Europeans And Australians Make Space Propulsion Breakthrough

[SteveMick]

Hohman transfers as you correctly state are minimum energy transfers. If more energy is used, travel times are reduced. Systems which supply more energy such as nuclear thermal and solar thermal can follow trajectories that get you there faster on a more direct path. I have never seen a proposal to use Hohman transfer for manned missions using NTR or STR because this is the slowest(with exceptions ) trajectory. The advantage higher Isp systems have is that they can get there faster than chemical which is less able to acheive the delta V necessary for these higher energy/shorter travel time trajectories. As I have pointed out, STR's can also use solar electric propulsion to speed things up even more. Aerobraking can improve things even more for whatever propulsion system you use.<br />Steve

 

[spacester]

Well I intentionally used the qualitative term 'very' so as to generate a discussion, simply to fix in people's minds that non-circular matters. Also IMO if I read something that's talking about circular orbits, I can safely conclude detailed study has not been done. It's amazing to me how many top scientists fail to do the orbital mechanics correctly.<br /><br />I called the orbit of Mars <i>very eccentric</i> on the Hohmann Transfer to Mars Reference thread and got away with it. You might want to check that out if you think I make such statements lightly. <img src="/images/icons/laugh.gif" /> {Link non-functional, duh what'd I do wrong?}<br /><br />There is no such thing as a Hohmann transfer to Mars that will in fact take 259 days anytime in the next century. I checked. I can do the math, my opinion means more than squat. <img src="/images/icons/laugh.gif" /><br /><br />Hohmann transfers mathematically assume instantaneous impulses at the start and the end of the trajectory. Of today's technology, only chemical propulsion has the thrust needed to have a short enough burn time to closely approximate that assumption to the degree that only minor corrections are required.<br /><br />In fact, in practical terms there's no such thing as a true Hohmann transfer, even if the minimum energy two-impulse transfer is selected. This is simply due to the fact that the tangent to an ellipse is not the same as the tangent to a circle at the same point in space. I'll let you ponder that, I gotta run.<br /><br />I am not an expert on continuous thrust trajectories but I've taken a couple whacks at it.<br /> <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

both this and the previous are wrong.<br /><br />Nuclear also approximates the 'instantaneous' dv change when you look at how long most nuclear systems are proposed to be operating. I generally see things along the lines of 5-10 hours of thrust somewhere in the region of 0.10 G acceleration. When you see a trip to Mars taking hundreds of days, acceleration periods of only a few hours amount to a fraction of a percent of the whole trip time, and thus should be considered for practical purposes 'instantaneous', though you are correct that this won't do for a navigational computer.....<br /><br />And while I agree that it is certainly POSSIBLE to accelerate longer with NTR, using more fuel, to obtain a more direct trajectory, requiring a deceleration period near the destination, just as electric propulsion accelerates and decellerates, the poor Isp of NTR compared to electric propulsion means, watt for watt, NTR takes 15-30 times more fuel than electric for such high speed transits. NTR only gains a bit on that ratio because it can put more watts into propulsion fuel thermally than it can generate electrically (typically 5-10 watts of heat for every watt of electricity produced using Brayton cycle), but much of that heat is also wasted in NTR propulsion from stagnation temperature losses, etc. I do not consider 800 sec Isp to be sufficient for interplanetary travel with anything but hohman-style transfers. Expect to pack a lunch and a few farm animals.<br /><br />In my trip calculations, I showed that VASIMR can do the trip in 90 days at high acceleration max dv with less fuel than NTR requires to even acheive the minimum dv Hohman transfer. If NTR were to attempt a similar high accel high speed trip, it would require many times more fuel than the VASIMR system requires. I never said it couldn't do it. Given the political will to launch millions of lbs of fuel into orbit, you can go anywhere, but the will is what is lacking, which is why we haven't launched any interstellar

 

[spacester]

Dude, I don't like it when people say I'm wrong when I'm not. <img src="/images/icons/tongue.gif" /><br /><br />Did you miss the “Of today’s technology . . .” part? Because I’d be thrilled to hear that we’ve got a VASIMR or NTR ready to go to Mars next year that I hadn’t heard about! <img src="/images/icons/laugh.gif" /><br /><br /><font color="yellow"> In my trip calculations, I showed . . .</font><br />Um, are you sure you want me to take a close look at those calcs? Did you account for gravity losses? Remember that anytime you’re thrusting in a gravity field, you have gravity losses. And I see a lot of handwaving with straight-line trajectories and no clear statement as to the dV required for the VASIMR constant accel/decel mission. A guy with your sig line really ought to try for more rigor and a clearer presentation, don’t you think? Are you interested in sharing your math skills or are you just trying to impress us all, figuring no one will challenge you? <br /><br />I don’t have time to click all your links until this evening but before I do I’ll say again that I am amazed at the lack of rigor I find on mission proposals. A first-degree approximation is fine for early mission planning, but IMO ignoring the eccentricity of Mars is not even good enough for that. IIRC the real-world trip times for Hohmann to Mars ranges between 236 and 280 days – that’s just too much to ignore if your trip time is an element in your trade studies.<br /><br />OK I clicked the 1st and last of your links, I’ve seen ‘em both before. Circular planetary orbits are assumed. Look at the ‘Definitions of Terms’ in the Stanford paper – do you see a symbol for eccentricity of Mars’ Orbit in the list?<br /><br />More later.<br /> <div class="Discussion_UserSignature"> </div>

