Magento: Redirect to Checkout after “Add to Cart”

app\code\core\Mage\Checkout\controllers\CartController.php

    protected function _goBack()
    {
        $returnUrl = $this->getRequest()->getParam('return_url');
        if ($returnUrl) {

            if (!$this->_isUrlInternal($returnUrl)) {
                throw new Mage_Exception('External urls redirect to "' . $returnUrl . '" denied!');
            }

            $this->_getSession()->getMessages(true);
            $this->getResponse()->setRedirect($returnUrl);
        } elseif (!Mage::getStoreConfig('checkout/cart/redirect_to_cart')
            && !$this->getRequest()->getParam('in_cart')
            && $backUrl = $this->_getRefererUrl()
        ) {
            $this->getResponse()->setRedirect($backUrl);
        } else {
            if ((strtolower($this->getRequest()->getActionName()) == 'add') && !$this->getRequest()->getParam('in_cart')) {
                $this->_getSession()->setContinueShoppingUrl($this->_getRefererUrl());
            }
            $this->_redirect('checkout/onepage');
        }
        return $this;
    }

https://indiestechtips.wordpress.com/2012/11/24/magento-redirect-to-checkout-after-add-to-cart/