For Questions That Don't Merit Their Own Thread

[peadar1987]

On 16/08/2017 at 9:40 AM, 1101 said:

One thing I've seen as a reason not to use NTRs is potential public issues with putting nuclear reactors at risk of launch failure.  How much worse for the environment than Hypergolics (already present in many, if not all, rockets), would it be?  How would the effects differ between say, a launch off the Cape and debris falling into the sea, as opposed to Baikonur?

So nuclear fuel isn't actually particularly radioactive until the reactor is switched on. The chemical toxicity is actually much more of a problem than the radioactivity, and uranium isn't that much more toxic than, say, lead. If you're using the NTR as an upper stage it's pretty much a complete non-issue.

One possible concern would be nuclear proliferation. To save mass you'd use very highly enriched fuel, which could be used to make a nuclear weapon were it to fall into the wrong hands

[DDE]

On ‎16‎.‎08‎.‎2017 at 5:40 PM, wumpus said:

Of course, the usual "public panic" about toxic substances always ignores the dose.  Ignore the dose and any toxicity report is mindless babel.

You sound a lot like Glushko's memo arguing for the deployment of an RD-301 ammonia-fluorine upper stage for the Proton.

[K^2]

12 hours ago, DDE said:

You sound a lot like Glushko's memo arguing for the deployment of an RD-301 ammonia-fluorine upper stage for the Proton.

Um... Maybe not on Proton. They don't have the best flight record.

[DDE]

4 hours ago, K^2 said:

Um... Maybe not on Proton. They don't have the best flight record.

That was exactly his point. Soviet safety limits of fluorine in the air were much, much higher than for UMDH or NTO, so what are you pansies complaining about?

Just line the launchpad with charcoal, see NASA TN D-3118, from which I'm currently blocked due to rampant Russophobia among .mil websites.

Edited August 22, 2017 by DDE

[_Augustus_]

On 8/21/2017 at 3:14 PM, DDE said:

You sound a lot like Glushko's memo arguing for the deployment of an RD-301 ammonia-fluorine upper stage for the Proton.

That was an idea? Did it have really high ISP or something?

[DDE]

2 hours ago, _Augustus_ said:

That was an idea? Did it have really high ISP or something?

It was more than an idea; it'd been test-fired and flight-certified.

It was Glushko's way of getting hydrolox-like performance (400 sec, only 8 sec fewer than the RD-401 ammonia nuclear motor) without hydrogen's deep cryogenics.

Edited August 31, 2017 by DDE

[Spaced Out]

The last topic was posted a while ago, so I am going to assume it has been answered. 

Could throttling down at max q, instead of just limiting aerodynamic stress, also decrease delta V loss by not just plowing through the atmosphere there?

[Grand Ship Builder]

Just now, Spaced Out said:

The last topic was posted a while ago, so I am going to assume it has been answered. 

Could throttling down at max q, instead of just limiting aerodynamic stress, also decrease delta V loss by not just plowing through the atmosphere there?

Yes. Terminal velocity.

[Spaced Out]

2 minutes ago, Grand Ship Builder said:

Yes. Terminal velocity.

Okay sorry but I am going to have to ask you to clarify what you mean by that. 

[Grand Ship Builder]

Just now, Spaced Out said:

Okay sorry but I am going to have to ask you to clarify what you mean by that. 

Terminal velocity is the max speed you can go in the atmosphere before the air starts slowing you down a lot. This doesn't only apply to Max-Q. Throttling down will help you up until you are below terminal velocity. Terminal velocity is higher the higher you are, due to thinner atmosphere and therefore less drag.

[Spaced Out]

1 minute ago, Grand Ship Builder said:

Terminal velocity is the max speed you can go in the atmosphere before the air starts slowing you down a lot. This doesn't only apply to Max-Q. Throttling down will help you up until you are below terminal velocity. Terminal velocity is higher the higher you are, due to thinner atmosphere and therefore less drag.

Is it therefore efficient to be as close as you can to terminal velocity then? Or is there a more efficient way?

[Grand Ship Builder]

Just now, Spaced Out said:

Is it therefore efficient to be as close as you can to terminal velocity then? Or is there a more efficient way?

It's efficient, but you should only worry about that when you're off the ground. Otherwise worry about TWR and stuff.

[Spaced Out]

11 hours ago, Grand Ship Builder said:

Otherwise worry about TWR and stuff.

I'd imagine they would try to have a TWR that would keep them near terminal velocity. I know TWR goes up over time but they probably adjust the ascent profile to make it so their acceleration curve always has the rocket near terminal velocity. They could also throttle during flight if they need to after adjusting the ascent profile.

Tell me if any of this is wrong or not.

Edited September 7, 2017 by Spaced Out

[Steel]

11 hours ago, Grand Ship Builder said:

Yes. Terminal velocity.

Can I just point out that terminal terminal velocity is just the velocity whereby force due to gravity on a falling object is equal to the drag acting on it as it falls. It has very little to do with rockets and their ascents.

[Spaced Out]

23 hours ago, Steel said:

Can I just point out that terminal terminal velocity is just the velocity whereby force due to gravity on a falling object is equal to the drag acting on it as it falls. It has very little to do with rockets and their ascents.

He probably meant the top speed you can reach wherever you are in the atmosphere before losing more delta-v to drag than you save by accelerating faster. Because, I mean, aren't those the speeds you are aiming for throughout the flight?

Edited September 8, 2017 by Spaced Out

[wumpus]

On 9/6/2017 at 11:17 PM, Grand Ship Builder said:

Terminal velocity is the max speed you can go in the atmosphere before the air starts slowing you down a lot. This doesn't only apply to Max-Q. Throttling down will help you up until you are below terminal velocity. Terminal velocity is higher the higher you are, due to thinner atmosphere and therefore less drag.

