Mars 9 tons at a time.

[kelvinzero]

Haha.. I love railguns but this is not really the right thread for it.<br /><br />This thread is about what we can do with existing rockets that can manage about 9 tons, from memory. ie specifically avoiding demands for new launch technology. There is always a railgun thread around somewhere <img src="/images/icons/wink.gif" />

 

[spacenutnewmars]

Along with the idea of using what we have such companies as Bigelow, Dreamchaser and others are now looking at this very seroiusly as a COMMERCIAL SPACE TRANSPORTATION SERVICES to the ISS.<br /><br />http://www.ulalaunch.com/docs/publications/ULA/ISS%20Cargo%20RFI%20Final%2009062007.pdf<br /><br />

 

[j05h]

The idea is "what can we do, starting today, with existing or near-term hardware?" <br /><br />It's not a pie-in-the-sky, give-us-billions-for-research plan, but simple, commercial steps that can be taken to open Mars up, with or without government space agencies helping. It uses as much existing and off-the-shelf hardware as possible. It's a fine-grained enough plan that we are discussing ideal landing sites and specifics of landing techniques. Not a railgun in sight.<br /><br />Also, on the 'railgun' post above, theirs a pretty big difference between mass-drivers and railguns in implementation. Mass-drivers (ie, mag-lev trains) use superconducting coils, a railgun (ie, what the Navy is developing) uses two electric rails and charges them with a fast-moving plasma that accelerates the round. Much different tech.<br /><br />It's also somewhat different from current plans to fly commercial cargo for ISS. The customer would be other companies buying water in LEO, paying visitors or building the Mars base for a specific user. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[abcole]

The Navy is not the only developers who understand Railgun technology. I think the Navy's Railgun technology is still inferior and in its infancy. And I know exactly what I'm talking about. So far they have a long way to go. But I do have the filling they will probably get their. It's too bad they only see a weapon. There are currently testing garbage and it still is amazingly impressive.

 

[j05h]

The Army is reportedly working on railguns for main battle tanks, too. The problem with all railguns is that the rails tend to warp under the magnetic tension. I would hesitate to call weapons that are about to be deployed as "inferior", though they are new. <br /><br />Now, you have to make a technology distinction. "Railgun" means a specific device with two rails and plasma charge. Most "massdrivers" proposed are of the coil/gauss-gun variety, which are very different than a railgun. <br /><br />Josh<br /><br /> <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[abcole]

A high current, Lorentz Force electromagnetic accelerator with multiple rails.. I think they are the way to go. At least for achieving Earth two LEO. I have managed to work out a couple little problems with these things. Think about It the projectile must slide out with incredible precision or the whole thing has to be rebuilt. Bigger ones who knows what could happen.<br />The coil models are interesting to. I just don't like winding them. <br /><br />

 

[thereiwas]

I am looking for the appropriate equations to calculate hypersonic drag loads during Mars reentry. I have everything else in place - centripital forces, temperature, pressure, and density profiles, rocket thrust models for both solid and liquid rockets to slow the descent. But I only have the subsonic drag formula. Hypersonic drag calculations need to take into account chemical changes to the air, so it is very specific to Mars. Can anyone provide a pointer?<br /><br />Just for fun, assuming the subsonic formula all the way down, 8t will decelerate at a gentle peak G of 2.0 with a conical ballute of 30m diameter giving a beta of just 7, reaching a terminal velocity of 62 m/s. At 150m altitude 3 seconds of 110kN thrust from solid engines slows us to 16m/s at 31m, and then throttleable liquid engines cut in to gently slow to a very soft touchdown. Humans could easily survive such a descent. Time from 150km to the surface is 936 sec, 2066 km downrange. Subtracting the mass of the solid engines, the liquid fuel, and the ballute we arrive with 6.3t at the surface.<br /><br /><font color="yellow">If</font>all my assumptions were correct, which I know they aren't. Hypersonic travel in the Martian upper atmosphere is nothing like subsonic. Can a 30m ballute keeps its shape? What is a reasonable mass of ballute material per sq meter (I assumed .4 kg/m^2, which is probably too optimistic. There has to be something to stiffen it. Anyway, increasing the ballute mass does nothing to the reentry - it just reduces the available mass of other stuff delivered to the surface. Remember, <i>everything</i> that hits the surface is intended to be useful once there - even the ballute fabric.)<br /><br />By the way, some interesting details about a balloon mission to Mars on this Mars Society page. They discuss some of the special packing requi

