No human will ever set foot on Mars. Religious extremism and islam in particular are growing at a much faster rate than scientific advances (neater cellphones with more gizmos notwithstanding). We’ll all be enslaved by sharia law long before we’re visiting mars let alone retiring there.
Moderator Note
kaltkalt, religious jabs are not permitted in General Questions. No warning issued, but don’t do this again.
Colibri
General Questions Moderator
Instead of Mars, I have wondered why energy (!) has not been used on either a manned Venus flyby (actually proposed post Apollo) or perhaps less likely (and perhaps less ambitious) sorties to Cruithne. To my space enthusiast views, they are simpler than Mars missions and have the advantage that they could add to knowledge on manned interplanetary spaceflight. Which in itself will be a major step on a Mars mission.
Like an uber Apollo 8 basically. Which was a mission planned and executed in about 4 months.
As an aside, any reason why ion thrusters cannot be used for manned spacecraft. I understand they are not practical in orbit. But once escape velocity has been achieved…
Anything is better than the lemon that is the ISS.
No reason why they cannot be used.
But the reason why they probably wouldn’t be (for any manned mission currently contemplated) is the extremely low acceleration that would be possible, leading to a very long time to reach a chosen speed and then to decelerate at the destination. Long flight times are costly (notably in terms of radiation exposure and the measures necessary to deal with this) and reduce the chance of mission success.
What would the purpose be? The crew would only have access to remote instruments, so they couldn’t gather any more information than a robotic probe. And the “learning to live in space” part can be done locally (i.e., on the ISS). A mission to Mars has the advantage of literal “boots on the ground”, and virtually requires use of local resources.
I did once read that there are places in Venus’s upper atmosphere where both the pressure and temperature are within comfortable human range. You could build a giant airship and only oxygen masks would be required.
A Cruithne mission would be interesting, but it’s still pretty much “living in space”, since its surface gravity is negligible. A sample return mission would be interesting, though.
More interesting than ion thrusters, for manned missions, is the VASIMR engine. It has much higher power density than typical ion engines. A Mars mission is likely to need a nuclear reactor to power it, though if you can make solar arrays light enough (thin-film, mostly plastic, with inflatable booms for rigidity), they might work as well.
Correct me if I am wrong, but Salyut, Skylab, Mir and the ISS have all been within the earths magneospehere so have some protection from radiation that Interplanetray spacecraft will not have.
Venus or Cruinthe missions can help develop technologies and procedures which will be useful for an eventual Mars sortie.
Well, that’s what I get by trying to do a off-the-cuff calculation between meetings. The original estimate wasn’t based upon such a rough order of magnitude estimates, but actually comparison to a similar type of vehicle (albeit a suborbital) and scaling the size and mass (the simulation of which is too complex to present or even discuss in depth here). For the Dragon capsule to carry enough fuel for a powered landing on Earth, it would have to have significantly more propellant than the entire volume of the capsule. This is not controversial or surprising; this is the reason why we don’t have rocket powered aircraft (except for experimental aircraft) and why we use non-powered landing profiles; the propulsive efficiency of rockets are very low, especially in a profile where most of the impulse goes to just fighting gravity, e.g. a soft landing (or slow ascent, which is why the STS and many heavy lift systems use solid propellant rocket boosters for initial ascent–despite the propulsive inefficiency of solid propellants, the high thrust to weight ratio ensures that the vehicle accelerates and gets above the densest part of the atmosphere faster, allowing it to spend less time subject to so-called “gravity drag”).
First of all, I am not sure what you mean by the term “suicide burn”, but if I interpret correctly that it is a trajectory profile which assumes optimal use of propellant at the highest allowable vehicle acceleration. Such profiles are possible on a body that lacks a significant atmosphere, but when you have to deal with reentry into an atmosphere aeroheating effects require significant consideration, as do aeroloads on the structure resulting in deceleration. It might seem that these effects should be less pronounced on Mars with its much thinner atmosphere, but in fact the reverse is true; because a vehicle is moving at a much greater effective airspeed at a given altitude, the heating rate–which is not caused by “skin friction” as often stated, but compression of the air forward of the vehicle at the shock boundary and subsequent radiation to the vehicle–is consequently higher. And given the higher total impulse requirements, it either takes longer to slow down (resulting in a longer heating duration and thicker, more complex thermal protection systems) or higher average acceleration to shorten the duration of heating.
