Rocket Self-Destruct?

[kerbiloid]

2 minutes ago, tater said:

so the crew version has a 60 ton capsule, 40 tons for crew, 20 tons for LES stuff. The vehicle can still carry the mass of an empty Space Shuttle into orbit as cargo, lol.

How will you land it?

[sevenperforce]

10 minutes ago, kerbiloid said:

Problems are:
1. Have enough powerful LES (weights up to 1/3 of the escaping mass)
2. Landing several dozen tonnes. Chutes are unlikely applicable, while rocket engines and fuel have some mass and must be ignited close to the ground, so if something went wrong there would be twin set of them. Which again means mass.
3. Heatshield. Mass.
4. If overturns on ground, the upper ones should hold their eyes with hands _(joking) due to huge turn radius.

So, anyway the capsule has to pe a lifting body equipped with engines. I.e. what they usually call a plane itself.

A separate escape capsule for BFS is certainly challenging, but it's doable.

Dragon 2, fully fueled, masses 7800 kg; you can probably budget 800 kg more for a crew of 7 but let's just round up to 9 tonnes. If, as a super-simple approach, all the passengers were placed in separate Dragon 2 escape capsules and nothing more, you could pack in 70 passengers and still have 40% of BFS's payload available for a common hab, airlocks, support structure, and additional cargo. 

A monolithic crew escape capsule built into the BFS airframe would probably be able to do much better.

[magnemoe]

3 minutes ago, kerbiloid said:

I'm afraid any vessel with >6..8 passengers has no another abort option except trying to land the whole ship.

Dragon 2 can hold 7, note that for an abort system you do not need an full spacecraft, think more of the canceled shuttle escape pod. Easier on BFR as you has more weight margins. 
But yes then you get up to +25 it start get complicated.  You might dual use  the escape pod as storm shelter, you will put engines towards sun anyway.
And forget about the trust of an standard escape system, this assume you have the crew and cargo part of upper stage between you and the explosion and to get high enough to use parachutes during an landing fail. 

One tricks you could do would be fuel missions with an crew module, filling up the deep space craft over time 

[magnemoe]

3 minutes ago, sevenperforce said:

A separate escape capsule for BFS is certainly challenging, but it's doable.

Dragon 2, fully fueled, masses 7800 kg; you can probably budget 800 kg more for a crew of 7 but let's just round up to 9 tonnes. If, as a super-simple approach, all the passengers were placed in separate Dragon 2 escape capsules and nothing more, you could pack in 70 passengers and still have 40% of BFS's payload available for a common hab, airlocks, support structure, and additional cargo. 

A monolithic crew escape capsule built into the BFS airframe would probably be able to do much better.

You don't need an dragon 2 its an fully fledged spaceship able to dock, do orbital burns and reenter. The trunk only hold cargo, solar panels and radiators. That makes sense if you reuse it not for an escape pod, think blue Shepard capsule but larger, more cramped and no windows. 
I would put the solid fuel engine below, it would give you more room, inside and an way to give aerodynamic stability. Also an crumble zone if you land on land because landing fail


 

[kerbiloid]

50 minutes ago, sevenperforce said:

Dragon 2, fully fueled, masses 7800 kg; you can probably budget 800 kg more for a crew of 7 but let's just round up to 9 tonnes. If, as a super-simple approach, all the passengers were placed in separate Dragon 2 escape capsules and nothing more, you could pack in 70 passengers and still have 40% of BFS's payload available for a common hab, airlocks, support structure, and additional cargo. 

 

46 minutes ago, magnemoe said:

You might dual use  the escape pod as storm shelter,

If it has 2 ft thick walls. But then it could hardly land.

46 minutes ago, magnemoe said:

use parachutes

To land ~50 t capsule? There would be enough big chutes, and any side speed would overturn the capsule like Soyuz (because it needs D2/Soyuz shape to fit the rocket proportions)
But while Soyuz passengers overturn from 2 m height, so nothing special, a 10 m wide capsule would throw its upper passengers from 3..4-storey building height.
So, no chutes, only rocket landing.

And you have to keep the fuel until the landing, so you have to escape a 50 t capsule full of fuel. Not very sure if cargo shielding is a good protection. LES should be there and have its own fuel.

But OK, the capsule has happily escaped.
It reenters and performs aerobraking. Mass/area is (square-cube) several times less than Soyuz or Dragon have. So it needs a thicker heatshield below the engine section.
The heatshield is probably ~10 m in diameter. Add 20 tonnes more.

And unless you have a lifting body, overloads are sigificantly greater than Soyuz/D2 would have. (mass/area, square-cube)
But it's not so simple to make a lifting body keeping D2 proportions because... mass/area, square-cube. 50 t capsule would be much flatter.

