scramjet to orbit (a way to do away with rockets)

[mrmorris]

<font color="orange">"Mars' atmosphere is too thin for the SR-71's wings to provide sufficient lift. "</font><br /><br /><font color="yellow">"Not true! Mar's atmospheric density is very similar to the SR-71's maximum ceiling."</font><br /><br />The SR-71 ceiling was ~85,000 feet. On Earth according to this NASA calaulator, that equates to .36 psi or 7.0E-5 slugs/ft3.<br /><br />Mars at 0 ft using the same calculator is .101 psi, or 2.0E-5 slugs/ft3.<br /><br />So I say again. No. This doesn't even take into account that at the SR-71's <b>max</b> ceiling that it was also going its <b>max</b> speed (i.e. when it was getting <b>max</b> lift from its wings).

 

[tap_sa]

Check atmospheric <i>densities</i>. Mars 0.020kg/m³, Earth at 85k ft 0.034kg³, it's that thicker CO₂. Not sure which is more important to generate aerodynamic lift, pressure or density but I'd assume that the relatively thicker Mars atmosphere would help. And like bobvanx already said, the 0.38g gravity helps a lot. Mars atmosphere might be a lot colder so less mass is required to cope with heating issues. Powering engines is the most difficult one, you could try to burn magnesium powder.

 

[mrmorris]

<font color="yellow">"Check atmospheric densities"</font><br /><br />I did -- and posted it -- just not in metric. slugs/ft3 *is* a density. But thank you for playing.

 

[tap_sa]

Oh, my bad, sorry. But the NASA calc gives me 6.0E-5 snails/box of 30.48cm per side or 0.317 psi for Earth at 85k ft, odd. Google coughed up several pages with the 0.020kg/m3 for Mars, which equals to...3.9E-5 slugs/ft3.<br /><br />Of course it makes a lot of difference whether the airstrip is on top of Olympus Mons or at the bottom of Valles Marinesis, latter being preferred.

 

[mrmorris]

<font color="yellow">"But the NASA calc gives me 6.0E-5 snails/box ..."</font><br /><br />It looks like I put in 84000 feet to get the 7.0E-5 slugs/ft3. <br /><br />Our Earth figures are fairly close, but your Mars one is about half of mine. I can't claim 100% confidence in the NASA calculator being correct -- but given a choice -- I'd bet on NASA knowing the atmospheric density of Mars better than anyone else. That doesn't mean the programmers of this app were worth a floop, of course.<br /><br />In any event -- I don't see why it matters. Both the pressure *and* the atmospheric density of Mars are well below Earth at 85k no matter <b>which</b> set of figures you use! Your statement <i>"...it's that thicker CO2. Not sure which is more important to generate aerodynamic lift, pressure or density but I'd assume that the relatively thicker Mars atmosphere would help."</i> seems to imply that you'd just shown that Mars' atmosphere is denser. I don't know why you're saying that when right before that you show <i>"...Mars 0.020kg/m3, Earth at 85k ft 0.034kg3..."</i>. <br /><br />.034 is greater than .020. Ergo -- even if .020 is correct, rather than the .010 that the NASA site shows... the density <b>at ground level</b> on Mars is still about half as dense as the atmosphere at the SR-71's ceiling at max velocity. So what exactly are you arguing?

 

[g_sat]

What do you think is the best method of getting from the surface of the earth to leo if not with the use of wings or lifting bodies? <div class="Discussion_UserSignature"> </div>

 

[vogon13]

Nuclear ram-jet was Project Pluto.<br /><br />Probably could be modified to work on Mars. (but would still need a rocket booster to get up to ram-jet operating regime)<br /><br />Low gravity would really help as the reactor was rather heavy. IIRC, 500 megawatts worth of US 1950s technology brute force aircooled atomic reactor.<br /><br />Advances in composites and CAD wing designs would make this pretty doable, at first glance.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[ve7rkt]

<font color="yellow">What do you think is the best method of getting from the surface of the earth to leo if not with the use of wings or lifting bodies?</font><br /><br />answer: A rocket.<br /><br />Now, what's the best way to get back down again? If not wings or lifting bodies, you're pretty much left with the semiballistic capsule and parachutes, or variations on that theme (Gemini's paraglider, Soyuz' capsule + chutes + rockets)...

 

[vogon13]

As for 'best' way to low earth orbit, the moon, anywhere in the solar system, and the nearer stars, with payloads from 4,000 to 25,000,000 <i>tons</i>, Orion is the best. Period.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[tap_sa]

<font color="yellow"> the density at ground level on Mars is still about half as dense as the atmosphere at the SR-71's ceiling at max velocity. So what exactly are you arguing?</font><br /><br /><b>If</b> (I don't know for sure) aerodynamic lift is directly proportional to density then the SR-71 has at least theoretical chances of flying (at very low Mars altitude) because martian gravity is only 0.38g ie. <i>less</i> than half.