Second, it isn’t just enough to cancel forward momentum at any point in the trajectory; you also have to negate the momentum you will gain during descent such that your normal velocity at landing is also zero, hence the need to consider the potential energy at your powered descent altitude. Again, realistically you won’t be able to optimize your trajectory so that you just reach a zero descent speed at touchdown; even setting aside the need for some cross range to correct for cumulative errors during descent and hover into a soft and damage-free landing.
If this were really so easy to do, we would have already been doing it rather than the highly complex and marginal reentry trajectory of s-curves that the Shuttle performed in order to slow down, or the use of parachute decelerators which–despite their long history of successful use–are always fraught with concern due to the highly transient dynamic loads during deployment, the inability to functionally test the system prior to use, and the large number of single point failure modes in every parachute system. It is worth nothing that the Chrysler SERV concept–an alternate STS proposal that was rejected not on technical grounds but because it wasn’t the spaceplane concept that NASA had envisioned–carried deployable jet engines for final descent and soft landing rather than reusing the ascent propulsion system. This means that they found it to be more efficient to carry an entirely separate propulsion system, including controls, fuel, feed lines, and attach structure, rather than carry sufficient propellant for powered descent using the main engines.
But you don’t need to take my word for it; the NASA Technical Reports Server is full of studies on Mars descent modes. It certainly isn’t infeasible to soft-land a large vehicle on Mars, but it will require a purpose-designed vehicle, not an adaptation of existing systems and will almost certainly not be using a fully powered descent profile.
Although what you are describing is pretty much how you would want to design a modular spacecraft to automatically assemble, the reality is that it would take a large development effort and significant proof of concept to make this feasible. This itself would entail significant development effort, especially when trying to dock massive payloads of supplies rather than small craft or hollow modules; a single failure could result in one module crashing into another with high momentum even at small velocity differentials. As you note, in space there is no drag to slow or dampen motion.
There is another consideration here as well; small spacecraft are often designed to be essentially rigid during acceleration phases, or at least have a small number of known structural modes (low range frequencies at which the structure tends to respond), with the propulsion system and guidance/inertial measurement unit located at points of minimal deflection so that any error between commanded thrust and resulting motion is minimized. (GN&C engineers talk about this as having “positive phase and gain margins”, which is a fancy way of saying that the amount of deflection and command delay in this modal response is accommodated in the autopilot.) A vehicle consisting of a large number of modules which are essentially joined at a few points is going to have a large number of modes and likely many at low frequencies where the deflections are comparatively large and the amount of phase lag is extreme. This would make a vehicle very difficult to control precisely, which is crucial for the high precision maneuvers required for planetary intercepts. It would really be better to send supplies and equipment which aren’t necessary during the transit on parallel or preceding trajectories rather than attempt to gang everything together.
Ion thrusters are very propulsively efficient (high specific impulse) but extremely energy inefficient, which means that they both take a lot of energy and produce a lot of heat. The thrust-to-mass ratio is also extremely poor (generally speaking, though concepts like the VASMIR have the potential to improve both characteristics) and so it would take a large wall of thrusters to exert any significant degree of thrust.
I’m sorry, but this isn’t how propulsion systems work. You cannot just dial down the rate of impulse over any range you like. I’m not aware of any conventional (i.e. chemical) propulsion engine of significant thrust capacity which can fire continuously for several hours, and even if it could, the buildup of heat due to combustion over that time would destroy any real world chamber, throat, and nozzle material. Nuclear thermal, fission fragment, or nuclear electric systems could all potentially provide thrust over an extended duration and with a higher specific impulse (reducing the mass of propellant that needs to be carried and expended) but the heating problem remains, which means you either need to develop unreasonably robust materials, highly complex regeneration systems (using the propellants as a coolant), or deploy enormous radiators to expel the excess heat. There simply isn’t an extant propulsion system which is capable of propelling a large spacecraft (sufficient to contain supplies and provide a habitable environment) for a crewed mission without having to carry an unreasonable amount of propellants into orbit and develop the means to transfer them onto your spacecraft. There is no off-the-shelf solution to this; this alone would require a substantial engineering effort to develop the systems and processes to do this beyond the current state of the art.