This seems to be outdated advice from the days of the "souposphere".  Outside of somewhere like Eve, it shouldn't be possible to hit terminal velocity.

It does, however, give you a great way to measure aero losses: terminal velocity = 1g of aero losses.  2*terminal velocity = 4 g losses (drag increases by the square of velocity).  The catch is that a rocket shape has the least drag possible for a given mass, and terminal velocity will get high pretty fast, and keep increasing as the atmosphere gets thinner (so you don't lose a lot of terminal velocity if you stage in the atmosphere).  So  if you somehow managed to accelerate to terminal velocity, TWR=2 should keep you there (but you will only accelerate as fast as your terminal velocity drops.  It would take an extreme atmosphere for this to be an issue (Eve, Venus,  or "souposhere Kerbin".  Certainly an outer planet if you got low enough).

A naive attack on the Goddard problem:

acceleration=a (in "gees"), v=velocity (speed really, I'm ignoring direction) V_T=terminal velocity TWR=thrust to weight ratio. e=efficiency, theta = angle of your prograde velocity (cos(theta)=1 until you start your "gravity turn").

a=TWR-cos(theta)-(V/V_T)²

e = (TWR-cos(theta)-(V/V_T )²)/TWR

I suspect that this will have to be solved numerically, but at least it will likely tell you that you can keep TWR arbitrarily high below terminal velocity.  In reality, maxQ (and max acceleration your crew and structure* can withstand) will limit TWR, as well as cost (especially if you are using liquid engines).

* note that the total weight of the rocket on the structure shouldn't increase beyond max thrust at full fuel no matter what the acceleration of the rocket.  Thrust = ma and the total weight = ma, so weight = thrust.

Edited September 8, 2017 by wumpus
close parens

[Spaced Out]

Okay guys I got all the answers I needed on my last question here is a new one: if Elon Musk is concerned with the air quality and want dossil fuels to stop being used, why is he using kerosene in his rockets? I know there isn't much carbon dioxide yearly compared to cars, but it takes it right to the upper atmosphere. What does he plan to do when the launch cadence rises and his rockets become a bigger factor in global warming?

[IncongruousGoat]

3 hours ago, Spaced Out said:

Okay guys I got all the answers I needed on my last question here is a new one: if Elon Musk is concerned with the air quality and want dossil fuels to stop being used, why is he using kerosene in his rockets? I know there isn't much carbon dioxide yearly compared to cars, but it takes it right to the upper atmosphere. What does he plan to do when the launch cadence rises and his rockets become a bigger factor in global warming?

Because there isn't much environmental impact right now, and kerosene is cheap and easy to handle, unlike any other fuel you could care to name. Plus, he doesn't plan to use it forever-as far as anyone can tell, he plans to switch to methalox for SpaceX's next LV family (whatever that may end up being).

[Spaced Out]

1 minute ago, IncongruousGoat said:

Because there isn't much environmental impact right now, and kerosene is cheap and easy to handle, unlike any other fuel you could care to name. Plus, he doesn't plan to use it forever-as far as anyone can tell, he plans to switch to methalox for SpaceX's next LV family (whatever that may end up being).

But isn't methane more of a pollutant, or does it turn into something harmless out of the nozzle?

[IncongruousGoat]

2 minutes ago, Spaced Out said:

But isn't methane more of a pollutant, or does it turn into something harmless out of the nozzle?

No, it goes to a whole bunch of stuff, but mostly CO2 and H20. Honestly, I think the answer here is just that chemical rocket launch cadences will never be high enough to have a major environmental impact. By the time we're launching enough things that it's a problem, we'll have come up with something cleaner.

EDIT: And, honestly, just emitting CO2 is remarkably clean for a rocket. Kerosene produces CO2, as well as CO and probably some fun sulfur compounds. Storables, of course, are a mess of exhaust products, as are solids. The only thing cleaner than methalox that I can think of is hydrolox, and SpaceX has no intention of using hydrolox.

Edited September 10, 2017 by IncongruousGoat

[Spaced Out]

16 minutes ago, IncongruousGoat said:

EDIT: And, honestly, just emitting CO2 is remarkably clean for a rocket. Kerosene produces CO2, as well as CO and probably some fun sulfur compounds. Storables, of course, are a mess of exhaust products, as are solids. The only thing cleaner than methalox that I can think of is hydrolox, and SpaceX has no intention of using hydrolox.

Actually hydrolox is worse because it it taking some H2O up to the upper atmosphere where it is sparse, and water is actually a worse greenhouse gas than CO2.

[IncongruousGoat]

Just now, Spaced Out said:

Actually hydrolox is worse because it it taking some H2O up to the upper atmosphere where it is sparse, and water is actually a worse greenhouse gas than CO2.

I stand corrected.

[Spaced Out]

Okay guys, next question. What are your ideas for bio-friendly fuels? By this I mean fuels that don't contribute to global warming etc.

[kerbiloid]

If a fuel doesn't contribute to global warming, it isn't a fuel.

Hydrolox is not a "clean" fuel because hydrogen is mostly produced not by water electrolysis, but from natural gas.
You just exhaust gaseous wastes at the plant, not at the launchpad.
And you have to process more natural hydrocarbons to get the hydrogen, than if just burn them in the rocket directly.

On 07.09.2017 at 6:09 AM, Spaced Out said:

Could throttling down at max q

If you have to throttle something down, this means that your flightplan is far from optimal.
Because this means you have to carry a dead weight instead of using it to push your craft.

Edited September 11, 2017 by kerbiloid

[kerbiloid]

(double)

Edited September 11, 2017 by kerbiloid