OK, aerobrake is finished, heatshield is separated.
Your (established? equilibrium?) velocity is greater than D2 or Soyuz has - by the same reason, square/cube, mass/area. Not ~150..200. m/s, but ~250..300 m/s.
So, more fuel needed.

As this is an escape capsule, not just a reentry one, you land on a random place, not on a flat and clear landing pad.
So, it has to perform a several kilometers side maneuvre to avoid landing onto a roof or falling into a river.
10 km / 0.25 km/s = 40 s to do this.
So, you need 200 m/s more of delta-V.
(100 * 40 = 4 km, and take it twice because of inertia and one time more because you have to stop the side movement).

Now you have a dilemma: you have to ignite the engines at enough high altitude to have some time to switch on the reserve set of engines if something went wrong.
But you have to ignite them at as low altitude as possible to save fuel.
So, you have to ignite them somewhere at 1..2 km and spend additional fuel on lowering.

As you are landing onto a random place you can't know your altitude so accurately that you could stop right on the ground.
You have to stop lowering at several meters altitude, zeroize h and v speeds, align your capsule and keep hovering slowly lowering.
Say, 10 s of hovering  = 10 * 9.81 = 100 m/s of delta-V more.

So, we can say that a massive capsule either would keep much greater amount of fuel than D2 would (if it were a real vessel rather than an abort test mockup) and carry strong people.
Also it would be a flying fuel tank with minimum survavibility.

So, the only way to avoid this madness is to have a lifting body with greater area/mass which can glide as shuttle with no engines.
So, this means - no capsule at all. Glider with high redundancy of systems.
And small auxiliary turbojets to reach the shore of an ocean. (Which it turn means kerosene as main fuel and probably (for orbital needs, not for turbojets) HTP as oxidizer/RCS monoprop)

Edited March 2, 2018 by kerbiloid

[sevenperforce]

A separate escape capsule for BFS is certainly challenging, but it's doable.

Dragon 2, fully fueled, masses 7800 kg; you can probably budget 800 kg more for a crew of 7 but let's just round up to 9 tonnes. If, as a super-simple approach, all the passengers were placed in separate Dragon 2 escape capsules and nothing more, you could pack in 70 passengers and still have 40% of BFS's payload available for a common hab, airlocks, support structure, and additional cargo. 

A monolithic crew escape capsule built into the BFS airframe would probably be able to do much better.

You don't need an dragon 2 its an fully fledged spaceship able to dock, do orbital burns and reenter. The trunk only hold cargo, solar panels and radiators. That makes sense if you reuse it not for an escape pod, think blue Shepard capsule but larger, more cramped and no windows. 
I would put the solid fuel engine below, it would give you more room, inside and an way to give aerodynamic stability. Also an crumble zone if you land on land because landing fail

Agreed, you don't need a full spaceship; I was just giving a worst-case-scenario where you do separate Dragon 2 capsules for each set of 7 passengers.

Having a few more capabilities (like a little on-orbit maneuvering) comes in handy if you have a major problem in space (think docking mishap or MMOD/debris strike to the main LOX tank). Sure, you could launch a rescue mission, but if you already have an escape capsule, you might as well be able to use it.

Why would placing a solid LES motor underneath be better? I mean, I get not putting a launch tower on top, but you could as easily put them on the sides. I would say to use a bank of hot-gas methane-ox thrusters (RCS thrusters for the BFS/BFR) for the escape engines, with pressurized tanks inside the escape capsule.

23 minutes ago, kerbiloid said:

To land ~50 t capsule? There would be enough big chutes, and any side speed would overturn the capsule like Soyuz (because it needs D2/Soyuz shape to fit the rocket proportions)
But while Soyuz passengers overturn from 2 m height, so nothing special, a 10 m wide capsule would throw its upper passengers from 3..4-storey building height.
So, no chutes, only rocket landing.

You're overcomplicating. Soyuz lands on land, with retrorockets, because they have no large body of water near their launch sites. US-based capsules always splashdown.

Plus, you don't need a D2 shape. You can go biconic. Your heat shield can be part of the main vehicle heat shield; the escape capsule is placed at the very tip of the BFS and uses pneumatic or pyrotechnic breakaway from the main body. The biconic entry vehicle has some degree of body lift, which avoids nasty ballistic entry, and splashes down, belly-first, with chutes. The passengers can egress using the same door used to move into the main hab area, and you can throw on some airbags for flotation if you like.

[kerbiloid]

20 minutes ago, sevenperforce said:

Soyuz lands on land, with retrorockets, because they have no large body of water near their launch sites. US-based capsules always splashdown.