You haven’t established anything of the sort. You’ve hand-waved an figure that is 1-2 orders of magnitude less than the estimates developed by people who have decades working in the field with detail knowledge of the current and projected state of spacecraft and space launch technology. You continue to repeat “5-10 billion dollars” as if saying it enough times makes it a real estimate, but have provided absolutely no basis of estimate or even a high level breakdown of the costs. Realistically US$10B is probably the minimum cost to ship this “mountain of spare parts” and “10 separate landers” plus all the necessary supplies, habitats, propellants, et cetera, plus the crew to low Earth orbit. You’ve provided no technical details on how such a spacecraft would be propelled, provide a sustainable habitat, protect crew from hazards, perform precision navigation, or any of the other of dozens of critical functions it would have to perform. The estimates I’ve provided are entirely in line with the mean of detail estimates developed by people actually working in the field with detail engineering experience (versus space travel popularizers and Internet moguls who work from animations and a PowerPoint-level of engineering detail). The notion of ganging together a bunch of commercial off the shelf parts and sending a scrappy group of socially maladjusted daredevils plays well in fiction and film but doesn’t represent the effort of any historically successful space program.
I don’t intend for this to come off as belittling or reflexively dismissive of you in particular, but this entire line of “it is just so easy to bang things together and go do it, how hard could it be,” flies in the face of logic and reason. Nobody suggests to a heart surgeon that his work could be done by a group of scrappy EMTs, or a mutual fund manager that his work could be done by a group of teenagers with a Magic-8 ball, but when it comes to space exploration there seems to be a large or at least extremely vocal community of people who think that this work is trivial and that the risks of crewed space travel are easily beat with a little ingenuity and some bailing wire. I suspect this comes from a couple of sources; one is how seamlessly many of our technical consumer products work without any insight into the engineering that goes behind them, and the other is how rapidly special effects in film and television have advanced to make it seem like any conceivable activity is possible.
The reality is that the space environment is extremely inhospitable to people; a few dozen seconds exposure to vacuum, a single burst of radiation from a solar flare, failure in an environmental cooling or carbon dioxide removal system, catastrophic loss of propulsion system can all be fatal in the time it takes a crew to even recognize that there is a problem, and no amount of ingenuity or bravura can change that. Even nominal terrestrial hazards, such as bacterial growth and distribution, moving large or bulky objects, or instrumentation hazards such as the recent “leaky” spacesuit which almost drowned Luca Parmitano are amplified by the free fall environment. We come from an environment which is seemingly so well designed for us (though it is more appropriate to say that we are well evolved for it) that the degree and amount of hazards in space simply don’t fit into the human experience.
This is not to say that there isn’t a place for people in space; certainly many of the exploratory efforts and decisions could be made faster by having controllers in relatively close proximity to probes and rovers, and as the infrastructure for in-situ manufacture and utilization of space resources, as well as propulsion, power generation, and habitation technologies advances, the costs and drawbacks of people in space will be reduced to the point that some combination of crewed and robotic exploration will be only only possible but desirable on a cost/effectiveness basis. But the notion that we must send people to the surface of Moon or Mars to do “something” regardless of the cost is a very 'Sixties mentality, stemming back from when computers were gigantic, room-hogging monstrosities capable of only doing relatively simple calculations with great slowness, and “sensors” were grainy video images sent at rates that make a 300 baud modem seem like a T-1 line. The comparison between what ‘robots’ can do and what actives a crew can perform are general (if unconsciously) baselined by assuming human capabilities in a Earth-like “shirt sleeve” environment, where in reality the limitations of a person (especially operating in free fall and/or in a pressure suit) grant far less advantage to crewed missions, and at costs that are orders of magnitude greater than robotic missions.
