and the consequences of that is you get a bunch of people finding ways to mass reduce and the inevitable consequences.
#79 Re: Human missions » Landing on Mars » 2012-12-25 00:29:57
Josh,
Just for starters, here is my starting point regarding CO fuel.
http://ntrs.nasa.gov/archive/nasa/casi. … 014990.pdf
Which gives you an Isp of around 260 for a pressure fed engine, and 290 for a pump fed engine.
I've assumed the lower figure for most of my calculations. That gives a mass ratio of just over 5 for ascent to Mars orbit.
I'm not, in case this has been confused, using CO as a fuel for anything other than the crew lander and fuel ferry.
My preferred fuel for the transit vehicle is methane/LOX.
I'll get onto the other issues once I've gotten through this pile of chocolate..
#80 Re: Human missions » Landing on Mars » 2012-12-24 06:16:30
Mery Xmas from the land of oz and may the new year bring you a damn good lander design!
#81 Re: Human missions » Landing on Mars » 2012-12-22 06:01:05
Josh,
Just to clear some confusions here. The germ of this thought experiment is to see how to refuel a transit vehicle in Mars orbit, such that that vehicle can do multiple trips from Earth to Mars and back. The next step was basically to design a fuel ferry for the purpose. Next I came across the same conclusion you do, that a vehicle that is scaled to transport a crew from Mars surface to Mars orbit would have to perform multiple trips in order to transport enough fuel. At one stage I figured as many as 6 trips for fuel. Certainly at least 3.
Whether that's a reliability issue or not has yet to be established. However, what I did next was to say, ok, lets build a vehicle large enough to transport fuel in one go. Then use the same type of vehicle to also transport crew from Mars surface into Mars orbit. Essentially in order to be a fuel ferry it is over-specified for a crewed ascent vehicle. That in itself is not a bad thing, and may give you needed overhead if what you're considering is safe abort from any stage.
I should add that we're talking about the same type of vehicle and not necessarily the same vehicle. I'm coming to the conclusion that inevitably it will specialize.
The real question is how much of the design can be common. But for the sake of the exercise I proceeded with the idea that at most we have one distinct type of vehicle, either docked to a capsule, or not. The crewed variant has a "permanently" docked capsule. The non crewed variant doesn't, but is able to fill its oxygen tank fully.
I hope that clears some potential confusion. Behind this is some thought being given to potential permutations. What if, for instance, you're simply interested in commonality of engine components? Well, then you specialize. You have one vehicle that is set up purely as a fuel ferry. And another purely set up as a crew landing/ascent vehicle. That has its advantages too.
For instance, if you discovered that inflatable heat shields were a viable technology you could design that into the crewed variant. Now you're in a position where you always have a crew around to do the necessary repacking/adjustment/cleaning.
The non crewed variant would then take a slightly riskier, faster, deeper approach and rely more heavily on a powered landing. But this would mean you could in theory avoid anything but a standard capsule shape - provided GW is correct and you can thrust through openings in the heat shield and you can get the lift correct. I'll add to this the thought that you might be able to create lift simply using a small amount of asymmetrical thrust from the point of interface.
The only reason I've been talking about more advanced configurations above (multiple pods, wings etc) is simply to push the edges of what can be done to bring the peak temperature down. But there's nothing stopping you building a heat shield for the fuel ferry from existing known technology that will survive many (perhaps a dozen or more) re-entries. Certainly you'd be worried about other kinds of ageing before the heat shield is exhausted.
Now, going back to the "all in one" design. It goes something like this. You've always got at least 2 of these vehicles either in orbit or on the surface. When configured as a crewed vehicle it is docked to a capsule. When its configured as a fuel ferry, its not docked to the capsule. So in the normal course of events you simply leave the capsule docked to only one of the vehicles.
The reason I left it detachable at all was simply a nod to the idea that under adverse situations you might want a form of abort where the capsule itself is free to land on its own.
On a normal landing the capsule remains attached and stays there for later use. Now, in orbit you may want to undock the capsule, swing it around and then dock it to the transit vehicle.
Of course your logistics get harder if for instance one landing/ascent vehicle fails. But at least then you still have options. I'll be happy to go into the details if you wish.
As for launching fuel and crew in one rocket. I've no problem with that since its just an extension of the same concept. Of course, there's things to worry about then too and one big issue is that now the dangers of in orbit refueling are occurring whilst there is a crew around. My mission design includes a spare transit vehicle so I can afford to take more risks with the hardware whilst reducing risks for the crew.
The difficulties in robotic refueling are there, but they are second order issues. And its something we have to get used to, and learn I suspect for other missions nearer to home. However, what I've done to simplify things is I'm simply transporting oxygen - which carries the bulk of the weight of the fuel. I'm also in a position to restock the transit vehicle with breathing oxygen at this point.
As for ballutes, towed drag etc. That's not essential. But I like the idea of something you can adjust at the last moment. Otherwise aerobraking is still a risky business. As for stability, well, its true that so long as the center of drag is behind (from the reference frame of motion) the center of mass, it will definitely work in terms of causing drag. How stable it is, on average, is a good question. We will have to test these things well, before having crew along for the ride.
Bit of history here. Where I started from I actually took a landing capsule along for the ride, along with the transit vehicle. That way you could plan to aerocapture the capsule separately from the main vehicle. Reason being its likely to be lighter relative to surface area to start with, and have more stability and have more control elements. That was then. I'm not sure now. For now I'm just as happy to let the crew aerocapture whilst inside the transit vehicle, but then give the transit vehicle some more safety features.
Stepping back from the landing problem, the overall mission design looks like this..
