4 minute de-orbit burn

[Seajay]

Tim Peak (and not forgetting  Tim Kopra and Yuri Malenchenko) will be coming back to earth shortly.  The thing which surprised me from the ESA report is that he'll need a 4 minute de-orbit burn.

By my rough reckoning it's only ~30 m/s to bring his periapsis down to 100km.  Even bringing it down to zero (which clearly they're not going to do) is only ~60 m/s.  4 minutes seems like a hell of a long burn to achieve that.  It implies an acceleration of 0.125 m/s² or ~1/80g

Is my reckoning wrong? do they do some other, more complicated manoeuvre to de-orbit? Or am I used to overpowered kerbal engines and a TWR of 0.013 is actually perfectly normal? 

 

Edited June 17, 2016 by Seajay

[magnemoe]

It might well be multiple burns, first an primary burn then ground checking trajectory and an correction burn perhaps multiple, this is how mechjeb does it but in RL you need trajectory data from ground 

[Bill Phil]

Actual engines used in space? In real life?

Do you know the TWR of the space shuttle's OMS? It's pretty low.

Apollo only had such a big engine because it was going to the Moon. Gemini had really small thrusters.

[Green Baron]

 

All i can find is that the Soyus TMA usually does a 4 min to 4.5 min deorbit burns. And that it takes around 45min. from end of burn to landing, g-loads in between are 4-5g. Should be enough data to calculate the dV (if only we had a nice atmospheric model). But 30m/s dV are not enough for a direct reentry from about 400km altitude.

But, yep, TWR of the Soyuz may well be in that regime ...

:-)

 

Edited June 17, 2016 by Green Baron

[Streetwind]

Juno's orbit insertion at Jupiter is going to take 35 minutes - using its "main engine", not its positioning thrusters.

This kind of thing is pretty normal in RL spacecraft, where no impatient player has to sit through the burn

[asmi]

 

2 hours ago, Seajay said:

Tim Peak (and not forgetting  Tim Kopra and Yuri Malenchenko) will be coming back to earth shortly.  The thing which surprised me from the ESA report is that he'll need a 4 minute de-orbit burn.

By my rough reckoning it's only ~30 m/s to bring his periapsis down to 100km.  Even bringing it down to zero (which clearly they're not going to do) is only ~60 m/s.  4 minutes seems like a hell of a long burn to achieve that.  It implies an acceleration of 0.125 m/s² or ~1/80g

Is my reckoning wrong? do they do some other, more complicated manoeuvre to de-orbit? Or am I used to overpowered kerbal engines and a TWR of 0.013 is actually perfectly normal? 

 

Soyuz does ~124 m/s dV deorbit burn, to bring orbit to about -2 km x 420 km.

Edited June 17, 2016 by asmi

[Lukaszenko]

...so it looks like they bring the periapsis even lower than zero. Not surprised, I didn't expect a direct translation from the Kerbal world here.

 

Edited June 17, 2016 by Lukaszenko

[Ten Key]

2 hours ago, Bill Phil said:

Apollo only had such a big engine because it was going to the Moon. Gemini had really small thrusters.

I have heard that the reason the Apollo SPS was so powerful was that it was a hold over from when NASA was looking at landing the entire spacecraft on the Moon. They kept the engine after they switched to using a separate lander, and it subsequently had a lot more oomph than it actually needed. 

[Bill Phil]

11 minutes ago, Ten Key said:

I have heard that the reason the Apollo SPS was so powerful was that it was a hold over from when NASA was looking at landing the entire spacecraft on the Moon. They kept the engine after they switched to using a separate lander, and it subsequently had a lot more oomph than it actually needed. 

Yeah. Later iterations of Apollo would've used LM engine(s).

[DDE]

On ‎17‎.‎06‎.‎2016 at 5:05 PM, Bill Phil said:

Yeah. Later iterations of Apollo would've used LM engine(s).

While I dunno about the proposed late short-SM Apollo I've seen mentioned, we have the Venus fly-by project, with an equipment container for the S-IV workshop in place of the LM requiring using a shorter dumbbell, leading to two LM engines proposed.

On ‎17‎.‎06‎.‎2016 at 4:53 PM, Ten Key said:

I have heard that the reason the Apollo SPS was so powerful was that it was a hold over from when NASA was looking at landing the entire spacecraft on the Moon. They kept the engine after they switched to using a separate lander, and it subsequently had a lot more oomph than it actually needed. 

The oomph however allowed the CSM to fully reverse its trajectory on the way to the Moon. By comparison, the Soyuz lunar mission stack had an entire fifth stage - the now-famous Block D - dedicated to lunar orbit insertion.

Edited June 19, 2016 by DDE

[Laie]

On 6/17/2016 at 1:08 PM, Seajay said:

Is my reckoning wrong? do they do some other, more complicated manoeuvre to de-orbit? Or am I used to overpowered kerbal engines and a TWR of 0.013 is actually perfectly normal?

For that purpose, the low TWR is actually 100% normal. They don't even have a proper engine at the back of their vessel to do the burn, only their RCS system -- and by KSP standards, even that is woefully underpowered.

Then again, it's fully enough for rendezvous, docking and de-orbit. So why add more? It's not as if they could timewarp between burns.

[DDE]

13 minutes ago, Laie said:

For that purpose, the low TWR is actually 100% normal. They don't even have a proper engine at the back of their vessel to do the burn, only their RCS system -- and by KSP standards, even that is woefully underpowered.

Then again, it's fully enough for rendezvous, docking and de-orbit.

I was surprised, seeing as there is a very engine-like hole at the back, along with a protective shroud animated in Tantares mods, and so I dug.

So what we got on modern-series Soyuz is a KTDU-80/S5.80 common-fuel system with 14 x 11D428 coarse RCS (14 kgf), 12 x S5.142 fine RCS (2.5 kgf) and a main thruster with four throttling valves for up to 300 kgf.

Otherwise it's a horrible mess of lackluster documentation and several dozen variants of spacecraft.

[Laie]

So I should at least have looked at a photograph before posting. I stand corrected... but: if I'm not mistaken 300kgf is ~3kN, right? For us KSP players that's still very much in the realm of ant drives and RCS.

[Reactordrone]

7 hours ago, Laie said:

So I should at least have looked at a photograph before posting. I stand corrected... but: if I'm not mistaken 300kgf is ~3kN, right? For us KSP players that's still very much in the realm of ant drives and RCS.

That's about right. The old Soyuz engine had a main engine with about 4kN but the integrated RCS/engine system used on Soyuz-T onwards was reduced in thrust (but with a better Isp) and has no backup engine since the RCS can act as the backup.

[DDE]

15 minutes ago, Reactordrone said:

That's about right. The old Soyuz engine had a main engine with about 4kN but the integrated RCS/engine system used on Soyuz-T onwards was reduced in thrust (but with a better Isp) and has no backup engine since the RCS can act as the backup.

Got any good source? Russianspaceweb told me there was a dedicated backup thruster  and I ended up trawling through a 2013 thread in some dank Russian forum.

[Reactordrone]

The Wikis on them aren't bad.

https://en.wikipedia.org/wiki/KTDU-80

https://en.wikipedia.org/wiki/KTDU-35

Edited June 20, 2016 by Reactordrone