The notion that we can go to Mars today, at a costs less than a major conventional construction project, using off the shelf components, and without a well-planned program with processes and technology which has been qualified and tested with any acceptable level of risk and expectation of success, is nothing more than high fantasy. And the idea of repeating such a mission “10-20 times [at] a 5-10 billion expedition, then, when the crew die on the way, you just pack more of the stuff that broke the last time and do another launch” is nothing short of preposterous from a technical, liability, and fiscal standpoint.
Stranger
Thanks for that Stranger. Love reading what you write. (And missed you in your month’s hiatus.)
Just as an aside, in your opinion, do you think it is a reasonable or sensible exercise to plan manned missions to Mars? Do you think it is even feasible?
J.
Stranger, thanks for the reply. A few things you’re wrong about :
- If you wanted to do a 2kps dV change over time, you don’t have to do it with 1 engine. You could have a cluster of small engines and use 1 at a time. I’m aware that it is difficult to throttle an engine down and still get the same isp out of it, because an engine is tuned for it’s rated thrust. So you run one for a few minutes, then let it shut down to cool, then the next one…
SpaceX is already using regenerative cooling. Also, their engines will last for several hours of cumulative burns - they are already designed and tested to that spec.
- These experts you alluded to were already wrong about what SpaceX has already done. They developed an entire rocket, a space capsule, the engines, the factory, the company, the back-end structure…for less than the cost that NASA has paid for a new launch pad they will probably never use. (about 1 billion USD)
I have already established in my OP that my “10 landers + mountain of spare parts” is approximately 20 launches of the Falcon heavy, at $120 million each.
A mountain of spare parts would be an entire launch crammed with ball bearings, circuit boards, space motors, etc.
- The Russians did in fact spend around 5 billion USD on the Mir. This is undeniable. Their costs are closer to what it actually costs to do space versus what a bloated infrastructure of contractors costs.
I will give you the problem with all the modes of vibration under thrust. My steel-cable “lashing” proposal doesn’t have any dampeners to prevent the oscillations from destroying the spacecraft. Undoubtedly there are other problems as well.
I still think your basic premise is wrong. If SpaceX were to higher several thousand more engineers, directly, and have people working in the flat management structure they use, directly on the individual problems, they are solvable. This is why America is falling behind : we cannot keep doing things the way they were done before because they are enormously inefficient.
I think the technology we need to execute a crewed mission to Mars (or another planet) doesn’t exist yet, and is unlikely to be in a mature state in the foreseeable future (i.e. within a timeframe we can reasonably budget a program, which generally ends up being a period of no greater than ten years). Rather than expend the effort on developing a system which will be overcome by advances in technology before it can even be executed, making it either obsolescent by the time it is available (i.e. the Shuttle) or cancelled due to poor performance and cost overruns (the X-33), it makes more sense to develop the necessary launch and orbital infrastructure to support both future interplanetary missions and the development of in-situ resource utilization so that when the requisite propulsion, power, and habitat technology for interplanetary transits are developed, the resources and assets to support use are already in place and the processes for handling materials and large components in free fall are mature.
The other issue with a ‘destination oriented’ program can be readily highlighted by looking at the Apollo program; once we got to the Moon, all incentive for even maintaining the infrastructure, much less expanding the scope of missions is essentially gone. Going to Moon was a prestige program, launched by the speech of a popular president, intended to challenge the technical lead in space that the Soviet Union enjoyed. There is no such incentive today, and even if there were, a program focused on going to a particular destination is likely to be abandoned if it either it falls substantially behind schedule or doesn’t provide any further benefit. Developing a sustainable infrastructure in Earth orbit, on the other hand, provides for a wealth of commercial opportunities which then provide incentive for continued development, regardless of the waxing of national imperatives or budget negotiations.
Most modern liquid propellant rocket engines use regenerative cooling. But they aren’t designed for continued operation for more than a few hundred seconds without refurbishment, and the plan for carrying “a cluster of small engines” is even more ridiculous than the “mountain of spare parts” and a squadron of landers. Chemical propulsion systems are very marginal in terms of the ability to get anywhere outside of low Earth orbit; hence why most spacecraft rely on perigee/apogee burns and phasing orbits to escape Earth’s sphere of influence. Adding more and more mass of engines, mounting structure, propellant feed lines and valves, et cetera simply increases the total propellant requirements to an absurd level, not to mention increasing the complexity of the system.