First step is to land on Mars anything you need there that is going to stay there. I'm going to gloss over this because aside from low thrust trajectories, ion drives etc you've basically got a shed load of gear to ship and that's the same in any mission, almost.
Second step is you deliver into Mars orbit one transit vehicle and two landing/ascent vehicles. Lets suppose for sake of this that these vehicles are specialized.
The fuel ferry descends and takes on fuel. The crewed landing/ascent vehicle is left in orbit and verified as functional. The fuel ferry ascends and delivers fuel to the transit vehicle and the transit vehicle is checked out. The fuel ferry returns the surface and beings to reload. So all of these systems are available and functional before you commit to launching crew. We're also assuming that the technology has been flown in a previous non manned mission.
Third step is to send the actual crew. This starts in Earth orbit with a second transit vehicle with Earth supplied methane/lox. On arrival at Mars, the crew transfer to the crewed landing/ascent vehicle and land on Mars.
During their stay on Mars, the fuel ferry delivers its next load of oxygen to the transit vehicle that arrived with the crew. So now you have a fully fueled spare.
At the end of the stay, the crew use the crewed landing/ascent vehicle to return to the original transit vehicle. If for any reason there is a problem, there is a backup, and spare supplies on the other transit vehicle.
Fourth step is the return to Earth and aerobraking. We then need an Earth return capsule which has been delivered into orbit previously. So another straight forward transfer.
I think you can see how this cycle repeats. The features of this design are that anything that doesn't land on Mars and stay on mars is reusable for a number of missions.
The transit vehicle requires the minimum mass for its trans Mars and trans Earth engine. It doesn't have an attached capsule so that is also a mass saving. Instead the crewed Mars landing/ascent vehicle remains on or near Mars, and likewise the earth return capsule has a similar life cycle.
That's a significant saving, because a lot of designs attach the Mars lander to each and every trans-Mars journey. And I'm happy to go through them with you one at a time and point out their shortcomings when it comes to mass/energy/cost. Point is, if you optimise well, you then have spare mass you can devote to crew safety and comfort. Believe me, one of the things I see going wrong with some mission designs is they consider the threats from externalities and hardware and then create another threat from the crew itself - bored, stressed, tired etc.
#82 Re: Human missions » Landing on Mars » 2012-12-21 07:54:07
I'd heard about IRVE, too. That will lead somewhere, but it's a long way from a usable technology. Give it time.
Reducing heating at low ballistic coefficient by decelerating higher up is a known phenomenon. To do it with a spaceplane requires a wing loading not unlike a Piper Cub. That's about 8 pounds per sq. ft, or about 40 kg/sq.m. A capsule shape would have to have a ballistic coefficient in that same range. That's an awful long way away from where we are right now in construction techniques. However, that's a place the inflatables could take us, once developed and "wrung-out".
Interestingly enough, it just might be possible to build a spaceplane with a wing loading that low, by building it the same way a Piper Cub is built: steel tube truss frame, covered by ceramic fiber (instead of linen) fabric. There are a host of problems to solve, such as low-conductivity stand-offs between the fabric and the tubing, and a gaseous atmosphere inside to be at least part of the heat sink. Yet the fabric can easily survive reusably at 2300 F ( 1260 C), and doesn't melt in one-shot use (it does embrittle, though) at 3200 F (1760 C). This is the same alumino-silicate fire curtain cloth they have long used in modern aircraft engine nacelles. I used it myself as the reinforcement in a very low-density ceramic composite liner for a ramjet, about 3 decades ago.
The nose cap, windscreen, and leading edges might need a more sophisticated treatment, who knows? But it's an intriguing idea, and most of the tinkertoys are already in place to try it. It'll take someone really innovative like an XCOR or a Spacex to try it, because it's so counterintuitive, otherwise.
Speaking of XCOR, take a good close look at the engines they offer. Some of these have the same safety and lifetime characteristics as ordinary certified aircraft engines. We're talking thousands of burns and thousands of hours of burn, here. With minimal repair/refurbishment. That's precisely the kind of engine you need for any reusable spacecraft. I think you might be surprised to learn that the best of these, already used in real aircraft, do not use turbopumps as the propellant pumps. Another really good tinkertoy available, that.
GW
You may have the freedom to design a low enough ballistic coefficient (and more importantly a high enough lift to drag) that the temperatures can be such that you don't need to protect the frame of your wing. Instead it quickly assumes the same temperature as the skin.
The point I'm making is that if your "wing" (which can be all kinds of shapes so long as its light and has the required lift) can be separate from the main part of the structure which then contains the things that actually need more thermal protection. Indeed the structural element that joins the wing to the main body itself provides a long enough thermal path that the rest is easy.
I've yet to take these notions and come up with something elegant. At least something elegant, and stable.
As far as strength goes we can take advantage of two things. First, modern aircraft wings are strong precisely because they can flex. The other is that the forces you're dealing with are still smaller relative to a landing on Earth.
Is there a link to those engines? As I understand it, without turbopumps you pay a penalty in terms of less Isp or more fuel. One other thought. Thinking back about cooling and nozzles that are behind a heat shield, are there any good known engine designs that also use the propellant to cool the nozzle?
Oh and I'd be more excited about taking the piper cub idea literally if I could figure out a way to do vertical take off and landing with it and manage to package all the extra tankage you need to turn it into an ascent vehicle too.
#83 Re: Human missions » Landing on Mars » 2012-12-20 04:32:25
Josh,
I've been following IRVE for a while and I'm glad they're getting somewhere. And I'd really, really love to combine inflatable with fully reusable and robotic.