 

[thereiwas]

This Hypercone looks very promising, though they only talk of deploying it at Mach 3.5. <br /><br />I found it from a reference in this Universe Today article that concisely describes the problems of landing larger loads (>1t) on Mars.

 

[gunsandrockets]

Nice stuff. Keep it up, I'm certainly interested.<br /><br />By the way, how does the basic ratios between ballute diameter, payload weight, and terminal speed work out? In other words, if your payload weight was doubled with the same ballute, what is the terminal speed?

 

[thereiwas]

Doubling the mass increases terminal velocity from 62 to 94 m/s in my model. But it also takes more than twice the thrust to slow down for landing (more mass in engines and fuel), and it goes subsonic at a lower altitude (9.5km instead of 17.8km). Peak aerodynamic G load increases slightly to 2.3G.<br /><br />I have found that changing one variable makes the optimum choice for all the other variables move as well. For example in this case you would want to move the solid motor firing point sooner to give more time to decelerate without raising the G's.

 

[j05h]

Your research is great, TIW. Keep it up, i'll comment more indepth later.<br /><br />josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[keermalec]

Yea great stuff TIW. At first glance a ballute should of course be much lighter than an equivalent aerobrake shell, but by how much?<br /><br />A triconic aerobrake shell will weigh 16% of the total lander mass for an entry velocity of 3.6 km/s, according to Borowski, ie 1.3 tons for an 8 ton vehicle. <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

 

[thereiwas]

Those "percentage of mass" calculations are suspicious to me because they assume a certain shield construction technique, thickness, structure, etc. The point about the ballute is that it can be <i>much</i> larger, and a traditional construction shield can't be that big. You just can't land large loads on Mars cheaply with high ballistic betas.

 

[gunsandrockets]

Interesting. It scales up better than I hoped.<br /><br />One more question... are your ballute calculations based on something like the IRDT (a large conical-shaped inflated forebody which combines the functions of heatshield and parachute)? Or is your ballute more like the older idea for the 1967 Mars Excursion Module (a half-sphere/half-cone shaped inflated drag-balloon which trails the lander)?

 

[thereiwas]

More like the former. A conical forebody. The idea is to spread the major heating over as large an area as possible, at as high an altitude as possible. A ballute can't take as intense a heating as an ablative shield, so it is important to deal with the deceleration as gradually as can managed.

 

[j05h]

The ballute works nicely with a small heatshield at the center. If it's metal/ceramic and transpires water, it can provide added protection to the ballute.<br /><br />Thanks for running first-order number, TIW. <br /><br />Materials I suggest looking into: Kapton plastic film and Nextel ceramic fabrics. From the center of the cone, the whole heatshield would be 3 circumferences: the center bulls-eye would be a non-ablative heatshield, surrounded by a Nextel shroud which is surrounded by a trailing Kapton ring. The Nextel can serve as packaging during transit for the larger Kapton ring. I'm not sure about Nextel but reinforced Kapton is amazingly tough. <br /><br />http://www2.dupont.com/Kapton/en_US/index.html<br />http://www.3m.com/market/industrial/ceramics/materials/fabric.html<br /><br />Kapton is my baseline material for Postcards To Space's STREET ring. <br /><br />Someone please clarify "triconic"? I understand biconic but do not see the advantage of three separate angles in the entry shape. The only advantage might be for protecting a weaker outer ballute ring, at which point we might as well talk about multiple stages to the ballute.<br /><br />Also, what do you all think about water transpiration? Max heating is only a few minutes, even a couple gallons of H20 could have a big effect in reducing heating. What effect would the added water plasma have on fabric/film ballutes? Does it increase buffetting and flapping? The machining of such a device is trivial, especially w/ the sphere-cone we've discussed. Thoughts?<br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[keermalec]