And no, the current Merlin 1D engine is not rated for “hours of cumulative burns”; according to the SpaceX website, “the Merlin 1D accumulated 1,970 seconds of total test time, the equivalent run time of over 10 full mission durations”. That is a total of about 32 minutes of operation, or approximately ten flights. (How much of time you consider “rated” depends on the standard you use for qualification; typically you qualify at a duration of 3X the maximum planned operating duration.) The Merlin VacD (upper stage) is rated for the 375 seconds of operation (since it is not planned for reuse, although it will perform multiple restarts).
It is unclear just how much SpaceX has spent (and continues to spend on development) since they are a privately owned. There is no question that they have done the development for substantially less than the (ridiculous) development cost of the SLS, and probably considerably less than it has taken other contractors (Boeing, Lockheed Martin) to develop their EELV vehicles. However, this doesn’t translate into order of magnitude reductions in cost in an interplanetary spacecraft and landing systems, which is an area where SpaceX has no practical experience. Aside from the costs of executing the mission is all of the technology development, systems engineering, and integrated testing which made programs like Apollo a success. It’s good to be lucky, but it is better to be thorough and prepared, especially with a highly complex system in which the failure of any number of components or systems can mean catastrophic failure of mission and loss of crew.
From this I infer that you have zero understanding of how launch systems and spacecraft are engineered, or indeed, how things are manufactured. A random collection of “ball bearings, circuit boards, space motors, etc.” would be about as much use to the crew of a spacecraft with a failing environment system or a propulsion system which just spontaneously dismantled itself into twenty thousand component pieces as a giant bag of marshmallows and a shipping container of Hershey bars.
I absolutely agree that the conventional way of business of aerospace contractors–which largely consists of appealing to politicians for bloated budgets based upon the number of jobs they’ll create in a wide spread of districts–is inefficient, as is the lack of incentives for program success (and the lack of penalties for failure). Certainly contractors like ULA have little interest in reducing launch costs and increasing launch rates, at a reduction in margins per launch. SpaceX has adopted certain features–such as horizontal integration–which serve to reduce the complexity of launch site operations, and are attempting to follow a more automotive-industry paradigm in manufacturing. How well this will work for them will be demonstrated by how they deal with the first major launch failure. (To be fair, contractors like Boeing have, of late, not dealt very well; the failures on the Delta III program, which was intended to be a low-cost, high production, COTS program was a clusterfuck of blamage and obfuscation.)
We’ll see whether they can actually launch at the advertised rates and make a profit; I hope they can, if for no other reason than that it will convince investors to support other efforts to develop launch and orbital deployment systems, especially in the smallsat and commercial telecom industries which has just been waiting for low cost access to space to take off. But the complexity and reliability of a system designed for interplanetary transit, and especially a crewed system designed to land people on Mars, is vastly more complex than just an orbital space launch system, and your assertion of a US$5-10 billion cost (which, as far as I can tell, is based upon the notion that is a whole lot of money so someone should be able to make it all work) has absolutely no basis of estimate or grounding in the real world complexities of developing and operating spacecraft.
You asked the question in the o.p. as to the feasibility of “retiring” on Mars; the answer is that unless your version of retiring is crashing headlong into the surface of the planet at several hundred meters per second, or missing it completely and flying off into solar orbit while trying to repair your guidance computer with an NVIDIA card and a pair of ball bearings, it is not feasible with any reasonable budget given the current and near-future state of technology.
Stranger
If Earth had no atmosphere… sure. But I’m still not buying that the same is true when aerobraking is taken into account, whether you’re talking Earth or Mars.
The Shuttle was so steeped in politics that I don’t think we can use it as a lesson in what had to be done. Among other things, one of the design requirements was significant crossrange capability, which dictated a certain aerodynamic configuration, which dictated all kinds of other things.
Capsules teach us things, though. Capsules use parachutes because parachutes are mass-efficient and because they have some good passive safety characteristics, not because retro-rockets are impossible.