Normally. A 2..3 m wide capsule with 2..3 special humans inside.
An escape capsule lands where it wants. And don't forget, it will return another 40 humans (mostly not acrobats, but weak nerds) from orbit. Or the same ones if docking has failed.
That's not a problem when you have one layer of astronauts in a small capsule. If it overturns, they just roll over head, nothing dramatic.
But a 10 m roller with several layers of people inside would land more carefully.

Spoiler

20 minutes ago, sevenperforce said:

Your heat shield can be part of the main vehicle heat shield; the escape capsule is placed at the very tip of the BFS and uses pneumatic or pyrotechnic breakaway from the main body.

The capsule itself will be still a cone-cylnider, not a saucer. So, this changes nothing in mass/area sense.
Unmanaged reentry of Soyuz/Vostok caused 8..10 g, and this thing would cause > 10 g. Poor nerds, poor heatshield.

A minute of history

Spoiler

 

Edited March 2, 2018 by kerbiloid

[sevenperforce]

52 minutes ago, kerbiloid said:

Normally. A 2..3 m wide capsule with 2..3 special humans inside.
An escape capsule lands where it wants. And don't forget, it will return another 40 humans (mostly not acrobats, but weak nerds) from orbit. Or the same ones if docking has failed.
That's not a problem when you have one layer of astronauts in a small capsule. If it overturns, they just roll over head, nothing dramatic.
But a 10 m roller with several layers of people inside would land more carefully.

BFR launches next to the water and does its ascent over water. BFS lands on a pad next to water. Any abort puts the escape capsule in the water. 

If the capsule needs to be used as an orbital lifeboat, then it can choose where to do its deorbit burn, which again will put it in the water. It has plenty of dV for a deorbit burn since it doesn't need to do an escape burn.

Chutes, splashdown, low center of gravity, wide footprint. No possibility of rollover.

52 minutes ago, kerbiloid said:

The capsule itself will be still a cone-cylnider, not a saucer. So, this changes nothing in mass/area sense.
Unmanaged reentry of Soyuz/Vostok caused 8..10 g, and this thing would cause > 10 g. Poor nerds, poor heatshield.

No, it won't be a cone-cylinder; it will be biconic. Forward CoM, wide entry area, aft CoP. Think less D2, more Dream Chaser. But bigger.

Passively aerodynamically stable on abort (all regimes), entry, and landing. Chute placement designed for splash down with a low forward velocity.

[kerbiloid]

1 hour ago, sevenperforce said:

more Dream Chaser

which in turn is the lifting body

[sevenperforce]

32 minutes ago, kerbiloid said:

which in turn is the lifting body

...which is a problem why?

[magnemoe]

4 hours ago, kerbiloid said:

 

If it has 2 ft thick walls. But then it could hardly land.

To land ~50 t capsule? There would be enough big chutes, and any side speed would overturn the capsule like Soyuz (because it needs D2/Soyuz shape to fit the rocket proportions)
But while Soyuz passengers overturn from 2 m height, so nothing special, a 10 m wide capsule would throw its upper passengers from 3..4-storey building height.
So, no chutes, only rocket landing.

And you have to keep the fuel until the landing, so you have to escape a 50 t capsule full of fuel. Not very sure if cargo shielding is a good protection. LES should be there and have its own fuel.

But OK, the capsule has happily escaped.
It reenters and performs aerobraking. Mass/area is (square-cube) several times less than Soyuz or Dragon have. So it needs a thicker heatshield below the engine section.
The heatshield is probably ~10 m in diameter. Add 20 tonnes more.

And unless you have a lifting body, overloads are sigificantly greater than Soyuz/D2 would have. (mass/area, square-cube)
But it's not so simple to make a lifting body keeping D2 proportions because... mass/area, square-cube. 50 t capsule would be much flatter.

OK, aerobrake is finished, heatshield is separated.
Your (established? equilibrium?) velocity is greater than D2 or Soyuz has - by the same reason, square/cube, mass/area. Not ~150..200. m/s, but ~250..300 m/s.
So, more fuel needed.

As this is an escape capsule, not just a reentry one, you land on a random place, not on a flat and clear landing pad.
So, it has to perform a several kilometers side maneuvre to avoid landing onto a roof or falling into a river.
10 km / 0.25 km/s = 40 s to do this.
So, you need 200 m/s more of delta-V.
(100 * 40 = 4 km, and take it twice because of inertia and one time more because you have to stop the side movement).

Now you have a dilemma: you have to ignite the engines at enough high altitude to have some time to switch on the reserve set of engines if something went wrong.
But you have to ignite them at as low altitude as possible to save fuel.
So, you have to ignite them somewhere at 1..2 km and spend additional fuel on lowering.

As you are landing onto a random place you can't know your altitude so accurately that you could stop right on the ground.
You have to stop lowering at several meters altitude, zeroize h and v speeds, align your capsule and keep hovering slowly lowering.
Say, 10 s of hovering  = 10 * 9.81 = 100 m/s of delta-V more.