GW is quite right about being able to land large payloads from low mars orbit and go directly to a powered landing. There's still some second order problems - getting the lift to drag right, and the packaging of the rocket nozzles and heat shield(s).
The open question for me though is whether some inflatable elements are worth the extra weight - they might be.
To me there are 2 places inflatables might be a good idea.
One is when you want to land something on Mars that is going to just stay there - one way trip items.
The other is supplemental drag for aerobraking. In this case the inflatable part would be consumable and you'd be in a position to replace it when you return to Earth orbit. Nice thing about inflatables for aerobraking is that again, you capture more drag higher up so a Km or so error isn't going to make as big a difference as it would further down. Also with inflatables you get to "trim" your aerobraking.
Going back to solid heat shields, I don't see why you can't capture the benefit of lower ballistic coefficient and thus lower heating through having a lighter and thinner heatshield. As I understand it, provided you keep below certain temperatures there is effectively no ablation at all. This is one of the underlying assumptions in building something big (surface area wise), even if it has to be assembled. Its to do what GW is suggesting and that's come in as slow and shallow as possible and avoid extremes of temperature even if the extra mass of superstructure seems at first glance non-optimal.
GW,
I'm aware of the impinging shock behind the craft and that's why I've moved away from a center tow line or tow structure towards an arrangement where the superstructure goes through the stream sideways at several points, before the stream gets to converge. Yes, I'm aware that that that's possibly the weakest point. However the more drag you can create the cooler (in relative terms) is the stream of gas you're talking about.
As far as having a trailing ballute, or a trailing anything for that matter, because of what you speak of, what you need basically is a donut.
Leaving aside nuclear solutions, if you simply built something that was a classic capsule shape with thrusters firing through 10 degree ports, how many would you have to position in order to provide enough redundancy? My feeling is somewhere between 8 and 12 in order to retain navigability with 2 or possibly 3 engine failures.
Here's another thing to keep in mind. Something that is designed to carry upwards of 20 tonnes (or more) of propellant cargo into orbit has a lot of margin left over when its only transporting 5 tonnes or so of crew and capsule.
As far as fuel goes, I'm quite comfortable with just plain O2 and CO. Both are reasonably dense.
As far as building something robust. Its not the frame I'm worried about, its the reusability of the rocket engines themselves given long periods of inactivity that bothers me. Again, this is what engineering is for. Besides you don't cut corners when you don't need to. You just burn more fuel. And since CO is reasonably "cheap" to procure that's not an issue.
In fact, would I be game enough to send a nuclear power source to Mars, I wouldn't use it as a primary power plant for a rocket. Rather I'd rely upon having lots of available energy to manufacture lots of spare propellant, making lots of problems easier - including having a more robust vehicle. And now we've got so much spare fuel we can afford to use it for longer distance trips around the planet.
Remember also that when you're thinking about inert mass for a mars landing/ascent/ferry vehicle one thing that counts in your favour is that if you have one, you then have a much lighter space hab.
Oh and btw. One thing that I have up my sleeve is this. If you build a Mars ascent vehicle with the tankage to act as a fuel ferry and its less than half loaded with crew on board, you could if you really wanted to use it as a boost vehicle, getting the space hab out of low mars orbit and up into a higher orbit. All of that and still being able to re-land itself after.
#84 Re: Human missions » Landing on Mars » 2012-12-19 09:00:12
Russel-
With regards to a Mars ascent vehicle (MAV), many mission architectures don't call for one at all. Mars Direct is the most obvious of these; It calls for an Earth Return vehicle (ERV) that launches from the surface of Mars.
Further, just because a heatshield is inflatable does not mean that it is not reusable. In fact, there is really no reason why an inflatable heatshield that was intended to be reusable would be any different from a solid one, so long as it was deflatable, which by nature anything inflatable is.
Having said that, it is considered a major plus to the safety of any mission that there are two vehicles to get back to Earth if it is found that the first one has failed. Mars Direct plans to have the second mission's ERV fully fueled and ready by the time the first mission would choose to leave the planet. Reusing a MAV eliminates this safety factor and also makes each mission less safe than the last. There is no orbital rocket on Earth that can be reused in the way you're suggesting; Though a Martian orbital launch vehicle will have an easier time of it than a Terran one, I don't think that reuse of an MAV is a safe choice. Please remember that the use of an MAV as opposed to an ERV mostly eliminates the potential benefits to be gleaned from producing the propellant on Mars.
If we're talking about standard, solid heatshields and launch vehicles with standard fairings, delta-wings would presumably be a problem. However, an inflatable delta wing heatshield is still very possible.
I've never liked the idea of a direct return purely because the mass of whatever it is that keeps your crew alive - the "space hab" function is a mass you now have to land on Mars, and then get off Mars again. Doesn't make sense to me. From what I've seen of mars direct, this also led to a design where they had to cut corners on the return vehicle. A bit ugly really.
So far as I can tell, there's only really two classes that fits every possibility. Either a) you take off directly from Mars so you go "direct", or some or all of you return vehicle is in Mars orbit. Correct me if I've missed something.
I'm focused on getting the crew down onto Mars in a manner that is as safe as possible and has as much margin/redundancy as possible. That goal before all else.
Linked to that is the concept that if you're designing a mars ascent vehicle, why not also design technology that has a dual purpose. One of those is to get your crew into some kind of orbit. The other is to get fuel to your return vehicle. Amongst other factors, what you find is that the level of thrust you need for Mars ascent is far greater than the level of thrust you need for your return vehicle if its going to start from orbit. Therefore a lot of the mass involved in the ascent vehicle would be dead weight if it returned to Earth.
This leads me to a strong feeling that this element of the overall mission, if not the space-hab itself should be fully reusable.