 

[spacester]

If someone wants to check my statements before I get to it, go to JPL's Horizons and enter in the date of Mars arrival for the U-Texas mission proposal when looking at sol-centered ecliptic ephemeri for Mars. I bet you'll find out that the orbital radius differs quite a bit from the 1.524 AU value they used. Nice mission proposal but they fail to account for the real solar system. <div class="Discussion_UserSignature"> </div>

 

[john_316]

Thanks for the links...<br /><br />Now as for studies on the NTR trip to Mars and back you can find some information (abiet briefly) at www.fas.org and in the past while googling it at different places I have come across studies during the NERVA studies and testing and even more recently with those of TRITON (Pratt & Whitney) coverage.<br /><br />So the studies for 120-150 day trips are out their in studies on the net.<br /><br />I think if we were to build a NTR system knowing it might cost between 3-5 billion to start out it could be very reusable in the sense that the MTV along with the reactor would be reusable and have perhaps 15-20 outbound trips and back. Depending on the various studies you can have a 150 day or less trip, a 300 day trip, and a low Dv trip for 900+ some days. All these studies of course depend on your reactor and ship sizes due to fuel etc etc.<br /><br />But of course the time you want in transit, you would like to be as fast as can be. You dont want rapid loss in the human body so you would have to make your flight short and productive with excercise.<br /><br />Now to take some of all you ideas and apply them here for a moment. Considering a 650+ ton ship (MTV) I think you could actually have a hybrid vessel which could include the best of VASMIR/ION and NTR (even solar). <br /><br />I think the idea of just one shot use of assetts for Mars visits is highly unacceptable. I propose a blend of technologies as this would make the MTV robust that it also isn't too complex. A VASMIR powered by Nuclear wouldnt be added complexity with NTR already on board.<br /><br />A MTV with 2+1 NTR (Gas Core) and a VASMIR/ION engines though expensive could be a testbed or in my arrangement the Initial capability of not just using a CORE Vessel thats transports the crew there but drops them off and returns back to Earth and goes back out again after a refuelling or refit as needed and any upgrades that can be done while

 

[spacester]

From JPLs Horizons:<br />Mars on June 25 2005:<br />a = 1.5236748 AU<br />e = .093444<br />nu = 345.642 deg<br />nu = true anomaly, this value is 14.358 deg before perihelion, so Mars is very close to its minimum distance from the sun<br /><br />Euler’s formula for an ellipse:<br />r = a * (1-e^2) / (1 + e*cos(nu) )<br /><br />plug and grind:<br />r = 1.385 AU<br /><br />1.524 – 1.385 = 0.139 AU = 20.8E6 Km<br /><br />Like I said, it’s a nice mission proposal. But the trajectory it’s based on misses Mars by over 20 million kilometers! BTW, it’s stuff like this that got me started on orbital mechanics in the first place.<br /><br />My point? If we want to go to Mars, we need real mission proposals that take into account the approx. 18-1/2 year cycle of varying energy requirements. We need to recognize the good years compared to the bad years if we want to get at all serious. BTW the cycle gets to the next lowest energy missions in 2016 or 2018 depending on your strategy.<br /> <div class="Discussion_UserSignature"> </div>

 

[SteveMick]