 

[mrmorris]

<font color="yellow">"...the SR-71 has at least theoretical chances of flying..."</font><br /><br />You're just determined to force me to work out the math. OK -- let's calculate the relative lifts involved here.<br /><br />Googling up an equation for aerodynamic lift, I find:<br /><br />L = Cl * p * V^2/2 * A<br /><br />L = Lift<br />Cl = Coefficient of lift<br />p = Air density<br />V = freestream velocity (i.e. airspeed at the lifting surface)<br />A is the surface area of the lifting surface<br /><br />Cl and A will both be constant -- since we're not changing the plane. The change in Lift then between Mars and Earth can be calculated using the following:<br /><br />L = p * V^2/2<br /><br />Since the SR-71 will only weigh 38% of what it does on Earth -- so long as lift is 38% of what it is on Earth, the SR-71 should be able to fly. The SR-71 has a max airspeed 'above 2000 mph', so we'll use 2,000 as the airspeed for maximum altitude on Earth. Using the above equation, and using dimensionless numbers for lift at max altitude on Earth:<br /><br />L = p * V^2/2<br />L = .034 * 2000^2/2<br />L = .034 * 2000000<br />L = 68000<br /><br />So on Mars -- to be able to fly, we must be able to produce 38% of 68,000 or 25,840. Since that and density are known, we'll solve for the minimum airspeed:<br /><br />L = p * V^2/2<br />25,480 = .020 * S^2/2<br />1,292,000 = S^2/2<br />2,584,000 = S^2<br />1607 = S<br /><br />So -- The SR-71 can get off the ground on Mars once it hits a speed of 1607 mph. Note this is using the 0.020kg/m3 figure that you found, rather than the 0.015kg/m3 figure that I get from the NASA calculator. If we were to assume that the NASA figure is correct, then the calculation would be:<br /><br />L = p * V^2/2<br />25,480 = .015 * S^2/2<br />1,698,666 = S^2/2<br />3,397,332 = S^2<br />1,843 = S<br /><br />So the SR-71 would need to hit 1,843 mph to make it off the ground. That better be one frigging long theoretical runway. <img src="/images/icons/smile.gif" />

 

[tap_sa]

<font color="yellow">That better be one frigging long theoretical runway.</font><br /><br />Indeed. Even if lift-off if solved by some sort of rocket sled the landing will be next to impossible. Undercarriage that survives nearly 2000mph landing speed? <img src="/images/icons/wink.gif" /><br /><br />I believe some sort of flying will eventually take place on mars, but probably with planes resembling more U2 than SR-71. While waiting for the real thing one can try it using X-Plane.

 

[mrmorris]

<font color="yellow">"...but probably with planes resembling more U2 than SR-71. "</font><br /><br />Ayup -- in my first post dismissing the SR-71 being used on Mars I said:<br /><br /><i>"Planes on Mars would require enormous wings. Think glider."</i><br /><br />The U-2 is about as close to a glider as you can get... without ditching the engines.

 

[nacnud]

This beasty might stand a chance, it can fly at 65Kft, so a little bit low for a Mars equivalent altitude.

 

[mrmorris]

<font color="yellow">"This beasty might stand a chance..."</font><br /><br />Getting the lift required all comes down to wing surface area, mass, and airspeed. That's a low mass plane with relatively lots of wingspan, but it's not intended for speed.<br /><br />Also -- getting airspeed is harder on Mars than on Earth due to the low atmospheric density. Props are <b>much</b> less efficient. For flying on Mars, I'd suggest hydrogen dirigibles. The lack of oxygen in the atmosphere would make them very much safer than here on earth, and the structural components and inflation (LH) are very low mass.

 

[spacefire]

an aircraft flying on mars would most likely look like a rocket propelled U2 with twice the wingspan <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[mrmorris]

<font color="yellow">"...a rocket propelled U2..."</font><br /><br />You might want to note the date in your diary, sf. I actually have no disagreements with this post.<br /><br />Of course that might have something to do with the fact that what you stated is a distillation of everything that I've posted previous to it on this thread about planes on Mars... <img src="/images/icons/smile.gif" />

 

[spacefire]

I actually didn't bother to read much of what you posted...it's just common sense.<br />A rocket will be the most practical mode of propulsion in a rarefied atmosphere with little oxygen.<br />High aspect ratio wings -another no brainer.<br />The airfoil shape should be similar to those used for model aircraft on Earth, because thye Reynolds number is going to be pretty low also. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[soccerguy789]

I'm gonna have to dissagree with you there, sort of. the problems with rockets is that they need to carry oxidizer, which is heavy stuff. but without oxygen in the atmosphere you can't have jets, so what do you do? you use something called a NIMF. it was suggested by dr. Robert Zubrin in his book The Case for Mars. It can be a rocket plane or a ballistic hopper but what it does is, pumps the atmosphere straight into a tank, and then uses a nuclear thermal rocket to shoot it out the nozzel. noo need for oxidizer, and you dont even need to carry return fuel, you can just suck it into your tank when you land. CO2 may not be the best fuel, but it will work, and the dry mass of the vehicle is unreal! this is likely to be the first flying vehicle on Mars, though I believ it will take the form of a balistic hopper befor it takes the form af a plane, simply because the hopper can then (thanks to 1/3 g) fly up to orbit and transport explorers and cargo to and from some klind of interplanetary transport. no need for expendable cargo landers!