It also happens that if you develop the engines needed for a Mars ascent vehicle for crew use, you've already done the work you need to build a reliable fuel ferry too.
I'd dearly love for someone to show me an inflatable heat shield that is re-inflatable - even multiple times. And one that can deal with the obvious practicalities. Either de-inflating it during descent, or else having to deal with it landing on the martian dirt and all the practical headaches that come with that.
Besides, I've also got the constraint that it must be fully automatic, because if I want to use inflatable structures they have to work on a fully robotic craft. Which is why I've toyed with fixed, but lightweight structures, including mesh.. even thinking about stuff that folds/unfolds.
Again, if someone can show me an inflatable heat shield that's reusable a number of times and avoids the obvious pitfalls I'd really love to see it. The furthest I've gotten down that path is a somewhat rigid drag-inducing structure above the craft, that also can then use inflatable technology. Just for the sake of a concrete example, picture a ring frame that attaches to the top of the capsule, the diameter of which is much larger than the slipstream of the capsule, and from that ring you inflate separate "balloons". The purpose of this whole exercise is to take a ring that has a diameter of about 8m and expand on it so that it has an effective diameter of say 14m - with a hole in the middle through which the superheated slipstream passes. Point is that if you can get it to deinflate properly at least it won't touch the ground. Anyhow that's the idea, but making it stow neatly and making it work again and again, is a toughy..
Stepping back to the issue of safety.
What I'm thinking is actually more redundant than you'd first guess. Firstly I'm parking a fully refueled space-hab return vehicle in Mars orbit. So when the crew arrives, they're actually parking a spare. Secondly, the ascent vehicle isn't one of a kind. There are at least two. Third the crew have the opportunity to both inspect and test the ascent vehicle. Fourth, the way I'd have it there would be full abort capability all the way to orbit.
As always when you consider the sum of all risks there are inevitable trade offs. I personally don't like direct entry to Mars. I don't like missions where the safety of the crew is reduced byt eh need to land the crew along with a large mass - I think they should always come first and have their own descent vehicle. And I don't like missions that suffer from redundant mass in places which means ultimately less food, less oxygen and less shielding.
I'm not sure what you mean by "the use of an MAV as opposed to an ERV mostly eliminates the potential benefits to be gleaned from producing the propellant on Mars."
When you do the math, both a direct ERV and an indirect MAV benefit from mars produced propellant. The major difference is that whatever you park in orbit that will fly home with you is mass you don't have to land and mass you don't have to get back into orbit. That's a substantial fuel saving all else being equal.
As for an inflatable delta wing - or inflatable aerfoils of some description. Not an entirely silly idea. If you can draw me a picture of the basic engineering that would help. As it stands I'm more comfortable with inflatable elements that trail - for stability reasons. You know, one suggestion put to me is simply a swing wing but the mechanism is simply driven by a simple gas cylinder. That could be fast enough. Even crazier ideas involve being able to use thrust into the airstream from engines hidden by a heatshield that are able to be retracted by similar means.
#85 Re: Human missions » Landing on Mars » 2012-12-16 17:13:10
Josh,
Clearly some things (like a Mars hab) are one way trip items. But others like the Mars ascent vehicle should be highly re-usable. So where I'm being led is the thought that if you're going to get satisfaction out of an inflatable heat shield its going to be in landing (and aerobraking) very large and otherwise one-way-trip things like the Mars habitat.
So the picture I keep seeing is a trailed inflatable simply there to buy you more altitude by the time you can go into powered descent.
As far as smaller, reusable craft is concerned. I think it would be a huge advantage to have a versatile, reusable craft that can do things like ascend into orbit, or travel large distances.
I keep thinking of going back to old fashioned glider shaped wings. Which could lead to a quite laid back, if lengthy descent - with temperatures down to a few hundred degrees. The trick of course is just enough thrust in the right places to get you vertical take off and landing.
Having a delta wing, especially for something potentially large enough to return to orbit around Mars, basically walks you back into the territory of having to do something special to get it off Earth. Mind you if the shuttle can do this...
#86 Re: Human missions » Landing on Mars » 2012-12-16 07:13:15
I'm quite deliberately taking advantage of the lower dynamic pressure in this situation. And I'm not particularly worried about stability. Trust in the computer and all that. After all, if the spacex people think they can take a regular cigar tube rocket and flip it over, control it while it goes through reentry, then flip it over again and land on its engine, I'd say that pretty soon the problem will be well understood
One could imagine other form factors that would tick the same boxes but part of the original motivation is to fly something with low(er) mass to drag than would be possible given the commonly assumed maximum dimensions. Remember also that the interconnecting truss isn't entirely dead weight since it contributes to drag, along with its own light weight heat shielding.
The point of having 3 or 4 "pods" and a common truss is that you've got commonality of design. Each "pod" can be lofted separately and its a relatively simple assembly task.
Remember also that in its Mars-landing configuration you're flying with relatively little fuel - that is relative to the thrust available. That fact also contributes to the lower mass to drag. Its different of course if you have just taken off from Mars with a full fuel load. Depend on the abort situation you might find yourself having to deliberately burn off fuel at some point. Again, the capsule itself is detachable. And for the moment I feel more comfortable with the capsule as a separate, flyable entity. Maybe as the concept develops we could reduce mass and make the capsule part of the overall structure. If so you then don't actually need a conventional "capsule" shape for the crew compartment.
Indeed, I'm still working on that idea. I just need to be sure the whole thing has "abort from anywhere" capability.