John 316 said: "I think if we were to build a NTR system knowing it might cost between 3-5 billion to start out ..." <br /><br />You're going to develop and launch a gas core NTR and a vasimir for 3-5 billion?<br /> This is patent nonsense!<br /> I think its telling that you have to resort to gas core technology once solar enters the picture. As you fail to point out: no gas core NTR has ever been built. <br /> You also assume that the system could also be used to generate electricity for a vasimir, although as I've pointed out: no such system has been proposed, but I suppose there is no reason such a hybrid couldn't be built. This was ruled out however for JIMO for some technical reason I don't remember exactly but having something to do with the operating temp. of the reactor I think. The costs are likely in any event to greatly exceed what was proposed for JIMO. Then let's factor in development time which is likely to be longer than JIMO and you'd be lucky to get flyable hardware in fifteen years. <br /> This desperate and illogical need to include nuclear tech defies reason and has become an end in itself - has it not?<br /> STR/solar electric tech is basically off-the-shelf and costs much less than 1% of any nuclear alternative while having a specific power around 50 times better (although this represents nukes that have actually been proposed not the theoretically possible).<br /> Also, solar thermal really kicks butt on conjunction missions to Mars.<br /> It is tough to let religious(pro nuke) positions go, but necessary if the scientific method is to be employed.<br /> Steve

 

[SteveMick]

"I do not consider 800 sec Isp to be sufficient for interplanetary travel with anything but hohman-style transfers. Expect to pack a lunch and a few farm animals. "<br /> Do the math then retract that ridiculous statement. Higher specific impulse means higher velocity. Higher velocity means a faster trip.<br />Also: "And while I agree that it is certainly POSSIBLE to accelerate longer with NTR, using more fuel, to obtain a more direct trajectory, requiring a deceleration period near the destination, just as electric propulsion accelerates and decellerates, the poor Isp of NTR compared to electric propulsion means, watt for watt, NTR takes 15-30 times more fuel than electric for such high speed transits" <br />This is only true if the electric system is able to accelerate at a rate sufficient to take full advantage of its Isp. At 1/1000 to 1/10,000 g, an electric system will be hard pressed to add more than a few thousand mph average to the velocity. The mass ratio increase required for a STR or NTR to add this much speed is not much of a problem. In addition, if the STR or NTR is used to provide the velocity for escape from LEO, their greater efficiency vs. chemical means more propellent is available. As if that weren't enough, the solar version in particular can use vasimir or any other electric system to reduce travel time just as effectively. <br />Steve<br />

 

[john_316]

Steve,<br /><br />What Electric system are you actually proposing? Your Electric propulsion can't do a 150 day trip outbound. And has it been built as well? What high speed electric system is there at the moment? Even the P&W TRITON NTR can be built and operated with better efficiency.<br /><br />The gas core sytem is thought to be more reliabe (efficient)in the NTR studies and yes 3-4 billion would be needed to get the GAS Core technoligies in swing and up running. But YES NTR (solid or gas core) beats any current electric propulsion hands down regardless of the weight implied. <br /><br />Solar has less than 20% effeciency even with a solar concentrator/collector what is it going to be? Still less than 50% even in 10 years its still never going to reach 70-80% efficiency. Has it been built as well and flight tested?<br /><br />Even if a solid core NTR is used it can be built as a hybrid and allow different cycles (brayton etc) to operate upon. It can also maintain power and propulsion with its weight consideration. I propose a hybrid but just a solid core NTR can do the job. And this system can be built and running in a few years with lead time. <br /><br />Nuclear propulsion could be built right now based on the previous NERVA program studies and get us to Mars way before your electric propulsion could in a trip one way out and back. By what I have seen it would 150 days travel out and 120 days travel back in many of the NTR studies? Does the electric system offer that or give that? Now nuclear rockets have been built and studied and there is alot of information to draw upon and design and build one as quickly if not faster than an electric system. <br /><br />Nuclear is going to be required for Mars surface stay as well. Solar isnt going to do it. <br /><br />Nuclear isnt as dirty as people make it out to be and it seems that you completely oppose it like some others. Its an option that they will look at and most likely use.<br /><br />Nuclear Power will power the MTV. I a

 

[SteveMick]

I apologise for not making myself clear. When I refer to STR, I mean solar thermal rocket and I'm talking about electric only as a possible flight time reducer once the vehicle is on its way to Mars. You also raise these issues:"Solar has less than 20% effeciency even with a solar concentrator/collector what is it going to be? Still less than 50% even in 10 years its still never going to reach 70-80% efficiency. Has it been built as well and flight tested? "<br /> The efficiency of the concentrator is about 90% give or take and when used to power a solar thermal rocket engine, a JPL study from 1986 I have by Joel Sercel estimates overall efficiency re sunlight intercepted to enthalpy in the propellent to be 33%. Since the sunlight is used directly to heat the propellent, the setup doesn't have a lot of mass unlike an electric system that must convert heat or light into electricity first with heavy conversion systems and larger radiators. The conversion efficiency to electricity is a little less than the STR since the currently available Spectrolab triple junction concentrator solar cells operate at about 30% efficiency so with the 90% from the concentrator, the overall eff. is 27%. While efficiency is important, specific power(elctricity per unit mass) is what counts and solar as compared with say the proposed JIMO reactor is about 100 times better. There is no reason that nuclear can't do much much better and part of what incensed me about JIMO was the horrible specific power which is way below what I think is possible. Did the fact that a builder of submarine reactors got the job have anything to do with this?<br /> As to what electric system I'm proposing all I can say is that it really doesn't matter since it won't save that much time anyway, but anything available will produce higher accelerations with solar cells making the power because the solar power system weighs so much less vs. any currently proposed nuclear electric power system.<br />Also you said:"The g