So, we can say that a massive capsule either would keep much greater amount of fuel than D2 would (if it were a real vessel rather than an abort test mockup) and carry strong people.
Also it would be a flying fuel tank with minimum survavibility.

So, the only way to avoid this madness is to have a lifting body with greater area/mass which can glide as shuttle with no engines.
So, this means - no capsule at all. Glider with high redundancy of systems.
And small auxiliary turbojets to reach the shore of an ocean. (Which it turn means kerosene as main fuel and probably (for orbital needs, not for turbojets) HTP as oxidizer/RCS monoprop)

The water or composite walls will remain, think of it as an launch tube. 
Don't see that simple escape will cover all settings, far into second stage burn the forces are less so less chance of something going catastrophic wrong. it will be hard to do an escape during reentry so you ignore that to. No capsules has an plan to survive an catastrophic fail of service module in orbit anyway. Just an engine + rcs fail will be fatal.
It will cover first stage burn, second stage separation and first part of its burn, while not covering reentry but it will cover landing.
It will be  lightweight, again the new Shepard is an good model, you want it larger and an bit stronger heat shield as new shepard just reaches mach 3 but you want to get past mach 6. 
You will splash down unless fail during touchdown. You might want to use some second solid fuel rockets for this landing, They will also be useful for pad abort settings. The solid rockets for braking uses radar triggering, has been used for artillery shells since ww2.
You will obviously have no control over there you land, you don't have with escape systems.
Like an ejection seat an escape system is not safe, its just way safer than riding to your doom. 

[kerbiloid]

6 hours ago, sevenperforce said:

...which is a problem why?

Is somebody saying this is a problem? This is the asymptote.
And if you have a 50 t glider for crew of 40, and several tens tonnes of expendables, either this escape is the ship itself (without an additional capsule inside) with expendable trunk for the cargo, or you face all same problems as listed above and have to build a 150 t glider around the 50 t glider, or make one reusable 150 t glider without any escape pod.

4 hours ago, magnemoe said:

far into second stage burn the forces are less so less chance of something going catastrophic wrong.

Soyuz-18-1 and lost Progress not so long ago had problems with 3rd stage.
This ship carries several tens not most useless people, btw, and is designed to fly regularly,. Every time when a spaceship with 1..7 people was lost, flights were stopped for several years.

4 hours ago, magnemoe said:

No capsules has an plan to survive an catastrophic fail of service module in orbit anyway

?
Any capsule can return if service module still can deorbit it in any way.

4 hours ago, magnemoe said:

It will be  lightweight,

it will be at least 1 tonne/person, as all of them.
And probably even heavier because large capsule can't rotate as fast as small one (stresses are several times greater) and has to be more soft and gentle, so needs additional fuel.
Like an airbus vs cessna.

4 hours ago, magnemoe said:

new Shepard is an good model, you want it larger and an bit stronger heat shield as new shepard just reaches mach 3 but you want to get past mach 6. 

Orbital speed is ~20 near-ground Mach.  At 3 Mach the heatshield has finished its mission.

4 hours ago, magnemoe said:

You will splash down unless fail during touchdown.

 

8 hours ago, sevenperforce said:

BFR launches next to the water and does its ascent over water. BFS lands on a pad next to water. Any abort puts the escape capsule in the water. 

Apollo, Gemini, Mercury were to be splashed. But every of them had an option do be grounded, though not so softly.
Escape capsule should work all the flight, otherwise it is not an escape capsule.
What if after getting into orbit the ship has problems and has to return asap rather than getting to a station?
Or what if it has reached 6500 km/s speed, has problems with booster, and has to perform a suborbital flight to a random point in Eurasia like ICBM?

[sevenperforce]

On 3/3/2018 at 12:05 AM, kerbiloid said:

And if you have a 50 t glider for crew of 40, and several tens tonnes of expendables, either this escape is the ship itself (without an additional capsule inside) with expendable trunk for the cargo, or you face all same problems as listed above and have to build a 150 t glider around the 50 t glider, or make one reusable 150 t glider without any escape pod.

As others have said above, abort is not "safe". Abort is a safety contingency, to allow survival when your primary means of survival fails. There is no need to insist on an infinite regression of nested escape modules, any more than there is a need to put ejection seats on a Dragon 2. 

Also, you're probably really overestimating mass. The BFS has a dry mass of 85 tonnes. The ITS Tanker was 60% the dry mass of the ITS Spaceship, so we can estimate the mass of a BFS tanker/cargo ship at 51 tonnes. This means that SpaceX is estimating around 34 tonnes for the entire crew and cabin area of the BFS Spaceship, not including OML structure.