There are of course alternatives to the upside-down configuration, and those include thrust-through-hole ideas. Some of those revolve (literally) around a gimballed engine/nozzle arrangement. So your "pods" rotate into the air stream. This has the advantage of being able to start off thrusting at an angle. And I'm sure you don't need 180 degrees to do this.
Remember also I'm trying to control the drag on the ascent too so that makes this more complex. Also I'm still giving some thought to being able to move the center of mass around in flight.
The one thing I'm most comfortable with though is the inherent modularity involved.
Oh and btw, even a four "pod" craft probably requires at least 3 engines per pod for the sake of engine-out capability. arranging those into a heat shield and forming a complete seal is a challenge in itself. You've also got the problem of which bits of your engine you want radiatively cooled. And that might leave you in a situation where the nozzle has to project. That and having more than one nozzle is the original motivation for the upside-down configuration.
And yes, speaking of thrust-through-shield I have considered the option of actually losing a little propellant (some liquid CO will do the trick) to both protect the engine and also to make life a little easier for the heat shield. But again you have to worry about cooling the engine in use.
Now going back to the overall mission design.
What I'd like to say is that unless someone can prove me terribly wrong, it seems to me that the whole problem of capturing propellant from Mars itself ignores that if you're using something like methane/LOX for the return journey, about three quarters of the mass you're trying to save in doing ISPP is actually oxygen.
Seems to me far easier to generate oxygen and ferry it than to go down the route of methane production on the surface. I'm using CO as a fuel for the ferry/landing/ascent vehicle which is "wasteful" but that doesn't matter that much because there's plenty of it to go around.
Here's the basic math. To make a tonne of propellant for the return journey (assuming methane/LOX) you need to make 777Kg of oxygen, and 222Kg of methane.
To make that methane you need 55.5Kg of hydrogen. (plus overhead for storage or boil off)
On a arguable typical return journey you need 40 tonnes of propellant and thus 2.2 tonnes of seed hydrogen. But, you must first land that hydrogen. However you do that you must pay for it in energy/weight. And allow for boil off. Conservatively you're getting closer to 4 tonnes of hydrogen/gear in Mars orbit. Perhaps more by the time you've landed the extra weight involved in the synthesis gear.
But instead, you could just send the methane along. The methane would have weighed 8.88 tonnes. So now you're talking about a modest saving, for a large increase in risk/complexity.
Now using CO as a propellant has its advantages/disadvantages too. But you'll probably find that the extra mass of propellant you need to manufacture at Mars (to get the ferry/ascent stage into orbit) is mostly balanced out by the extra weight/complexity/energy costs of managing hydrogen and methane synthesis.
So I'm actually quite happy with this approach. Now if someone comes up with some cheap way of getting hydrogen/ice off Mars, that changes everything.
There is actually one more wrinkle to this. And that's the option of using much less hydrogen and synthesising something that is mostly CO but with some methane - arguably worth the extra Isp but with the density benefits of a fuel that is mostly CO. I'll leave that to the guys who get paid for this
Point is, it wouldn't be a bad idea to design engines with the intent of at least running a mixture. Heck if you could design an engine that runs on any mixture from CO up to pure methane you're making things even simpler, but that isn't necessary or I suspect pragmatic.
Now, going a little further into, shall we say, half baked territory.
Lets suppose I develop the concept further with a crew capsule that integrates into the structure that bridges the "pods". Now you've saved more mass but you've also created something that lives its life out entirely between Mars surface and Mars orbit. It would also by its nature be fairly flat and have a low center of gravity (remember the size of the heat shields would be fairly large relative to the size/height of tankage needed). What I'm saying here is now the design leaves out a crew capsule as an element in the trans Mars and trans Earth journeys.
Now what do we have left? Well its pretty simple. Ignoring for a moment the Mars habitat and related gear, you've basically got a space hab that lives in space and has aerobraking capability.
So your mission looks like this. From Earth you ascend in a dragon capsule (no need to design anything new here). Dock with the space hab which is already in the correct orbit and head towards Mars. At Mars you perform aerobraking into a low orbit. From there you dock with the Mars ascent/descent vehicle, transfer crew and undock, then land. And of course the same in reverse. And back in Earth orbit you transfer to the dragon and return home. Its pretty simple and your space hab has the minimum of mass.
Bear this in mind though. This means that you've got no option but to aerobrake with the space hab. You can't climb into a capsule and take an alternate descent path. This may be a good or bad thing depending on your view of the risks of aerobraking versus direct. All I'm saying is if you refine the design of the ascent/descent vehicle to the point where the crew capsule is integral, that's what you get in return. And oh the other thing that motivated me towards having a crew capsule that is separate and detachable is that when this vehicle is acting as a fuel ferry you need it as light as possible.
One other idea I'll toss in here. The space hab does not need a large engine. But to make the most of the fuel one trick you can use is that you start with a relatively high orbit and then only thrust as you approach periapsis. This of course means spending a few days acquiring the velocity needed. Same thing for Mars.
Another related trick is that while the vehicle is unmanned, and you have spare solar power, you can also employ a small ion engine and gradually raise the orbit in advance of the crew.
So, lots to mull over there
#87 Re: Human missions » Landing on Mars » 2012-12-04 05:25:33
Hey. I think I've got a solution to all of this. And it involves thinking upside down.
Start with a conventional heat shield in the order of 4-7m diameter with the heat bearing surface facing downward.
On top of that heat shield place tankage for O2 and CO. Roughly 25 tonnes of propellant.
On top of the tankage place a cluster (2 to 3) of rocket engines, with the nozzles pointed up.
Extend a support leg, again pointed upwards.
So in normal atmospheric entry the heat shield protects the tanks and engines above it.
