New Mars Forums

Hi ejp,

Calculating a low thrust trajectory is more of an art than a science at the moment and requires mathematics typically not taught at the UG level.  There are a couple tools by NASA (SEPTOP) used as JPL for validating low-thrust trajectories.  It is very much an 'end first' operation and the more fleshed out your mission the better.  An associate did some calculations a few years back, I'll see if I can look them up for you (just moved, please be patient).

In the meantime, the following website should be useful: http://trajectory.grc.nasa.gov/tools/

edit:  Keep in mind that low-thrust trajectory is of little value for a manned mission, though it may be useful for cargo and larger unmanned missions.  You may want to check out VASIMR and contact Dr. Franklin Chang-Diaz.  He is a very nice guy and understand many of the practical challenges to such a system.

There are a few scenarios which can adequately explain the 'coning' that was witnessed before the feed was terminated.  First, it is possible that the lateral movement was initiated by stage separation.  This is fairly normal; the Atlas V typically experiences 3+ g-loading during staging events.  This may indicate that the ACS system was not robust enough to compensate for the forcing modes (the rocket did seem to hit an oscillational mode towards the end) extinguishing its ACS fuel early in the stage event.  This could easily be the result of a poorly designed feedback loop in the guidance/navigation control.  This was my initial thought before seeing the video and it still may have played a part in its failure.

The next explanation is that the second stage was experiencing fuel sloshing, which is fixable with baffles installed in the fuel tank.  This is a rather simple issue, though it would be considered amateurish if it was missed.

Ultimately, it was a failure of a systems analysis of the rocket with no single subsystem being the culprit.  The reliability of the Falcon I will be abysmal until an adequate systems analysis is done, even then it would be average at best.

Don't get me wrong, I would really like to see SpaceX succeed, and the guys working on the project are getting a rare opportunity to get their hands dirty with rocket design (a rarity in the industry today).  My concern is that they still have a few key piece missing before they will have a reliable launch system on their hands.  At least they're better off than most of these alt.spacers.

Just got back from a visit with some friends of mine a couple weeks ago.  From what I'm hearing P&W is pulling every trick out of the book they can think of to get performance out of the J2X.  The gas generator *may* need a redesign in order to get a higher chamber pressure; troubling considering the nozzle exp ratio is moving well above the 40:1 range.  Obviously they're looking at adding a second J2X, which will most likely work.

The biggest challenge NASA is facing now is that they (or anyone really) lack expertise with using methane for propellant; many of NASAs contacts are intended to build experience in the aerospace community.  Looking forward to discussing this further.

edit:  for context.

Your breaking one of the cardinal rules of rocket design with this statement; the Payload-to-gross weight should be proportionate for each stage.  It COULD be accomplished otherwise, but would not be very efficient system/stage-wise

This is only true if landing on a flat surface.  There are landing situations that would make a three leg lander unstable and highly dependent upon weight distribution, surface inclination, etc.

Also,

I recommend picking up a used copy of 'Rocket Propulsion Elements' by George Sutton.  Most of the back of the envelope calculations you are looking for can be founded there.  You could probably ignore most of the derivation and skip to the algebra (those this will give you greater understanding of the mechanics).  You'll probably pay $30 from barnesandnoble.com and would be a good investment.

To address so of the suggestion you brought up.  As you scale up your tank, some of the mitigated factors will become much more significant.  Structural imperfections will have a much greater likelihood of popping up and you'll quickly begin to increase weight as you try to reinforce the structure.  Deformations and the influence on adjacent structures is also of consideration.  There are also a host of combustion related issues that can influence the design of the rocket.

Anyhow, good luck and hope you able to get a look at Sutton.

Zubrin has no crew medic nor a dedicated mission commander.  The engineers are in charge of keeping them alive; while at least one mission scientist is needed.  I can understand the argument for 5, but 4?  Too many compromises, no margin.

^^^ agreed :-D ^^^

You'll have an extremely difficult time find any of my collegues agreeing with such a sentiment.  It is irresponsible to build a mission architecture on such an approach.  It is the goal of any responsible engineering to minimize risk, not assess acceptable levels of risk; these are the same conditions that created the "robust and adaptable" shuttle.  I know two astronauts who left the program because of such an attitude and it would be inconceiveable to allow that to happen again.