In answer to TIW and Josh, the triconic aerobrake shield looks something like this:<br /><br />....................XXX......................<br />................X.........X...................<br />.............X............. .X................<br />..........X.....................X.............<br />.......X...........................X..........<br />......X.............................X.........<br />.....X...............................X........<br />....X.................................X.......<br />...X...................................X......<br />..X.....................................X.....<br />.X.......................................X....<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />X.........................................X...<br />XXXXXXXXXXXXXXXXXXXXXX...<br /><br />Its diameter is the fairing diameter of the launch rocket back on Earth. Triconic in this case means three cones on top of each other, a flattish one at the top, a more vertical one, and then the cylindrical form of the main body.<br /><br />Its sole use is to slow the ship down from interplanetary speeds (typically 5.5 km/s!) to something (not given by Borowski) which can be further slowed do <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

 

[j05h]

OK, that's similar to the ballute I sketched out this morning. I'll post a pic soon. The "first cone" at the top of your ASCII art would be a planetary-class sphere-cone heatshield. The other two cones, moving downward in the drawing would be Nextel/Kevlar followed by Kapton and a set of stiffening rings as needed. <br /><br />Possibly instead of a "two stage" ballute, the trailing section could deploy fully. At lower velocity, further inflation of ring/ribs occurs, fanning the material wider. The whole ballute is ejected at engine-start, then recovered uprange. The heatshield is a severe cone, so it is designed to provide a crumble-zone under the payload. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[thereiwas]

Where do you get that a ballute only works below Mach 3.5? The one I referenced just above, the "HyperCone" had that limit, but there are other designs around that are intended for primary deceleration from orbital velocity. That is the kind I am talking about. That is the whole point - do the deceleration up high where the air is thin. Anything hard that fits in the launch shroud can't possibly be large enough to reduce the ballistic coefficient enough for an 8t load.<br /><br />

 

[solarspot]

<blockquote><font class="small">In reply to:</font><hr /><p>we are at 30m alt falling at 5.6 m/s.<br /><br />At that point the liquid engines (based on RL-10B-2) cut in at a gentle 36 kN thrust and slow us the rest of the way to the touchdown. Neat. Total liquid propellant used 1299 kg, liquid engine mass 100 kg, solid rocket mass 286 kg. Some fine tuning is possible to optimize the tradeoffs between solid and liquid. <p><hr /></p></p></blockquote><br /><br />ThereIWas, I'm curious how you got those numbers for touchdown engines. 5.6m/s is not all that high, it's under 20kph or 15mph. I don't recall the last time any vehicle had to expend 1/7 it's mass in propellant to achieve a delta-v that low. If those engines have an isp of 320 (I'd assume storable propellant engines, not cryogenic ones), then 1299kg of propellant would give over 4 million newton-seconds of thrust! That applied to a lander mass of 8000kg would provide (really quick calc, not even accounting for changes in mass due to fuel being spent) over 500m/s delta-v. Again, rough numbers, but I'm not sure how you could need that much liquid propellant to land from 30m alt. and speed of 5.6m/s.

 

[thereiwas]