More or less. You burn at the very last possible moment at full thrust. Suppose you are coming in at 200 m/s and you have 50 m/s^2 of thrust. Approximately speaking, you must burn for 4 seconds and start 400 meters above the ground. Of course, in that time you will pick up a bit of speed due to gravity. This is no more than 40 m/s on Earth, and a bit less in practice. So you must start the burn a bit higher.
At any rate, this is advantageous for two reasons: maximal use of the Oberth effect (which, in this particular case, is equivalent to “minimal gravity drag”) and aerobraking.
They’re advantageous either way. It how we’ve mostly landed on Mars already. Parachutes are not a special case: they’re just a method of artificially decreasing the ballistic coefficient and hence the terminal velocity. Eliminate the parachutes and you increase the necessary delta-V, of course, but it doesn’t necessarily follow that the propellant requirements have become unreasonable.
At any rate, all the Mars probes I know much about used a form of suicide burn. Viking had hydrazine engines with a wide throttle range (which could land softly). Pathfinder and the MERs used solid rockets and an airbag system. MSL of course went back to hydrazine and had its skycrane system (which wasn’t needed for the soft landing, but rather to prevent the rocket exhaust from contaminating the exploration site).
Viking is the closest to the Dragon proposal since the final landing was purely on rockets. It only had 180 m/s of delta-V, since parachutes did part of the job. But it only used 10% of its mass for that, and Dragon has higher performance engines, and could conceivably dedicate a higher mass fraction than 10%.
I’m aware that it’s not a trivial problem. But getting from several kilometers/sec down to several hundred meters per second is basically a solved problem. It’s been done, on Mars, lots of times. The remaining segment looks hard because NASA has tried to be very efficient with supersonic parachutes and the like. But it doesn’t have to be that hard, because it’s only several hundred meters/sec that we’re talking about, and current fuels can easily achieve that with smallish mass fractions (i.e., tens of percent).
Of course, you also want to cancel remaining forward velocity at the last minute. Not just for the reasons cited before, but also because of the vector addition gains: a single burn that cancels 1 m/s in X and Y only costs 1.4 m/s, while sequential burns cost 2 m/s.
You want some margin, certainly. Especially with a human crew. It doesn’t necessarily need to be a lot, though.
That’s all well and good, but the fact remains that SpaceX is already planning on propulsive landings for the DragonRider capsule (Earth landing, of course). NASA is paying them good money for development. It will carry a parachute, but only as a backup–propulsive landing is the primary system.
Maybe you’re right, and everyone at NASA and SpaceX can’t do basic math, but I’m inclined–based on the numbers I can run–that it’s quite possible and that the control system is the real long pole.
It’s a curious design. I’ll just say that achieving optimal efficiency (in the traditional sense) is not necessarily a design goal of the Dragon capsule or SpaceX in general. There are very good reasons why the Saturn V used RP1 on the first stage and LH2 on subsequent stages, while the Falcon 9 is RP1 throughout. SapceX’s definition of “efficiency” includes things like “storing a single mild cryogen is a lot cheaper than storing both a mild and strong cryogen”.
Thanks for the pointer. I’ll see what they have on parachute-free descent profiles.
Yeah, fully agreed that it’s not a trivial problem. What I had in mind were long cylindrical modules. At two points along the lengths would be reinforced sections with the docking points (each would have perhaps 4 of these points around the circumference), and so each module would connect with an adjacent via two clamps.
The hope is that this provides enough stiffness to the whole structure so that any vibrational modes are fairly high frequency. But it’s by no means guaranteed. I’m not really on-board with the idea that you need to assemble your craft in orbit, anyway. Fueling, sure. Something akin to the ISS with a big rocket… no so much.
I found a basic overview of Mars EDL here.
See page 14, where it talks plots a descent profile without parachutes. You can see that they estimate a final 600 m/s of delta-V–not far off from the 500ish that I had been assuming. They don’t say exactly what ballistic coefficient they used for that plot, but on page 11 they give a range of 150-600 kg/m^2, and Dragon is about 300, so it sits right in the middle.
The document mentions some challenges with supersonic retro-propulsion. I have to wonder if Dragon’s system, where the nozzles are outside of the airstream, sidesteps some of the issues.