Modelling rocket burn in a software simulator

[evilfer]

Hi all,
I keep the development of Trajectories, a simulator of space missions. I'm implementing finite thrust maneuvers, and I have a couple of questions about how to model rocket burns.

I'm working with the following assumptions:

- Thrust [measured in Newtons] is constant during a burn, mass variation rate (fuel consumption) [kg/s] is also constant. Isp is thus also assumed constant during the whole burn.
How realistic is it to consider these parameters constant during a burn? I've read that specific impulse depends on chamber/nozzle pressions, temperatures and exhaust gas parameters, but I don't know much about this...

- In the case of lower-than-maximum thrust maneuvers, so far I'm assuming that fuel consumption is proportional to thrust.
This would mean that the rocket's Isp is always the same for any thrust in the range [0-max thrust]. How realistic is this?

Thank you!


PD. I did search the forum, but in all cases the results were mostly unrelated topics :S.

 

[Notebook]

ISP varies with altitude, there are a couple of threads here, I'll see if I can dig them out.


This one:
http://www.orbiter-forum.com/showthread.php?t=9589

must be more, but can't find them yet.

N.

 

[evilfer]

ISP varies with altitude, there are a couple of threads here, I'll see if I can dig them out.


This one:
http://www.orbiter-forum.com/showthread.php?t=9589

must be more, but can't find them yet.

N.

Thank you for the link, very helpful!

My simulator is only for maneuvers in space, and does not include liftoff (a ship always starts in orbit around a planet).

I have the intention of including aerobraking, and thus a burn could take place inside the atmosphere. But even in this case, in the thread you linked it's said that in Earth you can ignore the pressure above 20km, in order to calculate Isp. I guess you cannot do aerobraking below that, right? For instance, Wikipedia talks about a Hiten's aerobraking maneuver at 125 km over Earth surface.

So, my questions refer only to Isp in vacuum .

 

[Wishbone]

For injection, capture and deep-space maneuver burns you can safely assume constant Isp and constant thrust as a good first approximation. It is fuel-optimal to have as short burn time as possible, so all imaginable and realistic craft will most likely have maximum thrust selected.

One exception of particular interest for Martian missions is a nuclear thermal rocket where one has ramp-up, period of constant thrust and then propulsive cooldown. Yet this is a complex case, and I haven't done the required maths myself, so cannot recommend this option to others.

 

[Notebook]

Thank you for the link, very helpful!

My simulator is only for maneuvers in space, and does not include liftoff (a ship always starts in orbit around a planet).

I have the intention of including aerobraking, and thus a burn could take place inside the atmosphere. But even in this case, in the thread you linked it's said that in Earth you can ignore the pressure above 20km, in order to calculate Isp. I guess you cannot do aerobraking below that, right? For instance, Wikipedia talks about a Hiten's aerobraking maneuver at 125 km over Earth surface.

So, my questions refer only to Isp in vacuum .

You're welcome, evilfer.
If I said "Ignore pressure above 20km", don't take that as accurate, I was probably relating to one of my own projects!

Don't know anything about aerobraking, but its a popular topic here, and you'll get lots of help regarding that.

N.

 

[evilfer]

You're welcome, evilfer.
If I said "Ignore pressure above 20km", don't take that as accurate, I was probably relating to one of my own projects!

Don't know anything about aerobraking, but its a popular topic here, and you'll get lots of help regarding that.

N.

It's ok if it's not accurate, it gave the idea that only when close to the surface pressure affects specific impulse significatively. If I can assume this, burns during an aerobrake are not so important, and modelling atmospheric drag will introduce much more error probably than variations in Isp. Thus I'll keep it simple and assume constant Isp .

Thanks!

---------- Post added at 03:00 PM ---------- Previous post was at 02:56 PM ----------

For injection, capture and deep-space maneuver burns you can safely assume constant Isp and constant thrust as a good first approximation. It is fuel-optimal to have as short burn time as possible, so all imaginable and realistic craft will most likely have maximum thrust selected.

One exception of particular interest for Martian missions is a nuclear thermal rocket where one has ramp-up, period of constant thrust and then propulsive cooldown. Yet this is a complex case, and I haven't done the required maths myself, so cannot recommend this option to others.

Nuclear thermal rocket will have to wait, at least multi stage rockets have more priority . Constant Isp it is.

Also thank you for the clarification about burn times. I think all maneuvers will be done at full thrust.

Eloy

 

[Loru]

From my point of view you can ignore atmospheric pressure for ISP calculation over 20km altitude (it will be quite match to real values) but for aerobraking you can't do that.

Pressure above 20km alt is small enough to ommit in case of engines but still aerobraking starts much higher. Look at atmospheric parametrs in earth.cfg file and study real data to determine where to start.

Also play with "aerobrake mfd" which gives you nice diagrams of decelration vs altitude.

 

[evilfer]

From my point of view you can ignore atmospheric pressure for ISP calculation over 20km altitude (it will be quite match to real values) but for aerobraking you can't do that.

Pressure above 20km alt is small enough to ommit in case of engines but still aerobraking starts much higher. Look at atmospheric parametrs in earth.cfg file and study real data to determine where to start.

Also play with "aerobrake mfd" which gives you nice diagrams of decelration vs altitude.

I agree with you, Loru .

What I meant is, in the case of a burn during an aerobrake, I can ignore the effect of the atmosphere pressure in Isp. Actually I was thinking in small burns to help aerobraking (not to cancel it).

But of course, I'll take into account the atmosphere to calculate drag and thus brake the ship.

I'll try aerobrake mfd when I decide to implement the atmosphere drag model... Won't be in the next version though :S.

Thank you!