Take four of these, and place them into a square configuration with some distance between them.
Build a truss structure that binds the four into one overall vehicle.
Now, take a conventional crew capsule (some variant of the Dragon is fine). Place it with its heat shield pointed downwards (the conventional orientation).
Attach its docking port at the top, to the center of the X shaped truss formed above (just the center for now, we can refine this).
Now you've got a formation of four rocket engines and one crew capsule bound by a truss. Now you've got he combined drag of all 5 units.
Ok, for the fun part. After the majority of heating has passed, you perform a 180 degree flip using a small amount of asymmetric thrust.
Now your heat shield are pointed up (and your capsule is upside down relative to where you entered).
The next step is fairly obvious - powered descent, landing on the four landing legs that were originally oriented up.
This now solves a number of problems. Least of which is you can use very well understood technology and there's a minimum of in-space assembly.
There are a couple of minor problems to be solved, but are not hard problems.
One is creating lift to drag. The simplest and most robust is to mount the crew capsule off center. A more complex mechanism might allow you to gimball the crew capsule, giving you more control. Even further you can use the rockets themselves for fine control. One option (though not essential) is to allow the center portion of the truss to rotate so that the capsule can be 180 rotated. Of course if you don't do that its no big deal. There aren't normally huge G forces and if you're really keen you can just have rotating seats.
As you probably guessed this keeps your options open. Even though the platform has a lot of redundancy, you're still in a position to abandon it and simply use the capabilities of the crew capsule. Of course I could refine this further and take that out for saved weight, but I don't feel like that because this platform serves another purpose.
Here's the bigger picture. The platform itself (four engine pods and truss) has excess tankage. Without the crew capsule it has the ability to deliver in the order of 20 tonnes of oxygen.
With the crew capsule it has the ability to arrive in low Mars orbit with excess O2/CO fuel on board.
This excess fuel is there for a return to surface, but if that option isn't taken then the fuel provides an extra boost for the whole system (including space hab) into a higher orbit. I did some rough figures and that put it about 300-400m/s boost.
The platform itself has the option to do a empty (no crew) landing at this stage. Whereas the space hab fires up its own propulsion and goes home.
So in conclusion, I've taken my "fuel ferry" concept and refined it.
It only needs to make 2 trips into Mars orbit (one for fuel, one for crew) per mission.
Its now powered on CO. The space hab instead carries its own methane, but the bulk of the mass of return fuel (the oxygen) is sourced from Mars itself.
I returned the space hab into low Mars orbit because its neither here nor there when you add it all up. High mars orbit requires less oxygen refueling but also requires more CO burned to get it there. In the end I plumped for simplicity and crew safety which is provided by the low orbit.
You can refine this further if you do find that its less energy intensive to produce a CO/methane mix on Mars. Personally I'd go for simplicity and worry about methane on a later mission when we find a good source of hydrogen on Mars itself.
The nice thing about my mission design is that basically everything can be re-used. The lander/ascent platform (if not the capsule as well) remains at/around Mars. The space hab can be re-used. Etc.
Seriously would like some comment/refinement, or suggestions as to where to take this?
#88 Re: Human missions » Landing on Mars » 2012-12-03 19:56:41
Oh and btw, I hasten to add, I'm talking purely about crew descent/ascent. Nothing wrong in landing the bulk of the non-crew items in just a big ugly blunt body.
#89 Re: Human missions » Landing on Mars » 2012-12-03 19:55:09
GW,
Having thought it through, I can roughly intuit how it works. It does break down as you lose speed, thus lift.
Which is what is making me think more carefully about weird and wonderful lander designs.
Here's another curly one.
Supposed you decoupled the need for blunt-object type drag from the actual crew space?
Concrete example. Supposed the crew traveled in something that's actually designed to have as little as possible drag, and thus heating - in other words more like the nose of a Concorde. You could even imagine a thermally detached leading nose that induces a shock cone.
Attached to this craft, but at a suitable distance are 2 or more other major structures which do contribute blunt-object drag. Think of them as retro rockets sitting behind a regular circular heat shield but you could imagine other forms. So the bulk of the heat goes nowhere near the crew.
You could refine this by adding elements that are steerable so you can control overall lift/drag.
Anyhow, before anyone's got an optimal mission they've got to stop cringing and start designing novel landers. That's my thinking.
#90 Re: Human missions » Landing on Mars » 2012-12-03 07:27:09
On another wild side track...
This is spurred by the thought that one of the basic numbers that affects the entire scale and mass of the "getting back off Mars" problem, is what's the least mass you can get away with for a simple capsule with half a dozen people in it? I've seen lots of figures out there but none that break it into irreducibles. Like, how much volume, the minimum thickness of metal etc..
And take that proposition even further, what about personal descent craft? How about ultra-minimal cocoons with a hypersonic, high temp drag tail.. and then a chute.. and finally the whole thing pops off and you're left with an astronaut with a small rocket pack on his back left to slow up the last few hundred m/s.
Completely mad?
My gut feeling is its workable. If you're going to throw stuff away it might as well be the barest minimum entry hardware. Of course, landing accuracy might be a problem so you'd need to have a reasonably fast recovery. Maybe you could add a teensy "segway".. hmm..
While I'm at it, one thing that keeps going through my head is a big glider. A few hundred Kgs of hardware with heat resistant fabric for wings. In some ways pretty conventional. Thing is you'd be going for the maximum lift/drag so serious heating (>300C) wouldn't be an issue.
At the end of the journey when you're down to ~200m/s you engage small rockets and land vertically. Too crazy? My point is it wouldn't be *that* hard to assemble these things en-route and for a 6 person crew you'd be talking in the vicinity of 2 tonnes of hardware.
