Landing a rocketship

[DDE]

@shynung, interesting, I thought it didn't do anything to drag and only kept the really hot plasma away.

[shynung]

Just now, DDE said:

@shynung, interesting, I thought it didn't do anything to drag and only kept the really hot plasma away.

It's a magnetoplasma heatshield, really. How they managed to do that, I have no idea.

[StrandedonEarth]

1 hour ago, shynung said:

MSNW LLC was recently putting forward that idea.

[kerbiloid]

17 hours ago, magnemoe said:

It will still protect engine as it pushes the shock front away from the ship, this has been seen on Falcon 9 braking burns.

According to http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/
Falcon's first stage separates with Mach=10, i.e. speed ~3000 m/s.  Not much greater than ISP.
Normal deorbit reentry means 7900 m/s inital speed, i.e. the gas flows twice faster into the bell, than out of it. Not sure if the engine would spit the fireball out rather than swallow it down the throat.
Falcon's engine restarts on speed 1300 m/s, which is much lower than ISP.

13 hours ago, Nibb31 said:

would combine the Falcon 9 upper stage with a Dragon-like design

Which would mean very low center of mass and huge fuel loss to keep the rocket from staying vertical.

Edited August 15, 2016 by kerbiloid

[magnemoe]

2 hours ago, kerbiloid said:

According to http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/
Falcon's first stage separates with Mach=10, i.e. speed ~3000 m/s.  Not much greater than ISP.
Normal deorbit reentry means 7900 m/s inital speed, i.e. the gas flows twice faster into the bell, than out of it. Not sure if the engine would spit the fireball out rather than swallow it down the throat.
Falcon's engine restarts on speed 1300 m/s, which is much lower than ISP.

Speed of the air close to the rocket is much lower this is why air is compressed and you got friction and heat. 
Yes the expansion is lesser, its not like airspeed inside the rocket engine is high

[Rune]

On 13/8/2016 at 5:51 PM, DDE said:

Let's assume we have an honest-to-Heinlein VTVL SSTO, with a bell-like shape for optimized hypersonic aerodynamics.

How would you go about landing such a machine? Would you aerobrake engines-in, or aerobrake with your bow and then flip?

Depends, but the classical answer is "bottom first".

You can of course go the DC-X route, and reenter nose-first before flipping for terminal landing. No need to be afraid of the turn, because it would be done at subsonic speeds by stalling the whole thing, as @Tex_NL says. That was done so that the thing could maneuver in the high atmosphere, since an engine-first reentry is mostly ballistic. It did carry a structural penalty due to having to build the thing so it flew.

If you go bottom first, however, you should take some steps to protect your engine instead of your nose. This is the reason all of Bono's SSTO concepts used aerospike engines: he intended to use them as actively-cooled heatshields by bleeding fuel trough them. This is cool, because it means that the heatshield subsystem is almost weight-free. A separate heatshield would be a mass hit a SSTO can ill afford. It is also cool because an aerospike nozzle is perfect for a SSTO.

 

Rune. And for boop's sake, chutes are heavy. Mt per landed mT, one of the heaviest landing systems!

[magnemoe]

20 hours ago, Tex_NL said:

One does not need to re-enter either nose or tail first. A belly first shuttle style re-entry with a tail landing is still very much within the realm of possibilities.
Re-enter just like the shuttle and glide towards the landing area. Pull the nose up to vertical until it stalls to bleed off horizontal speed. Open nose mounted airbrakes and/or grid fins to come in for a controlled tail landing.

This should work too, note that this would not be an classical rocketship, more like the one pictured in the first post, you want something who is stable aerodynamic as an lifting body. 
 At some point you would lit the engines and use the gimbal to take you vertical for landing. 

[DDE]

31 minutes ago, Rune said:

If you go bottom first, however, you should take some steps to protect your engine instead of your nose. This is the reason all of Bono's SSTO concepts used aerospike engines: he intended to use them as actively-cooled heatshields by bleeding fuel trough them. This is cool, because it means that the heatshield subsystem is almost weight-free. A separate heatshield would be a mass hit a SSTO can ill afford. It is also cool because an aerospike nozzle is perfect for a SSTO.

That reminds me... http://www.astronautix.com/s/sassto.html

[cantab]

I re-enter the atmosphere like a shuttle or spaceplane and descend until I'm gliding at around 500 m and 130 m/s. Then I pitch up hard, gradually easing on the power, until I'm pointed vertically and at near zero speed. If I fly it well I won't gain more than 1000 metres altitude doing this manoeuvre. Then I descend vertically, controlling my speed because it's aerodynamically unstable flying backwards, and touch down gently.

The powered descent does take some time though, and is only practical with efficient jet engines. Doing it on rockets would be too thirsty. On the other hand this spaceplane has big wings, a rocketship with much smaller fins would be less unstable flying backwards and so could descend and land more quickly.

[tater]

20 hours ago, kerbiloid said:

According to http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/
Falcon's first stage separates with Mach=10, i.e. speed ~3000 m/s.  Not much greater than ISP.

I_sp is not a velocity, the units are seconds, it is c/g, where c is the exhaust velocity. 

 

[Bill Phil]

27 minutes ago, tater said:

I_sp is not a velocity, the units are seconds, it is c/g, where c is the exhaust velocity. 

 

ISP = specific impulse. Impulse = Ns. Specific impulse = Impulse per unit mass. Ns/kg = m/s. Isp can be said to be either, and thus the confusion...