Here is a link to a graph of the entire descent profile. The horizontal scale is velocity and the vertical scale is altitude. Both are logarithmic, so you can see the appropriate amount of detail at each stage. The upper right corner represents 3550 m/s at 150 km altitude. The vertical pink line over to the right represents Mach One.<br /><br />The vertical section of the black descent plot near the center is where it reaches terminal velocity. The sudden deceleration at 100m is the solid engines. The bent straight line below that is the liquids, first descending more quickly (to save fuel) and then increasing a bit at 1m altitude to slow to a gentle stop. From solid rocket fire to touchdown is 8 seconds elapsed time.<br /><br />In this particular run (I keep tweaking it) the solids fire for 3 sec at 110 kN, which is about 1.75 G. When they burn out we are at 36m dropping at 14 m/s. The liquid engines average about 47 kN most of the way down. Fuel consumption for this run was 1021 kg. Perhaps I am figuring the fuel consumption wrong - I have a value of 0.004528 kg of fuel used per second per Newton of thrust.<br /><br />Velocity plotted here is magnitude, not direction. Up at high altitude it is primarily horizontal. Touchdown is vertical. I assume the ballute/heat shield is kept at right angles to velocity at all times.

 

[solarspot]

I don't think my post implied a touchdown speed of 5.6m/s, I used that as the initial velocity down to calculate the amount of fuel needed to land at under 1m/s.<br /><br /> But going on your more recent model with the liquids starting at 36m up and falling at 17m/s, I found the lander would need to accelerate up at about 4m/s/s to reduce it's touchdown speed to near zero. I added 3.7m/s/s to this, to account for the downward acceleration due to Mars's gravity, for a total of 7.7m/s/s acceleration those liquids need to apply to safely land. That might need a little bit more than 47kN thrust, as that applied to an 8000kg lander would give an acceleration of 5.875m/s/s or 5 & 7/8ths m/s/s.<br /><br /> With that acceleration, the initial velocity of 17m/s down, and the change in position of 36m, the liquid engines would need to provide a delta-velocity of 24m/s. To change the landers velocity by that much would require 192 000 newton-seconds thrust. I found the propellant mass by dividing that by 9.8 times the engine's isp, which I assumed to be 320s, and got 62kg of liquid propellant.<br /><br /> Actually I think your point about fuel consumption may be why we got different numbers. If 0.004528kg of propellant gives 1 newton-second of thrust, then dividing 1/0.004528 gives an exhaust velocity of 220.8m/s. This equates to a specific impulse of 22.5seconds. This is actually worse than the average cold gas thruster working on Nitrogen gas. With this specific impulse, the propellant mass would be closer to 870kg, so the change in acceleration per unit thrust due to loss of mass through the engines would become significant...<br /><br /> Other than the point about the liquid fuel required I can't find any major problems with what you've said. I don't think you've mentioned using a parachute in parallel, but that seems to be a non-issue from the decent speeds you've shown. And if the liquid engines are more efficient than the solids (might not be, dependin

 

[thereiwas]

Thank you for pointing this out. If I understand the definition of ISP correctly then, the mass flow figure I need for my simulator, kg/s/N = 1/(ISP*9.806). This is good news, as I was clearly using pessimistic numbers. I will update the code.<br /><br />Another question though - how fast can an engine like this (the RL-10B) change its thrust level? The graph I mentioned above assumes 100ms for any change, including startup.

 

[gunsandrockets]

<Another question though - how fast can an engine like this (the RL-10B) change its thrust level? The graph I mentioned above assumes 100ms for any change, including startup.><br /><br />Aside from throttling issues an RL-10b is overkill for a final settling touch-down engine. But even worse, the engine bell of the RL-10b is so large it will interfere with ground-clearance.<br /><br />Might I suggest using somewhat oversized RCS thrusters for the job instead?<br /><br />You know if you added your ballute to an old Surveyor lunar lander, the landing technique would be very similar to your Mars lander. <br /><br />

 

[thereiwas]

I wasn't thinking of using actual RL-10B engines - they are way too big. I just needed a ballpark efficiency number, and an engine mass. As was pointed out, I need to use storable propellants anyway, which the RL-10B doesn't use. So I was using it as a guideline as to dry engine mass for a given peak thrust.<br /><br />It doesn't have to be throttleable actually - a pulsed engine would work as well and I think some Mars lander does use them. A set of RCS thrusters spaced around the rim would be fine, provided they can deliver the required total thrust.<br /><br />I would need over a dozen of the shuttle orbiter's main RCS thrusters. What do those weigh and what's their ISP?