GCNR, I as I was alluding to there fundamentally aren't any problems with the technologies presented in MD.  I do have issue in designing an architecture around technology that hasn't been proven on the scale needed for a manned mars mission.  Should it prove to be a viable alternative, the mission would benefit from great power capabilities as well as a larger science payload.  These technologies have a low TRL (tTechnology Readiness Level) with little indication of development.  Anyway, keep it up.  Mars Society can definite mobilize general public, but needs more vetting of ideas and information that come through.

Unfortunately, we cannot go to Mars with "the aerospace establishment that we have" to paraphrase Donald Rumsfield.  You must develop the hardware that will competently achieve the mission requirement to be undertaken by a manned mission to Mars.  This can not be done with Mars Direct.

The most glaring error of Mars Direct is that the survival of the crew depends on unproven technologies.  In-situ fuel production and inflateable structures may prove viable for successive mission, but an evolving exploration architecture cannot depend upon them.  The lack of redundancy and low factor of safety pose too great a risk to mission and crew, forgiving the fact that there is no margin for a mission commander, physician or necessary overlap in task functionality.  Honestly, if the Mars Direct method is not used on a precursor mission - like the moon - there is little chance that any of its technologies will be used as a framework for a manned Mars mission.

Finally, Zubrin has done little to garner the support of the aerospace community outside of a few enthusiast groups.  No government, corporate or academic organizations have any seriously involvement into the Mars Direct architecture.  Mars Society seems unable to pick its batttles, and in several ways is more restrictive than the aerospace establishment it detest.  Mars Direct does have a couple of good ideas in there, but there are too many single points of failure engineers would have to agree to in order to go forward with MD as is.

Please listen to GCN.  The skepticism he present is a strong part of the engineering process; if your ideas can't get pass this sort of critisism it will never become workable.  Further, the Russians lost may valuable ground crew members during their failed launch attempts; do not discredit there contribution.

Zubrin has done little more than say "we'll add a nukeclear rocket later" with no real study into integration, environmental consideration or development cost and schedule.  It is not worth mentioning and should be irrelevant when discussing the merrits and shortcomings of Mars Direct.

The reason a 6 mo return trip would be unacceptable for a 500 day mission where only have the time is spent in a non-zero g environment should be obvious.  The body of work regarding the effects of long-term microgravity (as well as prolonged exposure to radiation) are to immature.  Further, the very nature Mars Direct give the astronauts NO 'abort' option other than a cyanide capsule.  If the astronauts can not be return with minimal effect to their health than the cost are too high.

I personally would love to see a clean sheet rocket (more interesting work for me), but the reality is the general public would not have the attention span to endure the cost and time of such a project.  We are competing with iPods, celebrities and other areas of science that have a more direct impact on the well being of most individuals.

Though you are 'annoyed' by the assurance of astronauts safety, it is a moral obligation of those of us who have chosen to make engineering career.  There are alway risk, but ALL reasonable measures should be taken to mitigate the risk to the personnel involved.

In-situ fuel production.  It may work, but the mission design should progress as though it will not be feasible.  It is hard enough to build, test, prep and launch a rocket here on Earth; try adding the complexity of doing so 0.5 to 2.5 AU away?  Good luck.  At best (from an engineer stand point) your launching a Falcon I; more than likely you'll be launching something akinned to the Atlas V or the stick.  Fun!

Space exploration is hard, there are no clever shortcuts that will make it easier.  The complexity is there to mititgate risk to crew and personnel, not to inconvience them.  Mars Direct would be more palpable for a sample return mission AND would prove techologies that can be adapted to a manned mission

That would be be my guess; the fact that the thing crumbled like an aluminum can and still managed fly for 40 seconds or so hopefully bodes well for a "clean" rocket straight off the line.  This particular rocket has been exposed to the elements, moved numerous times and be decompressed/recompressed.  I sincerely hope this represents a "worst-case senario" for the Falcon I.  We are in desprerate need for a low cost alternative to space, and hopefully Elon will be able to provide it.  Otherwise, we will be depending one ULA for a long time to come.