Or even in Earth orbit assembly. And strap them onto the sides of your space hab. Now that would look truly interesting
Oh and you'd have enormous range
#91 Re: Human missions » Landing on Mars » 2012-12-03 02:30:09
Hmm..
'The analysis here is similar to that of reference 1. Entry requiring heat protection is assumed “done” at Mach 3, as before. This velocity is at the same 1.63 degrees entry from low Mars orbit as in reference 1, leading to a vertical velocity component of 19 m/s, and a horizontal velocity component of 675 m/s. '
I find that a little hard to intuit. I'm sure the model is fine, but I need some intuitive explanation of how you can come through a large gravity field pointing down, with little lift to drag, and still end up flying basically horizontally.. or am I reading this wrong?
#92 Re: Human missions » Landing on Mars » 2012-12-01 20:23:24
One other lingering thought..
If you're at 5Km and you're travelling down at 700m/s, what rate of deceleration do you need?
My calculation puts it at 50m/s/s
So, some serious thrust, no?
#93 Re: Human missions » Landing on Mars » 2012-12-01 20:03:08
In no particular order..
The baseline here is a fully pressurised capsule with its own abort capability. Much like the proposed Dragon/Super Draco. Since the cradle is sized to deliver a reasonable sized payload in a reasonable number of trips (that means to me about 6-8 tonnes per trip) its not that hard in its crewed version to give the capsule a reasonable amount of fuel, and even a backup parachute.
I guess you'd have to think through where and under what circumstances the cradle itself might become a dead weight and how to maneuver from under it.
I'm a bit reticent to consider a crew travelling on a bare cradle with no more than (perhaps) a faring. It would be an interesting trip, especially if you're chasing the space hab into high orbit.
As far as bailout goes. I think that's possible but you don't get a lot of time to make decisions. You'd need both a reasonable chute, and a personal rocket pack to actually land, methinks. Baumgartner would be jealous that's for sure.
Going back to plume stability. There's a bunch of questions here. First, at what speed ranges are we dealing with exactly. Where does local Mach 3 put you? Closer to yep, we can do this easily? Or closer to OMG that aint gonna work?
With the cradle I was considering a slight angle - around 10-15% for the thrusters. Why? Just gut feeling. Some ideas I have floating in the back of my head involve thrusters that can be maneuvered, along with the arm holding them - which dovetails into ideas that are related to packaging. Its possible the capsules angled thrusters may play a useful role. So you can steer a nozzle to suit the phase of the flight.
Does anyone know when SpaceX will test its Dragon/Super Draco in real world hypersonics? Or if they even plan to do so, thoroughly? By that I mean starting the Super Draco even at speeds that aren't really necessary to prove safety of human flight, but which reveal more data about what goes on in hypersonics.
Going back to the 5Km altitude for a heavy craft. How sensitive is that figure - what's the error bounds given every conceivable variable such as atmospheric variability. And where does that place you on Mars. I'm pretty sure its doable. You're going to run into the same problem I have which is where are the nozzles, and how are they configured or deployed (wrt to the shield) and how are they covered, if at all. After all, with a 60 tonne craft you're also dealing with fairly sizeable motors.
Here's a really goofy concept. Suppose you have a nozzle where the edge of the nozzle is aligned with the surface of a heat shield, in the middle of the shield. Leaving aside the question of whether entry will bother a nozzle at all, what if you were to pump a small quantity of fuel through the nozzle (no ignition). Would that have a cooling effect?
As you know I'm reluctant to land people with big cargo. I'd rather land them in a smaller craft, with lots of margin. Having decoupled the crewed landing from the larger items - presumably habitat - then I don't mind if the larger items are slightly riskier.
And speaking of robotic missions, I think its also an opportunity to test ideas like the cradle too.
Going back to large craft, since you're relying heavily on a gentle landing from low orbit that commits you to prior aerobraking, does it not? How confident do you feel about that problem being solved? The point I'm making here is the bigger craft with the higher ballistic coefficient also has the greatest challenge doing a satisfactory aerobrake.
#94 Re: Human missions » Landing on Mars » 2012-12-01 07:47:08
Here's another design factor I've not seen widely considered.
Would you rather
a) Aerobrake into high Mars orbit, then commence entry and landing.
b) Land directly.
Point is a) has its hazards - you'd need to be pretty confident in your navigation
And b) has it hazards - you come in faster meaning all else being equal you end up with less margin for error in final descent
I wonder if anyone's thought about this?
With reference to the mission idea I posed above. You get a choice, provided you're travelling with a fresh landing/ascent vehicle. If you don't you've only got one choice - aerobrake and catch up with your descent vehicle in orbit. Personally, I'd have to be convinced that either my landing vehicle can handle the higher speed with plenty of margin, or that someone's made aerocapture foolproof.
Of course, its possible that you get an option there too. If your approach into aerocapture isn't good, or your fuel isn't where it should be, you've got a window into which you can jump into the landing vehicle. It may even be possible to abandon the space hab, and use the remaining fuel to aerocapture, orbit then descend in the landing vehicle.
Btw, the above considerations is part of why I like missions like the above - there's always another workaround if at all possible - apart from getting hit by an asteroid.
(Yes, I was an asteroids addict once.. )
#95 Re: Human missions » Landing on Mars » 2012-12-01 07:36:45
http://spacecraft.ssl.umd.edu/publicati … hieldx.pdf
Interesting read..
#96 Re: Human missions » Landing on Mars » 2012-12-01 06:58:15
This starts with a question.