Here's Wikipedia on the subject:

Quote

If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g₀) before dividing it into the thrust, converts specific impulse from the mass basis to the weight basis.^[2]

 

Edited August 16, 2016 by Bill Phil

[tater]

I never see rocket I_sp in any units but seconds in practice, not just KSP which uses this convention. I'd call the other formulation effective exhaust velocity.

[kerbiloid]

3 hours ago, tater said:

I never see rocket I_sp in any units but seconds in practice

(In Russian literature ISP in seconds is obsolete, used in 1960s books. Main unit is m/s, this also looks very informative, as it is an effective exhaust speed.)

As here we speak about speeds, not engines comparison, m/s seemed for me more actual in this case.

Edited August 16, 2016 by kerbiloid

[tater]

Yeah, In English the trouble is that seconds vs m/s with the same term used is pretty confusing, hence "effective velocity" vs "specific impulse" usually being used for the two terms. I was trained in the 80s, and my professors were certainly of the 60s  . I can certainly see the utility of the mass formulation, but I'd be worried about mixing up figures with available dv (in my head). 

[Bill Phil]

10 hours ago, tater said:

I never see rocket I_sp in any units but seconds in practice, not just KSP which uses this convention. I'd call the other formulation effective exhaust velocity.

That's mostly due to us Americans forgetting about our own unit of mass: the slug. We "mass" everything in pounds. Kind of crazy, but people also say that something weighs a certain number of kilograms, so... *shrug*

In practice it's used in a lot of different things. Seconds is useful to allow for conversion to m/s and ft/s, or anything really.

[tater]

43 minutes ago, Bill Phil said:

That's mostly due to us Americans forgetting about our own unit of mass: the slug. We "mass" everything in pounds. Kind of crazy, but people also say that something weighs a certain number of kilograms, so... *shrug*

In practice it's used in a lot of different things. Seconds is useful to allow for conversion to m/s and ft/s, or anything really.

We never used slugs in physics. Or pounds. Ever. We didn't even discuss Imperial units as a thing, not even in the early 80s, just SI. Someone did, I'm sure, but not anyone I ever saw.

[cantab]

I think those unusual units are more from an engineering background than a physics one. And in the UK I'm pretty sure "pound" is legally defined as a unit of mass, and pound-force is secondary.

[wumpus]

5 hours ago, tater said:

We never used slugs in physics. Or pounds. Ever. We didn't even discuss Imperial units as a thing, not even in the early 80s, just SI. Someone did, I'm sure, but not anyone I ever saw.

Which (in the USA in the 1980s) meant you could only "do" physics on paper.  Sure, all your calculations and (hopefully) all your measurements will be in metric*, but don't count on being able to buy parts defined by metric figures.  US commerce is typically imperial.  Oddly enough, even printed circuit boards are typically manufactured to mills (thousandths of inches) instead of metric even though there don't exist any imperial electric units so *everything* else on the board *has* to be metric, and the designers are presumably more used to metric than mils.  Presumably the housing the thing fits into is defined by imperial, so all length units are defined by imperial.

* The place I was an intern had a french engineer pointing out that calculating imperial is easy: just convert to metric, calculate, and convert back.  Even the pro-imperial mechanic admitted that such was exactly how even pro-imperial americans did it.

[tater]

I think the pope that built the Space Shuttle and all our space probes "did" physics in the real world. Regardless, in astrophysics all the measurements are at scales small and large that are outside human experience, anyway (and we used metric, exclusively).

It's absurd that military/NASA contractors kept using imperial as long as they did, but they had a workforce and tooling in place, so it was easier to just go with what they had and were used to. For actual hardware it doesn't matter, arbitrary units are arbitrary. Of course maybe cognitive science people have determined that imperial negatively affects cognition  .

[wumpus]

Doesn't the "backburn" (out of atmosphere) slow the falcon 9 booster down to 1000m/s to 2000m/s (with 2000m/s being a "hot landing")?  I'd also assume that a falcon heavy will stage significantly faster and have to backburn down to at least 2000m/s to land.

PS: I'm pretty sure NASA [Goddard] lists the speed limit in imperial (miles per hour).  I don't expect anything else to be in imperial.  There is at least one door on that campus that says [unofficially]: "Open slowly (1rad/sec)", but that still seems pretty quick to me.  I was working for a company that exclusively worked on subcontracts for Lockheed Martin when they lost the Mars Climate Observer [1999], everything was in imperial.

[Bill Phil]

9 hours ago, tater said:

We never used slugs in physics. Or pounds. Ever. We didn't even discuss Imperial units as a thing, not even in the early 80s, just SI. Someone did, I'm sure, but not anyone I ever saw.

Have you seen any NASA design documents? They all have imperial... And they're not for public viewing, either. We have used Imperial. And the tools they currently have are from the 80s, and they're imperial. Now, of course, they use metric, but they still use legacy software.

[tater]

I said in (astro)physics, not at NASA. Like I said, I'm sure that engineers, even after training otherwise use imperial for the simple reason you state, legacy tools (and training/staff used to those). 

[Bill Phil]

On ‎8‎/‎16‎/‎2016 at 7:18 PM, tater said:

I said in (astro)physics, not at NASA. Like I said, I'm sure that engineers, even after training otherwise use imperial for the simple reason you state, legacy tools (and training/staff used to those). 

As far as I can tell the most common astrophysics units are AUs and various radii of celestial bodies, like Earth radii, Jupiter radii. And Earth masses. Of course there's also plenty of metric used...

[tater]

Yeah, SI, it's physics. Certainly not Imperial, even in the early 80s.