Resultant Force and Acceleration of Space craft

[Rapid A]

Homework Statement

https://isaacphysics.org/questions/gcse_ch2_13_q10
https://www.physicsforums.com/attachments/213735
https://i.imgur.com/WnZkt83.jpg

The thrust generated by a rocket engine is equal to the mass of propellant burnt each second multiplied by the exhaust velocity of the gas. The Space Shuttle (with booster rockets and external tank) had a total mass of 2 040 000 kg at launch. In this question we shall assume that the exhaust velocity of the gas was 3 000 m/s
https://www.physicsforums.com/attachments/213736
https://i.imgur.com/atgZFfP.jpg
My problem here is that I've tried my best in solving this but it won't accept my answer of 20400. I've tried other methods which gave me answers like 61200.

2. Homework Equations
Acceleration = Resultant Force / Mass of Object
Thrust = Mass of propellant burnt/s x Exhaust velocity of gas (m/s)

The Attempt at a Solution

Since I'm supposed to work out the mass of propellant burnt per second, I did:

• Thrust = 20 400 000 N

Because that's the amount needed to equal the weight of the rocket hence the conversion from KG - N (at my stage g = 10 m/s²)

• Velocity of gas = 3 000 m/s

20400000 = Mass of propellant burnt/s x 3000
Mass of propellant burnt = 6800 kg

I worked out the mass of propellant burnt per second for lift off; so to work out the mass of propellant burnt per second to make the rocket accelerate at '3g' (i.e 30 m/s²) I tried multiplying 6800 by 3 (20400), by 30 (204000), by 9 (61200), by 900 (6120000) and none of them seem to be the right answer.

For lift off I sort of guessed so I'm not sure of the formula I've used. It would be helpful if I was given the formula to use and some assistance on working out the answer to part B of the question. To test the question on the website, here's the link:
https://isaacphysics.org/questions/gcse_ch2_13_q10
and please let me know if you've managed to get the right answer.

 

[.Scott]

The question specifies acceleration "from the launch pad".
That pad is already accelerating at 1g.

I responded used 2 significant digits and was marked "correct".

 

[Rapid A]

The question specifies acceleration "from the launch pad".
That pad is already accelerating at 1g.

I responded used 2 significant digits and was marked "correct".

Did you answer the two questions and if so what formula did you use?

 

[.Scott]

Did you answer the two questions and if so what formula did you use?

I answered both questions - and they were both marked "correct" - with a note on the first one saying that using g=9.8m/s^2 (vs. 10m/s^2) was not required.

I didn't give you the formula, because I wanted you to work it out.
My hope is that my hint would be enough:

The question specifies acceleration "from the launch pad".
That pad is already accelerating at 1g.

As additional hints:
What is the total acceleration on the spacecraft at launch?
You closest response was 20400 Kg.

 

[Rapid A]

I would assume the total acceleration at launch would be 10 m/s²
I just got the answer thank you soo much. What I did was calculate how much would be burnt for 4g instead since you said the launch pad is already accelerating at 1g.
How would one know that since it wasn't specified in the question?
Thank you again. Is there anything I can do to show my appreciation?☺

 

[.Scott]

You are accelerating at 1g - or at least that is the force that needs to be applied to you to keep you from falling.
So 1g gets you nowhere.

 

[Rapid A]

I understand now. Thank you.