Imagine a landing and ascent vehicle for Mars. The core is a conventional blunted cone capsule. Attached to the docking ring at the top is a framework that extends beyond the slipstream of the capsule. On the ends of the framework are tankage and rocket motors. Suitable heat shielding is then applied to protect the tankage/plumbing. In addition there may be light weight mesh structures designed to add drag.
So basically you're aiming for a low(er) ballistic coefficient vehicle. Not a glider by any means, but a step above just a regular capsule.
The intention here is to simply slow the vehicle enough for a propulsive final descent.
Now, the question is, given that you've built something that's fairly ugly and definitely not streamlined, how much does this matter on ascent? Presumably there is a fuel penalty. You've got to find a compromise between gravity losses and aerodynamic losses. But presumably also, once you're past the first few Km of atmosphere you can accelerate more or less how you like. Anyhow the question is, has anyone given this thought?
Ok, now lets consider this further. We're building a fairly hefty craft. Gross mass in the order of 40 tonnes, fully loaded. Able to deliver a mass of about 6 tonnes into high Mars orbit.
If I have my sums right it will be able to transport a crew into high orbit, or abort from any point in the process. Just. It will also make a pretty decent surface hopper.
One tricky bit - its LOX/CO powered. The trade-off here is "wasting" more propellant against a simpler and more efficient production process. After all, the one thing that comes for free is the Martian atmosphere, and LOX/CO is as simple as you can get without abandoning ISPP altogether.
The tankage allows for an excess of oxygen to be carried into high orbit - minus a crew of course. About 6-8 tonnes worth.
Now, imagine that we've sent a space hab to Mars well in advance of a manned mission.
On top of that we send at least one such lander. Plus a very minimal ISPP plant that only knows how to make Oxygen/CO
Yep, I'm revisiting my fuel ferry concept. However now that I've had the chance to step away from it and come back and think about it, I think I like it better this way.
This time there will be no hydrogen transferred to the surface. Instead, the space hab is itself carrying the necessary methane (about 8 tonnes worth). Additionally the space hab carries its own propulsion unit, but scaled to the task - so maximum acceleration is in the order of 1m/s/s - about that of a commuter train. Which makes for a reasonably light propulsion unit.
You also have the freedom here to go for a non-cryogenic fuel, even old fashioned RP1 or some other storage-friendly liquid.
What you save here, is about 32 tonnes (or more actually since you can also stock up on breathing oxygen) which translates to about 130 tonnes saved in LEO terms.
It would take about 6 return journey's for the fuel ferry to completely refill the oxygen tank on the space hab. But the risks involved are mission critical not life critical. You more than likely would like some redundancy which could be easily provided - landing two complete fuel ferries and support systems from the word go.
Before any human leaves Earth you have one fully fueled space hab waiting in high Mars orbit.
This time you bring another mars lander/ascent vehicle, this time fully configured for crew. As well as the second space hab. That way you've always got one fully fueled spare in orbit.
Eventually as you develop confidence in the hardware you can avoid sending a new lander/ascent vehicle with every mission.
The space habs are recycled as well. In theory they could last several missions with refurbishment.
If you really wish to be frugal you could avoid taking a separate crew capsule on the journey and simply aerobrake into Earth orbit, then dock with the final landing capsule in orbit.
In this way the entire process can run on chemical fuels, and use at most a Falcon Heavy plus a bit of in orbit refueling.
Over to you guys..
Whip me! Beat me! Force me to use C++
#97 Re: Human missions » Landing on Mars » 2012-11-30 22:08:16
I know it appears daft, but its really trading landed mass off against security.
In this case water will store indefinitely. Whilst (deep) cryogenics are pretty well understood, its still another part that can fail.
The bigger picture is using a propellant that is only part methane, mostly liquid CO so you get the benefit of density whilst getting higher isp. You're still using ISPP Oxygen for the bulk of the oxidant.
In that context you only need a few tonnes of water, the hydrogen can be extracted as-needed, and you're not stuck with large volume tankage either.
And if you really want to you can treat the Oxygen that comes with the water as a bonus - yet another backup system.
I wonder if Mars needs more marshmallows?
#98 Re: Human missions » Landing on Mars » 2012-11-27 23:20:30
Has anyone considered taking seed hydrogen to Mars in the form of liquid water?
#99 Re: Human missions » Landing on Mars » 2012-11-27 05:12:37
Ever so slightly off topic, since this is about getting off Mars, but..
What is the minimum piece of hardware you need to get 2-6 people off the surface of Mars and into Mars orbit?
I originally thought about re-using the Dragon or something similar. But the problem there is the dead weight. So with methane/LOX you'd be launching with the better part of 20 tonnes.
Then I realised that there just isn't the space for tankage inside that thing so I put the whole idea aside for a while.
Recently I came back and in a moment of not-having-a-life I started to think about this again.
Here's 3 approaches.
One is basically no capsule at all. Just a fairing and a seat sitting on top of a platform with rocket motors. You'd be relying entirely on your spacesuit for life support. Nice thing is it cuts the fuel needed to a third. Yes, it has obvious safety concerns.
Another was more or less what I've seen before. A cradle of sorts onto which the Dragon sits, with its own motors - able to provide enough Delta-V but then able to re-land itself. I stole this from what I believe is a SpaceX idea to have an adjunct to the Dragon that itself is useful for small hops around the planet.
Third approach is simply a big brother to the Dragon with enough tankage to go into orbit and/or return to the surface or at least abort from most of the way.
Any thoughts on which of these takes your fancy?
BTW, seriously considering using CO/O2 fuel for the sake of simplicity (on the assumption that cheap/light solar cells are on the books)
#100 Re: Human missions » Landing on Mars » 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.