 

[john_316]

Even though I like those ideas like Orion I think I am a little more founded to with current state technologies and what can be accomplished at the moment and in the near future.<br /><br />I feel the MTV/ERV should be nuclear powered regradless of gas core or solid core NTR to allow fast transit time and long surface stays. The 150 day outbound trip. I dont want to see the Astronauts in Zero-G over 6 months or them build the system to create a tumbling spacecraft to create artificial gravity. It complicates matters.<br /><br /><br />I dont oppose solar as it has its merit but I think a small snap reactor would be suffient for the Mars Hab rather then investing in Microwave and beam power technologies for Mars. Maybe start that here to it matures then later on apply it there.<br /><br />Also a nuclear reactor can help augument a small smelter or solar oven on Mars or Moon to start mining operations to expand and build the base there. But INSITU will be heavily relied on in that case either way.<br /><br />If Solar Power can offer the same speed and capabilty as Nuclear can at the moment then hey I am for ther other clean energy. People dont realize that cosmic radiation is just as deadly as gamma rays. <br /><br />I think a GC/NTR with 5-10 uses before Refuel or repacement would be a good space vessel. It provides reusabilty and helps augument construction and fabrication in space and gives the future workers the skills for the next 100 years.<br /><br />Of course I do envision larger spacecraft this way such as 100+ people rather than just as they call them footprints and flag planting on Mars.<br /><br />But I wont stop there. I think Titan needs further investigation and possible exploitation. Thus a robust long range space craft and lander for there as well....<br /><br />I guess I am dreaming bigger than they want to go. But by that time all the nations will be on board so we can build bigger craft for further Solar System discoveries.<br /><br />So 2001 (movie) would be nice to

 

[mlorrey]

Steve, what propulsion system are you talking about here? JIMO was a spacecraft, not an engine. Stands for Jupiter Icy Moons Orbiter, and was under the Prometheus Project, thus it used the VASIMR engine, and NO, it would not take two years to get out of LEO. At the 10k sec Isp level, it would take 30 days to get out of LEO, whereupon it would shift to the high Isp/low thrust setting.<br /><br />I have never heard of anyone but you say JIMO would take two years to get out of LEO. As I've amply demonstrated, without a minimum of 30+ HLV launches to boost sufficient fuel, no NTR propelled mission of any size is going to get into the Jupiter system, and won't get back out again without refuelling from one of those icy moons.<br /><br />Oh, and KAPTON? Are you CRAZY? Don't you know that stuff is explosive? What are you looking for, a repeat of Apollo 1? One overcurrent situation and your spaceship is gonna blow, well, higher than sky high, if you use kapton. Might as well just put DDT in the astronaut rebreathers and get it over with.

 

[SteveMick]

I would apologise for equating JIMO with its propulsion system, if it wasn't obvious that you are being disingenuous and knew exactly what I was talking about as was clear from your post. I can understand you using VASIMIR as your choice since it can operate at lower Isp than the ion engines that were being developed for JIMO and which would acheive a less ridiculous acceleration rate, although at the 100KW electric and 58,000 lb. total mass of JIMO even VASIMIR would take several months. That it wasn't the system chosen is perhaps because they didn't think it would ever work or they were working against limits on what can be launched in one piece and didn't want to consider the increase in propellent mass lower Isp requires. I don't know, but the two year figure came straight from the JIMO website.<br /> As to your comment about NTR, I find it puzzling since I have never advocated or had much nice to say about NTR's. However, since chemical rockets have sent payloads to Jupiter, I believe at twice the Isp, NTR's could handle getting into the system and there was never any plan for it to "get back out", although there was considerable delta V required for the visits to the various moons. It continues to amaze me that no NTR/electric reactor has been proposed. I advocated a solar thermal/electric system for the purpose.<br /> As for kapton, the particular material to be used as the support for the rectenna is relatively unimportant and I only said kapton because it has been proposed for this purpose in many things I've read over the last few decades. I'm curious though - exactly how does it explode in an O2 free atmosphere? Does it contain its own oxidizer like solid fuel?<br /> Now do you withdraw your ridiculous statement that STR and NTR tech is only suitable for Hohman transfers to Mars?<br />Steve